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RoSPA_man

Crossing fuel gauge wires?

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RoSPA_man

Guys

I have a faulty fuel gauge unit. It shows empty always, thereby triggering the yellow warning light and the flashing gas pump.

 

I have both covered on the dash display with black tape to avoid distraction while riding and I reset the trip at each gas fill so I’m happy, but….

 

…could I disconnect the connector to the fuel gauge unit in the tank and then cross some wires to fool the bike into believing it always has a full fuel tank, thereby extinguishing the warning light an the flashing gas pump in one hit?

 

If so , which wires?

 

Reminds me of that movie…Juggernaut – “Cut the blue….”

Cut the blue

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dirtrider

Afternoon Hugh

 

You can try to fool the fuel gauge system but so far as I know nobody has come up with a way to do it reliably.

 

 

One thing you can't do is just disconnect it as there are 2 sides to that gauge strip in the tank. A heater side & fuel level resistance side.

 

You can try removing the green/red wire & brown/blue wire from the connector then plug the connector back in. Then try adding some resistance between the removed green/red wire & brown/blue wire. Maybe start with 2.5 ohm 1/4 watt resistor & if that doesn't work try maybe a 5 ohm then try a 10 ohm.

 

This is assuming the sender side is what is causing the issue, if the problem is on the heater side then maybe try adding a 30 ohm 1/2 watt resistor across the heater side (of the disconnected from the tank connector) as well as trying the " adding some resistance between the removed green/red wire & brown/blue wire. Maybe start with 2.5 ohm 1/4 watt resistor & if that doesn't work try maybe a 5 ohm then try a 10 ohm".

 

One thing I can say is, fooling that electronic fuel gauge won't be an easy slam dunk.

 

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RoSPA_man

Excellent -will try that and hopefully I can dial in my preferred tank level!!!

Thanks DR -how does any one person know so damn much- you could win Mastermind (uk quiz) with BMW boxers as your specialist subject!

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dirtrider

Afternoon Hugh

 

Unfortunately I'm pretty well guessing on this one based on the resistances I have measured on a working fuel strip.

 

This thing works by the bike's electronics supplying power TO the fuel strip heater to heat it then it monitors the resistance change in the sensor side resistor. That makes at a crap shoot to fool it.

 

If you end up getting something to work let us know what it took to get the light to go out.

 

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ESokoloff

Just a thought... Would using a pot(variable resister) be a better way to try this?

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Hall Vince

Be intersting to if there was a fix for those of us out (or soon to be at my mileage) of warranty.

 

Hello Hugh,

\v/

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dirtrider
Just a thought... Would using a pot(variable resister) be a better way to try this?

 

Morning Eric

 

Sure, I can't see how that would hurt on the level sensor side. I don't think you would want to use a pot on the heater side or at least use a pot on the heater side without some type of load resistor in series.

 

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Bob_Sheehan

Since there are two circuits, one for heating and the other for resistance, it seems entirely reasonable that the reported quantity of fuel on board could be a function how much that fuel has cooled the second circuit and effected its resistance.

 

But what if it works the other way? What if the current required (in the first circuit) to maintain a specified resistance in the second circuit is used to calculate fuel quantity?

 

Bob

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dirtrider

Afternoon Bob

 

Possibly, but on the one & only working strip I tried to measure the voltage on the heater side didn't change (I assume voltage would change with current change). I didn't try to measure the current into in either side though.

 

To me that sensor strip looks a lot like a simple Wheatstone bridge type affair.

It looks like possibly passing current through the heater side heats the sensor then the high resistance side is measured as it changes with how much of that resistor is submerged in the gasoline.

 

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RoSPA_man
Afternoon Bob

 

Possibly, but on the one & only working strip I tried to measure the voltage on the heater side didn't change (I assume voltage would change with current change). I didn't try to measure the current into in either side though.

 

To me that sensor strip looks a lot like a simple Wheatstone bridge type affair.

It looks like possibly passing current through the heater side heats the sensor then the high resistance side is measured as it changes with how much of that resistor is submerged in the gasoline.

 

OK I bought a "Lucky dip" bag of potentiometers today and several of the 20 or so in the bag seems to be 0-4 ohm range so it might get some sort of reading! I will let you know.

 

Reading this thread, I'm wondering why the hell BMW didn't stay with a simple float!

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w2ge
Afternoon Bob

 

Possibly, but on the one & only working strip I tried to measure the voltage on the heater side didn't change (I assume voltage would change with current change). I didn't try to measure the current into in either side though.

 

To me that sensor strip looks a lot like a simple Wheatstone bridge type affair.

It looks like possibly passing current through the heater side heats the sensor then the high resistance side is measured as it changes with how much of that resistor is submerged in the gasoline.

 

OK I bought a "Lucky dip" bag of potentiometers today and several of the 20 or so in the bag seems to be 0-4 ohm range so it might get some sort of reading! I will let you know.

 

Reading this thread, I'm wondering why the hell BMW didn't stay with a simple float!

 

They changed back to a float... (DR, I believe it is a simple whaeastone bridge design, too.)

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EddyQ

Here is an interesting thread LINK

 

So, according to them, there are two circuits. A heater across pins 1 and 4 which measures 32-35 ohms. Then a resistor across pins 2 and 3 which measure about 2.4kohms. What always fails is the resistor. So keep the heater wired properly because the computer checks. Then replace the connections to pins 2 and 3 with a 2.4Kohm, may work. So far though, nobody got it to work yet.

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dirtrider

 

Reading this thread, I'm wondering why the hell BMW didn't stay with a simple float!

 

Afternoon RoSPA_man

 

 

That's a good question. With the shape of the tank I'm sure some of the reason for not using a float was packaging concerns while maintaining accuracy & a linear reading.

 

Might also have something to do with the reserve amount on the originals being programmable.

 

 

It sounds like they (BMW) finally got a float working as the newer 1200RT's are supposed to have a float instead of that darn fuel strip.

 

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RoSPA_man

OK guys

I put the GS-911 on the bike and there was a fault code for the fuel level circuit (resistance circuit) but not for the heater circuit. Disconnecting and reconnecting toggled the heater to fault/no fault so we can assume we are dealing with a fault at the resistor side of things I feel.

 

With connector back in to have the heater circuit working, I tried a number of pots of varying resistance but failed to see any change whatsoever on the gauge itself. But I confess I haven't tried a sufficient range of resistores so far.

 

Then it struck me that maybe I need to wait 90 seconds to allow samplings. Or does the bike has to move? I know that when I had a working gauge way back then, no matter how long I sat after filling up with gas i.e. after a significant change to fuel level, the gauge would not register until I had moved. Could it be that I would need to apply a resistor, then move the bike etc. or should I see a change immediately?

 

In other words, what is the drill to ensure I am doing everything I need to in order to get (any) reading on the gauge?

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WestyLancs

Hugh,

if it's any use, on occasion I have topped up the tank on my RT from a fuel can in my garage after a ride.

If left for a good few seconds (no idea how many) the gauge does indicate the new level.

To answer in part your question, wait - yes, move - no

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dirtrider

Afternoon Hugh

 

 

If your heater is working & you show a fault on the fuel level circuit side then unplug the connector & measure the resistance in the fuel strip, the cold sensor resistance should be between 2.2K to 2.8K. If it is then your problem might lie some other place like the thing lost it's calibration or ?????

 

Or maybe this resistor thing just won't work. As I mentioned someone else tried the resistors & could get the light to go out.

 

Definitely try different resistances as more resistance sure won't hurt the system. Also try adding resistance as it heats as maybe it is looking for not only a (certain) resistance but a changing resistance as it heats the strip.

 

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RoSPA_man
Afternoon Hugh

 

 

If your heater is working & you show a fault on the fuel level circuit side then unplug the connector & measure the resistance in the fuel strip, the cold sensor resistance should be between 2.2K to 2.8K. If it is then your problem might lie some other place like the thing lost it's calibration or ?????

 

Or maybe this resistor thing just won't work. As I mentioned someone else tried the resistors & could get the light to go out.

 

Definitely try different resistances as more resistance sure won't hurt the system. Also try adding resistance as it heats as maybe it is looking for not only a (certain) resistance but a changing resistance as it heats the strip.

 

Hi DR

The resistance circuit is open for sure (it is also open on the old strip that I still have on the shelf).

 

So I will persevere with the resistor approach - it's easy and doesn't stop me using the bike. I have lengthened the wires out so I can refit the Tupperware and still play with that circuit.

 

One question arises - originally you mentioned using resistance like 10,20,30 ohm but then you say above that the resistance circuit should be 2.4K, ie 2,400 ohms so should I not be using that sort of amount of resistance instead of,say, 30 ohms?

 

Many thanks

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dirtrider

Morning Hugh

 

Definitely try using more resistance. The empty strip resistance is (supposed) to be in the 2.2k-2.8K range. My thinking was a full tank would leave very little strip resistor exposed above the fuel therefore much lower resistance.

 

I have a new strip hanging on my wall here as well as a few failed ones. I guess I really should take the time one of these days & throw the new one in a bucket of gasoline (er mineral spirits), then power up the heater side & measure the resistance across the level side as I lift the strip out of the fluid.

 

One of the things that is very baffling is on 2 of the failed strips I have here the dry resistance on both the heater side & level sensing side ohms out to very close to a new un-used strip. So those strips are either not really failed but something else caused the system to not read them correctly or they only act up when submerged in the fuel.

 

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Jameseo

This is how it was explained by JoelWisman on ADV. He claims to have worked for BMW in the past...

 

"The 2,400 ohms is the tank FULL reading because the strip is all at room temperature.

 

When the strip is in the bike, a current is applied to pins 1 and 4 which heats the strip. The portion of the strip that is above fuel level warms greatly which changes the resistance to a higher value.

 

The portion of the strip that is immersed in fuel does not appreciably change temperature because the fuel wicks away the small heating produced in the strip.

 

 

For calibration, the MOSS is attached to the bike, and the completely dry strip is attached to the bikes connector while the strip is outside of the fuel tank.

 

The bike reads the 'un-heated' resistance of the strip and records that as 'tank full', then heats the strip and records the higher heated resistance as 'tank empty'.

 

A dry unheated strip will have the same resistance as a strip fully immersed in gas with heating current flowing, so all wee need to do is plug in a resistance that equals a dry strip (possibly a little lower, strips do vary some), leave the heater plugged in, and I think we have it."

 

So far I've been trying to spoof the strip with a 10k pot on the strip circuit and leaving the heater hooked up. Tests have been unsuccessful so far but I still have things to try. Will advise on results when I have them.

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w2ge

for the future.. it would be nice if we could calibrate these strips with the GS-911. (In case we have a dealer, factory unwilling to replace under warranty)

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RoSPA_man

Yo!

That which was a mystery is now solved through a very lucky series of coincidences and a little deduction. Thanks DR for the pointers which led to the lucky breakthrough.

 

As you know, in the saga so far, I had added some cable to the resistance circuit's 2 wires so that it was easier to manage and work on. I did the connection (luckily) with those little connectors (with screws) that are normally used for joining wires in domestic situations.

 

I cobbled together a 2.8K Ohm resistor (3w) and added in a variable resistor in series so that I could increase resistance easily. Naturally I was using a multimeter to take readings.

 

Lucky bit 1: Having spent over an hour with no luck, I decided to hard wire in my Draper multimeter to free up my hands and so I could have a continuous readout of the resistance settings.

 

Lucky Bit 2: That I happened to put the multimeter probes/ pins into the connectors the right way around (it matters…read…on)

 

Suddenly I was getting readings - and every 60-70 seconds the gauge would indeed change if the resistance was changed! The meter was reading around 2.8K ohms to start. NB That was the reading WHEN the meter was probing the circuit - i.e. connected in to the wires going to the innards of the bike i.e. those two leads that used to connect to the resistance side of the fuel gauge in the tank. OK so far?

 

If I was to measure just the resistance in isolation, the reading would be higher by about .6K ohms. By this I mean that say the resistance is 2.8Kohms; when that resistance is attached to the wires on the bike, the reading drops to 2.24K approx.

 

Anyway, the big Eureka moment was that the gauge works great and reliably and I can now even dial in the fuel level,all warning lights go out etc. – as long as the meter which is reading the resistance is in the circuit. I realised that weird fact when I removed it!As soon as I did, the gauge went back to its old trick of failing - and staying failed.

 

Furthermore, the polarity of the meter matters – it took me 20 mins to realise that after I removed and refitted the meter probes and nothing worked!

 

So it seems that a)the meter must, by definition, be pushing out a voltage (to do its job of measuring resistance) and b) spookily that that voltage is exactly/roughly what is needed by the bike’s electronics to activate the gauge!

 

I was about to ask that question yesterday on this forum – i.e. does the resistance circuit not get some power from the heater circuit – the answer would now seem to be – Yes it does!

 

What I couldn’t measure, frustratingly, was what voltage is at the meter’s probes when it is in Resistance measuring mode. I need a second meter in order to measure the first! But I will measure it tomorrow!

 

Then if that voltage/”juice” needs to be provided, I will have to establish some way of providing that voltage – I’m assuming it’s not going to be 12V (that would be too easy!) so I am postulating at this stage (it’s late here) that it may need something like a tiny battery to be added in to the mix to keep the gauge energised.

 

Anyway, that probably explains why others got mixed/no results. The lucky bit was "hard-wiring" in the meter's probes and being lucky with polarity - they also need to in the circuit for at least 60-70 seconds - something that doesn't happen if you just take readings and remove the probes!

 

Summary: We are almost there! Tomorrow, I will see what resistance exactly is needed to achieve half, full readings etc. - I had great fun dialling in my endless supply of gas though!

 

Edited by RoSPA_man

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Bob_Sheehan

Hugh,

 

Thanks for sticking with this. You might end up saving many of us significant money and/or aggravation.

 

Bob

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DR  Major

Curious, what polarity did you find worked on which wire?

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Jameseo
Lucky Bit 2: That I happened to put the multimeter probes/ pins into the connectors the right way around (it matters…read…on)

 

Right on!!! I got the same results!! Positive probe of my meter has to be connected to green/red and negative to brown. This is outstanding that it is repeatable across multiple bikes with multiple probes. I measure zero volts when trying to the meter measuring resistance. Hmmm...

 

I'll keep working at it.

Edited by Jameseo

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RoSPA_man
Lucky Bit 2: That I happened to put the multimeter probes/ pins into the connectors the right way around (it matters…read…on)

 

Right on!!! I got the same results!! Positive probe of my meter has to be connected to green/red and negative to brown. This is outstanding that it is repeatable across multiple bikes with multiple probes. I measure zero volts when trying to the meter measuring resistance. Hmmm...

 

I'll keep working at it.

 

Update and question for an electrical expert

 

This morning, I repeated the tests and I can say that it is fully repeatable. With the resistors in place, gauge doesn't work. Tickle the circuit with the meter probes - meter set to 20k resistance mode and negative probe to brown, positive to green/red - and the gauge swings into action.

 

I found an old analog meter (let's call it meter 2) which works on a single Duracell 1.5V battery. Using Meter 2, I measured the voltage at the probes of my Draper digital Meter (Meter 1), set to measure up[ to 20k resistance. Not altogether surprisingly, it reads 9V which indeed corresponds simply to the battery in meter 1.

 

I then tried using meter 2 (with its 1.5V battery) as the "tickler" in the circuit and the gauge does not work.

 

So, I am beginning to believe that 9V (the voltage of Meter 1) was enough to cause the gauge to work and I am beginning to therefore believe that applying the bike's 12V will be OK (and extremely convenient!!) but I'm nervous about connecting in 12V - should I? I don't want to undo all my good work by frying the electronics with 12V. Any opinions?

 

Will the circuit just take what it needs if I connect 12V etc.

 

Back to Juggernaut again - connect 12V or not?

 

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Boffin

Hugh, I think you may be close to cracking this one.

 

1. There will need to be current flow through the resistor for the circuit to work.

2. Replacing the strip seems to fix the system

3. If I was designing this, I would add some sort of isolation to the circuit to protect against miss-connection such as a diode in the sensing line - hence polarity matters.

4. I would also use a diode to feed the power to the strip for the same reasons.

5. Diodes are relatively low-reliability devices if marginally specced.

 

I would add a diode to feed power from the heater circuit to the strip. One of these should do the trick. You may also need to provide a ground on the other lead, the ohmmeter was doing that in effect.

 

Andy

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RoSPA_man
Hugh, I think you may be close to cracking this one.

 

1. There will need to be current flow through the resistor for the circuit to work.

2. Replacing the strip seems to fix the system

3. If I was designing this, I would add some sort of isolation to the circuit to protect against miss-connection such as a diode in the sensing line - hence polarity matters.

4. I would also use a diode to feed the power to the strip for the same reasons.

5. Diodes are relatively low-reliability devices if marginally specced.

 

I would add a diode to feed power from the heater circuit to the strip. One of these should do the trick. You may also need to provide a ground on the other lead, the ohmmeter was doing that in effect.

 

Andy

 

Hi Andy

Thanks for the guidance. Just checking what you mean. I am going to apply 12V from say the accessory socket so that it is switched, to the circuit. With a resistance of 2.8K ohms, the current flow will be tiny. Or could I take the supply from the wires already going to the heater circuit (remember that the resistance circuit OF THE FUEL STRIP is completely out of play, wires to it are cut etc.- the new resistors I am adding in take the place of the fuel gauge strip.

 

Where does your diode go? Is it in series with the 12V supply or across the 12V supply? If in series, is it in series on the + or - etc.?

 

BW

Hugh

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lkraus

Hugh,

When you were doing your tests, was the heater strip powered? From the ADV link above (post #120) I found this:

 

"I happen to have a new strip in my garage. Pins 1 - 4 is 32 Ohm. I believe that's the heater circuit. My RT feeds it with 9 Vdc at key on and after ~ 5 sec. it drops to 7.5 Vdc."

 

Looks to be about the same voltage you are applying with your meter...

 

Larry

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Boffin

diode-L.jpg

 

I am guessing that the fuel strip is energised from the heater circuit in the strip with some form of isolation - probably a diode. What I am suggesting is to emulate that - I have sketched what I have in mind below.

 

If I am right, it may be possible to resurrect a failed strip with an external diode feed.

 

There is of course, the chance that this could put power where power should not be and damage things. The output from your ohmmeter is 9V but from a very high impedance source. In fact, it may be worth putting a 1 megohm resistor in series with the diode first time for safety (not shown in picture - this is an afterthought)

 

Andy

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RoSPA_man
Hugh,

When you were doing your tests, was the heater strip powered? From the ADV link above (post #120) I found this:

 

"I happen to have a new strip in my garage. Pins 1 - 4 is 32 Ohm. I believe that's the heater circuit. My RT feeds it with 9 Vdc at key on and after ~ 5 sec. it drops to 7.5 Vdc."

 

Looks to be about the same voltage you are applying with your meter...

 

Larry

 

Here's several updates in one go:

 

Yes the heater is still connected in - i.e. the fuel gauge plastic connector is plugged in, albeit with the 2 inner wires (2&3) cut so that he resistance circuit is completely out of play

 

I have resistors fitted (rather crudely at this time)as described to the 2 wires that used to be connected to the resistance circuit of the fuel gauge.

 

Now, when I want to get the gauge to work, I simply apply the 2 probes of my meter to the 2 wires (physically I am applying them where the join to the resistors is made (because it's where the connector block is!) That works every time

 

Since then, I have applied 12V from the accessory socket to the same place- nothing

I applied 9V from a Duracell battery - nothing

I applied 1.5V from a battery - nothing

I applied 3V from 2 batteries - Nothing

 

 

Further update:

I tried 5,4,3,2,1 AA batteries ie 7.5,6,4.5,3 and 1.5V-nothing

 

But apply the meter set to 20K resistence and all is well!

I think I will will have to wire in my multimeter and locate it under the seat!

 

We are nearly there- someone must know teh answer to what is so special about applying the meter as against a power source of any voltage?

 

Boffin

I'm not trying to save the gauge in any way - I'm solely trying to fool the gauge display. So if you can tweak your excellent advice to tell me how I can emulate the "meter effect", we might get the result! Thanks

 

Edited by RoSPA_man

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dirtrider

Morning Hugh

 

Not much time here as I'm just about to leave.

 

In any case a couple of thoughts.

 

First, on your findings- EXCELLENT!

 

Next, most ohmmeters work by placing a small current across what is being measured then measuring the voltage produced. So to see what your ohmmeter meter is producing voltage wise place it across the load of your added resistors + bikes resistance (just like it is hooked up & working) then use another (high impedance) voltmeter to measure the voltage at the probes of your ohmmeter.

 

Next, in looking at my BMW wire schematic for the 1200 hexhead it shows absolutely no interconnection between the heater circuit & the level sensing circuit (at least at the sensor area). What it shows is a thermistor as the designation for the level sensor side.

 

What is strange (to me anyhow) is in the schematic there is a double arrow next to the thermistor that (I believe) is saying current flows (or can flow) in both directions.

 

So maybe try a small diode across your added resistors (both directions) as maybe the gauge circuit is looking for a different return than sent. Then try (I have no idea on size) a capacitor across your added resistance. Maybe the bike's gauge is looking for some capacitance change as the strip heats & level changes.

 

Might not be the voltage/current of your added ohmmeter but something else inside it like a cap or diode that is actually making your system read. Maybe try with the ohmmeter hooked up & switch turned on with but with battery removed.

 

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Jameseo

I do not think the heater probes will work. Someone else measured them and said they're 9V when the bike is started and then drop to 7.6V shortly there after (if memory serves correctly). I have not confirmed this but it should be verified if that's what you're trying to use as a source.

 

9V from your meter also seems like a strange reading to me. I don't think your meter is applying a full 9V to test the resistance of a circuit. Did you perform that measurement with the circuit and meter 1 in normal operating position? Again, I measured the voltage of the meter I used and read zero volts (but was using a cheap meter to read voltage). However, a buddy measured voltage with his Fluke and read 0.178V using a 2.2k resistor and 0.09 volts with a 1k resistor. He did not have a bike to work with so was strictly measuring with resistance and two meters.

 

I had the same effect as you where the resistance was pulled down once in the circuit (i.e. I set the pot to 2.4k and it would read something like 1.7k in the circuit). So for fuel level I had the following results using a 10k pot measured in the circuit: 2.0k ohms = full tank, 2.8k = 1 bar from top, and 5.4k = half tank (note that I didn't try minute changes between these values due to the time delay it takes to read the change). Strangely, I cannot get the pot to increase the resistance from 5.4k and I did verify that it would supply 10k when out of the circuit. But this is getting a little ahead of myself.

 

Fingers are crossed for a breakthrough today!

Edited by Jameseo

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RoSPA_man

Hi DR

 

OK I think I have got as far today as is possible - it quite fun and quite interesting.

 

So, right now, I have done as follows:

 

1. I set up a 3.9k resistor (a 4.74K and a 22k in parallel - both wire-wound and 3W)

 

2. I extended wires 2&3 back to under the pillion seat (so I could put back the Tupperware and easy to work back there) and simply put the 3.9K resistor across those two wires.

 

3. I also plumbed in my multimeter (which now reads 2.86 K when in place) as you can see here and shoved the lot into the tail section of the bike!

 

4. Not pretty but it works until we solve this last piece - help! PICTURES

 

I think you are absolutely right that there is something else in the multi-meter which is in play - everyone speaks of diodes so I'm thinking diodes! If someone could tell me where and how to fit it, I'll try it- does it go inline and is the direction - to + or + to - etc... I know nothing about diodes.

 

If it is something like diodes in the multi-meter, that begs the question as to whether there is a need for any "voltage" or current or whatever from the multi-meter at all? BTW, I used a digital meter today to measure my meter that is plumbed in to teh circuit. The voltage across the probes (in resistance mode 20K) and when free-standing is exactly 2V - not 9V as I said earlier that I got with a cheap analog meter yesterday

 

Edited by RoSPA_man

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RoSPA_man
I do not think the heater probes will work. Someone else measured them and said they're 9V when the bike is started and then drop to 7.6V shortly there after (if memory serves correctly). I have not confirmed this but it should be verified if that's what you're trying to use as a source.

 

9V from your meter also seems like a strange reading to me. I don't think your meter is applying a full 9V to test the resistance of a circuit. Did you perform that measurement with the circuit and meter 1 in normal operating position? Again, I measured the voltage of the meter I used and read zero volts (but was using a cheap meter to read voltage). However, a buddy measured voltage with his Fluke and read 0.178V using a 2.2k resistor and 0.09 volts with a 1k resistor. He did not have a bike to work with so was strictly measuring with resistance and two meters.

 

I had the same effect as you where the resistance was pulled down once in the circuit (i.e. I set the pot to 2.4k and it would read something like 1.7k in the circuit). So for fuel level I had the following results using a 10k pot measured in the circuit: 2.0k ohms = full tank, 2.8k = 1 bar from top, and 5.4k = half tank (note that I didn't try minute changes between these values due to the time delay it takes to read the change). Strangely, I cannot get the pot to increase the resistance from 5.4k and I did verify that it would supply 10k when out of the circuit. But this is getting a little ahead of myself.

 

Fingers are crossed for a breakthrough today!

 

Yep sorry, the reading is actually 2V - cheap meter used yesterday! Cheap things are of no value, valuable things are not cheap!

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RoSPA_man
Morning Hugh

 

Not much time here as I'm just about to leave.

 

In any case a couple of thoughts.

 

First, on your findings- EXCELLENT!

 

Next, most ohmmeters work by placing a small current across what is being measured then measuring the voltage produced. So to see what your ohmmeter meter is producing voltage wise place it across the load of your added resistors + bikes resistance (just like it is hooked up & working) then use another (high impedance) voltmeter to measure the voltage at the probes of your ohmmeter.

 

Next, in looking at my BMW wire schematic for the 1200 hexhead it shows absolutely no interconnection between the heater circuit & the level sensing circuit (at least at the sensor area). What it shows is a thermistor as the designation for the level sensor side.

 

What is strange (to me anyhow) is in the schematic there is a double arrow next to the thermistor that (I believe) is saying current flows (or can flow) in both directions.

 

So maybe try a small diode across your added resistors (both directions) as maybe the gauge circuit is looking for a different return than sent. Then try (I have no idea on size) a capacitor across your added resistance. Maybe the bike's gauge is looking for some capacitance change as the strip heats & level changes.

 

Might not be the voltage/current of your added ohmmeter but something else inside it like a cap or diode that is actually making your system read. Maybe try with the ohmmeter hooked up & switch turned on with but with battery removed.

 

BTW, DR are you postulating here that you think the diode/capacitor treatment might work alone i.e. without "juice" being provided from the meter or somewhere else?

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dirtrider

 

 

 

 

 

 

Afternoon Hugh

 

I have been thing about your gauge issue while riding today & the more I think about it the more I think there is way more to these 1200 gauge circuits then I first thought.

 

Unfortunately I'm not much of an electrical engineer as the little I learned MANY years ago I have forgotten most of it. I basically learn what I need to get my job done or when I need to know something for a project I'm working on & that's about it.

 

In any case the more I think about your gauge issue & what you are finding the more I believe the gauge circuit isn't just a simple resistor gauge with fluid level changing the strips resistance. If it was then you would have make it work by adding those resistors.

 

I'm now thinking the circuit is some sort of a capacitance type system with the fluid level changing the capacitance of the strip. Maybe (big guess on my part here) the thing works by pulsing low current into the level sensor side of the strip, then timing how long it takes to bleed off the charge through the basic resistance. I have seen similar level sensors being used in some future automobiles but never took the time to see how they functioned & so far I haven't needed to know to get my part of the development work done.

 

 

In any case that would probably mean some sort of pulsed or square wave into the strip with a varying time square wave returning.

 

So another "guess" here is you have the resistance about right & somehow your ohmmeter is supplying the capacitance to make it function. Maybe the diodes in your meter are also doing something.

 

I guess the first thing I would try is leaving your meter in the system just as you have it now but remove the meter battery. Then turn the meter switch to on. See if the gauge still works. If so your meter is probably supplying the capacitance it needs to work & that resistor you added is correct to allow the correct bleed off time.

 

The more I think about the BMW schematic the more I'm thinking that two way set of arrows is saying current in, then stored current back out through the resistance (maybe or maybe I'm all wet here)

 

If the above is anywhere near correct then you might be able to take your meter to an electronics store & see if they can measure the capacitance of your meter as you have it set & sell you a capacitor that is close to the same value.

 

We really need an EE to help us understand this thing.

 

I'm done with supper so off to hopefully get another 100 miles in before days end.

 

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Jameseo
I guess the first thing I would try is leaving your meter in the system just as you have it now but remove the meter battery. Then turn the meter switch to on. See if the gauge still works. If so your meter is probably supplying the capacitance it needs to work & that resistor you added is correct to allow the correct bleed off time.

 

It does not work with the meter powered off. I'm guessing the same holds true if the battery is removed entirely.

Edited by Jameseo

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ESokoloff
I don't want to undo all my good work by frying the electronics with 12V. Any opinions?

 

Relax.

 

If you do let the smoke out of the electronics just use this. :grin:

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smiller

You guys are over-thinking it. Thanks to RoSPA_man's fortuitous observation the puzzle has been solved, only takes two resistors. James will be posting the details shortly.

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Jameseo

RoSPA_man really made an outstanding discovery when he realized that the multimeter enables the circuit!

 

After I was able to repeat his results on my bike I was discussing them with smiller. He locked on to the very small voltage generated by the meter (mV) and recommended wiring the positive side of the fuel strip circuit to 12V with a very large resistance between to emulate the meter. It turns out that a 1M Ω resistor comes very close to supplying the same voltage as the meter. With this high resistance there is very little current flow; a mere 0.001mA (yes, one or two micro Amps). With that little current it may not even be necessary to ignition switch it.

 

Couple this small voltage with the 2k Ω that it takes for the tank to read full and you have an infinite full tank and, more importantly, no error code flashing at you constantly! Here is what the final circuit looks like (in my kindergartener’s mind):

 

FuelStripFix.jpg

 

This is it people! We no longer have to repeatedly pay BMW for their poor design!!! I give you... The Infinite Tank

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RoSPA_man

Wow!

I've just gone ahead and ordered those resistors and will collect them this morning. Is this still a theory or have you actually got it to work? Is a ground not needed for flow or is there already a ground somewhere in the system? I guess I'll find out easily but is 2k the new value to use and not 3.9 as I used up to now? I can't wait to try this!

 

I think the forum should take out a patent on this. With the price of gas being what it is, billions of people will want to carry out this modification and have the infinite gas tank! I'd short oil futures while there's still time.

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RoSPA_man
I don't want to undo all my good work by frying the electronics with 12V. Any opinions?

 

Relax.

 

If you do let the smoke out of the electronics just use this. :grin:

 

Excellent!

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RoSPA_man

IT WORKS! IT WORKS! Well played!

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Ebbo

Well done guys with the progress, it’s a big step forward in understanding how these strips work.

 

Just a thought, normally these strips work without need of an external supply, so would it work if a feed was taken from Pin 1 or 2 (depending on which is Positive +) to supply the 1M resistor, maybe a small resistor would be needed as I’ve read the voltage on the heater circuit varies between 9v-7.5v, maybe 700k, but i'm just guessing.

 

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TestPilot

Looking at James' circuit raises some questions. James provides an external current to the 2kOhm resistor that is simulating the fuel measuring strip. I would normally expect this current to come from pin 3. So on Hugh's bike, is there an open circuit between pin 3 and the ZFE that prevents current this expected current flow?

 

Or, since Hugh's results have been duplicated elsewhere, is there an interaction between the heating element and the measuring strip that has not been accounted for? In the BMW design, does the heating element induce a current in the measuring strip that wo't be present if an external resistor is used?

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Jameseo
Is a ground not needed for flow or is there already a ground somewhere in the system? I guess I'll find out easily but is 2k the new value to use and not 3.9 as I used up to now?

The brown wire (pin 2) in the fuel strip circuit is not ground but it's pretty darn close: I measured 7-ish ohms to ground. Either way, ground is not needed. And 2k ohms just happens to be the resistance I used to create a full tank. I suspect you could used more or less resistance depending on what you have laying around. I will note that 2.8k ohms provided me with one bar less than full. Just remember that fuel level and the resistance between the two strip wires are inversely proportionate.

 

Just a thought, normally these strips work without need of an external supply, so would it work if a feed was taken from Pin 1 or 2 (depending on which is Positive +) to supply the 1M resistor, maybe a small resistor would be needed as I’ve read the voltage on the heater circuit varies between 9v-7.5v, maybe 700k, but i'm just guessing.

This may be worth looking into for a cleaner installation. Then a jumper to 12V wouldn't be necessary. BTW, the source should be either pin 1 or 4. That's the heater circuit.

 

Or, since Hugh's results have been duplicated elsewhere, is there an interaction between the heating element and the measuring strip that has not been accounted for? In the BMW design, does the heating element induce a current in the measuring strip that wo't be present if an external resistor is used?

smiller and I were discussing the same thing. Perhaps there is some voltage supplied by the heater circuit. We're not really sure at this point. That's the magic in Hugh's discovery. Perhaps smiller will take a stab at that question...

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smiller
Just a thought, normally these strips work without need of an external supply, so would it work if a feed was taken from Pin 1 or 2 (depending on which is Positive +) to supply the 1M resistor, maybe a small resistor would be needed as I’ve read the voltage on the heater circuit varies between 9v-7.5v, maybe 700k, but i'm just guessing.

Yeah, that thought occurred to me last night. All you need to do is apply a positive bias of about 0.1 volt to pin 3 of the fuel strip connector and that could probably be taken from the supply to the heater (as long as there isn't something we don't know about this voltage, such as it varies by an even wider range than we think, etc.) I would just try using the same megohm resistor and see if it works, if not you could try dropping the value a bit.

 

BTW I wanted to pot the whole thing up as a 'fuel strip eliminator module' and sell it for $89.95, but then James had to go and spill the beans.

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TestPilot

From these discussions, it's obvious no one really knows how the fuel sensor operates. The two units in the sensor are a heater and a thermistor (temperature-dependent resistor). For a fixed heater current, the thermistor will get hotter as fuel level decreases due to less cooling from the fuel. The sensor could measure fuel quantity in a couple of ways. Heater current could be held constant, and fuel level determined from the resistance in the thermistor. Or, heater current could be varied to maintain a constant thermistor temperature and resistance, and the heater current could be measured to determine fuel level. Anyone have an idea how the system actually works?

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smiller

I don't think anyone knows for sure exactly how it is intended to work outside of BMW's engineering dept. but there is no end to speculation in countless threads in just about every BMW forum so take your pick. Regardless, we now know how to make the gauge read anything we want so it wouldn't be all that difficult to design an interface between the combi display and any fuel sensor you could come up with, permitting an aftermarket replacement of the trouble-prone stock system. But I'm not sure why it would be worth the effort as a simple trip odometer works better as a fuel gauge than just about anything else anyway.

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TeSmSp

I still don't understand how the fuel strip does its job, but I googled "fuel level measurement" and this came up: a no moving parts system ...

http://www.sapconinstruments.com/level-measurement/translite-fuel-level-sensor/70

 

For you guys that have played with your bikes studying this issue, does BMW and this company use a similar method?

 

The author of this article has a good argument for the strip versus the float; ethanol corrosion and fire hazards.

http://en.wikipedia.org/wiki/Fuel_gauge

 

Edited by TeSmSp

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