How does the fuel warning light work?

[ncstatecamp]

So the yellow dummy light is a bit confusing. Mine comes on when it's not empty and doesn't come on until it's really empty.

 

I get that sometimes it that the bike is on the sidestand but it'll come on when it shows 2 bars on the side stand, then won't come on until it's flickering between 0 and 1 while riding. What gives?

[dirtrider]

So the yellow dummy light is a bit confusing. Mine comes on when it's not empty and doesn't come on until it's really empty.

 

I get that sometimes it that the bike is on the sidestand but it'll come on when it shows 2 bars on the side stand, then won't come on until it's flickering between 0 and 1 while riding. What gives?

 

Evening ncstatecamp

 

The low fuel light is controlled by a separate float in the fuel tank so it works independently of the main fuel gauge. The main fuel gauge float is more centered so sees the fuel level differently than the low fuel light float while on the side stand.

 

While riding straight up both floats see about the same fuel level in the tank.

 

The main fuel level gauge on the oilhead BMW's can have some resistance issues so they are not always totally accurate on the older bikes. The low fuel light part is a simple yes/no so is inherently more accurate (unless) the float itself is degraded & crumbling, or someone bent the float arm during a fuel filter service, or the float is hanging up on the in-tank wires or hoses.

 

 

 

[ncstatecamp]

Thanks for the reply. Yeah I've noticed the main fuel gauge is a bit finicky, I've never been able to get the top bar on the gauge to light up, any suggestions?

[Michaelr11]

Thanks for the reply. Yeah I've noticed the main fuel gauge is a bit finicky, I've never been able to get the top bar on the gauge to light up, any suggestions?

 

Easy suggestion first. There should be an electrical connection for the gas tank on the right side. Open it and use electronic contact cleaner on both sides. This could correct the fuel gauge. If it doesn't, there are other repairs you can make to improve the ground connection.

[Roger C]

Michael, don't hold us in suspense. My gauge has never shown the top bar. What are the fixes for this?

The caveat is "how full is full?"

Edited May 6, 2017 by Roger C

[Michaelr11]

Not trying to hide anything, I just stay away from topics that I haven't tackled myself. I'm a firm believer in cleaning that electrical connector to the fuel pump plate. This has been the only maintenance that I have used for my fuel gauge levels. I was getting a full tank indication (10 bars) for only a few miles, and then it would drop to 9. After thoroughly cleaning the electrical connection. I get a full tank indication for about 30 miles, and then the gauge loses a bar at each additional 30 miles. Whenever I have the right side fairing off the bike, I open that connector and spray quick-drying contact cleaner.

 

Some other contributors here have suggested adding a separate ground wire for the fuel gauge connection. The factory harness has one ground used for both the gauge and the fuel pump. I have never needed to look at that.

[dirtrider]

Thanks for the reply. Yeah I've noticed the main fuel gauge is a bit finicky, I've never been able to get the top bar on the gauge to light up, any suggestions?

 

Morning ncstatecamp

 

Getting the top bar to work on the BMW 1100/1150RT bike can be a test of patience. (many things in the system can effect the fuel gauge reading correctly)

 

The top bar not working is almost always a resistance issue, problem is there are MANY spots in the fuel gauge wiring/connections that can be the, or stack up to be the, resistance issue.

 

It starts right at the float tube as the main gauge float rides up & down on two thin wires that are resistance wires so if those are not making a clean contact, or the wires have some corrosion, or the wires have a varnish build-up, then the top bar will never light up.

 

There is also the float itself-- If the float is slightly heavy due to varnish on it, or partially fuel logged, then it won't fully float so it will never float high enough to get to the top of the resistance wires.

 

Next-- That internal float assembly plugs into the pump pass through assembly inside the fuel tank with a small 2 terminal plug. Any resistance in that 2 pin plug due to varnish buildup or fuel additives effecting the terminal coating can add resistance to the fuel gauge circuit.

 

Next-- The main float tube wires need to pass through to the outside of the fuel tank through the fuel pump pass through. There are solder joints inside the fuel tank that are exposed to the fuel additives & alachaol so those can degrade & add resistance to the fuel gauge circuit (I have seen a number of BMW 1100/1150 bikes that have a little resistance in the solder joints with just enough resistance to keep the top bar from lighting correctly)

 

From there the main fuel gauge low side circuit (basically a ground) exits the fuel tank then splices into the fuel pump low (ground) then they go on to chassis ground as a SHARED or combined ground. This also means that ANY resistance in that shared low (ground) allows the fuel pump low side current to back feed the fuel gauge low (ground) & raise it's potential enough to keep the top or even the upper couple of bars from lighting. (there was an old BMW service bulletin to split the pump ground from the fuel gauge ground then add a new connector & return the fuel gauge low (ground back to the battery (-) post.

 

To sometimes tell (IF) the fuel pump pass through pig tail (connector resistance) or the combined ground is the problem just put the bike on level ground then see if fuel gauge reads the same with or without the fuel pump running. If the top bar lights without the fuel pump running but doesn't light with the fuel pump running then there is probably a resistance issue in that shared ground so splitting the brown wire ground & running a separate ground wire from the smaller brown fuel gauge low side brown wire back to the battery (-) post will probably help.

 

One somewhat decent test is next time the fuel tank is removed is to invert the tank (probably best done with little or no fuel in it) then use a precision ohmmeter & measure the resistance between the brown double wire pin in the fuel pump connector & the yellow wire in the pump connector (you want to see under 4.5 ohms & under 4 ohms is better yet). If above 4.5 ohms then the problem is more than likely inside the tank, or at least part of the problem is inside the tank, or in the pump pass through solder joints, or in the connections, or in the float tube resistor wires.

 

[ncstatecamp]

Damn dirtman hell of a write up! Thanks! I'll start with the harness connection and work from there. Thank you

[dirtrider]

Damn dirtman hell of a write up! Thanks! I'll start with the harness connection and work from there. Thank you

 

Afternoon ncstatecamp

 

First thing to try is to add some Techron concentrate to the gasoline for a few tank-fulls.

 

That can help to clean the float sensor wires, & help to clean the internal main sensor connector. It can also dissolve some of the accumulated varnish on the main float.

 

At least that is an easy place to start as it requires no tupperware or tank removal.

[ncstatecamp]

So I had the tank off and just replaced the fuel filter. Dirtrider I did a few of the things you suggested. I started by measuring the ohms and got a beginning reading of 12ish.

 

After cleaned all the contacts and everything else I could I got down to 6 ohms.

 

After filling the tank I was able to get a full tank reading, atleast flirt with it. When I first filled it, it didn the same as before showing just one bar below, but after a few minutes of riding it showed the top bar. We'll see how long it shows that top bar for now.

 

O and if anybody cares to know the RSL has about a 6 gallon tank (I assume usable is 5.5 b.c of the lobes?), this is great news as I was always filling up with about 4ish before.