R1150/R1100 Fuel Gauge Reading Low -- A Fix

[roger 04 rt]

For about the past two years, every time I fill up my R1150RT, the fuel gauge only reaches 8 bars out of 10. I've read in the past that this is due to the use of a common ground for the fuel pump, low-tank warning and fuel level circuits. Today I cleaned the connector and the gauge now reads 10 bars. The common ground wire is indeed the root of the problem but because I wanted to know for sure that the contact of the ground wire was the problem, and that after cleaning the connection was better, I made several before and after measurements that shed light on what's going on.

 

The first thing I tried was turning the bike on and not starting the engine. The RID came alive, the fuel pump ran for 2 seconds and shut off, and the RID showed 8 bars. I then waited about 15 seconds and the RID showed 9 bars and then waited another 15 seconds and the RID displayed 10 bars. The waiting time is because the RID averages readings before changing so that the display doesn't jump around. The behavior of the bars going from 8 to 10 is consistent with the Common Ground theory. Once the pump stops, its effect on the gauge is eliminated.

 

Before Measurements:

Fuel Pump Current, measured at the fuel pump relay-- 5.25 amps

Fuel Level Sensor Voltage with pump running-- 285 millivolts

Fuel Level Sensor Voltage with pump off-- 85 millivolts

Resistance of Fuel Level Sensor, full tank-- 4 ohms

 

Before Calculations

Drop Across Ground Lead at Fuel Tank Connector: 200 millivolts (285-85 millivolts)

Resistance of Connector Pins in Tank Connector: 40 milliohms (200mV/5.25A)

Current in Fuel Sensor Circuit: 20 milliamps (85mV/4ohms)

Full Scale Voltage for Fuel Sensor Circuit 1.2V (20mA*60ohms, I found the 60 ohms in a white paper on the RID)

 

Interpreting the above data shows that the voltage at the RID varies from about 0V with a full tank to about 1.2V with an empty tank. The 0.2V drop across the ground pins in the connector from the Fuel Pump current adds 0.2V to voltage the RID is measuring, leading to a measurement of about 80% full, just what I was seeing.

 

Although 40 milliohms resistance in the ground lead is a very low, it is too high in the application where the BMW engineers ran two sensors and the fuel pump through one ground wire. And who can blame them for saving about a buck with with this engineering shortcut? ;)

 

The solution was to clean the ground connector pins and get the resistance of those pins below 10 milliohms. I didn't have contact cleaner so used isopropyl alcohol and a clean cotton rag. The pins were mated and unmated several times to get further scrubbing action and then the connectors were plugged together.

 

After Measurements:

Fuel Level Sensor Voltage with pump running-- 120 millivolts

Fuel Level Sensor Voltage with pump off-- 80 mV

 

 

After Calculations

Drop Across Ground Lead at Fuel Tank Connector: 40 millivolts

Resistance of Connector Pins in Tank Connector: 7 milliohms (40mV/5.25A)

 

Now that the contact resistance is under 10 milliohms and the voltage drop only 40 mV, the circuit is stable and the excess ground drop insignificant, and the RID fuel display reads a full 10 bars. The moral of the story is that the Fuel Tank electrical connector has to be very clean for the gauge to read correctly give BMW's cost-cutting.

[roger 04 rt]

I've done some more investigating and it turns out that 5 wires come through the fuel pump plate. There are two groups of wires. One pair of two wires is Ground for the fuel pump and the other is Switched +12V to turn it on and off. The other group of three wires is Ground for both fuel level sensors, a wire for Low Fuel, and another for Fuel Level.

 

The two ground wires are separate right up until the tank side of the fuel pump electrical connector where they are joined together. One could simply cut the smaller ground wire at the connector, and connect it directly to the engine case (through a spade terminal connector) or another low current ground wire. That would permanently solve the problem.

 

Here is a post that shows the actual wiring of the fuel pump plate. It's more accurate than the BMW Electrical CD. http://advrider.com/index.php?threads/fuel-gauge-stopped-working-on-r1150gs.241775/.

 

 

And here's a photo of the two ground wires coming together. The burnt one (not from my bike) could be disconnected and wired separately.

 

[roger 04 rt]

In the schematic in the prior post, the mechanism for the low fuel and fuel level is from a R1150GS which you can see if you follow the link. And although the low fuel/fuel level mechanism is different in the RT, the wiring is the same at the connector on my '04RT. I included the schematic only to show how the grounds were separate through the fuel plate wall and therefore one could separately ground the smaller wire, eliminating the crosstalk and error caused by fuel pump current.

[RPG]

Interesting data, Roger.

 

Last winter, I did that exact upgrade on my '04 RT (routed the ground wire in your photo to engine case, bypassing the connector) and saw my fuel gauge jump from 8 - 10 bars.

 

I was ecstatic. I was elated, overjoyed! Ok, I wasn't ecstatic but I was pleased.

 

Anywhoo, the fix lasted only a few months. Around May of this year, (and through this morning), I can only get 9 bars on a fill-up.

 

Mind you, I sometimes stand there for an extra five minutes trying to squeeze in as much fuel as possible, after the handle trips, and still only 9 bars are ever visible.

 

I remain defeated, spent and have fully succumbed to this issue as unsolvable. (for me anyway)

 

RPG

[roger 04 rt]

Next time you fill up, measure the voltage on pin 3 of the fuel tank connector. Then pull the connector apart and measure the resistance between pins 1 and 3. Resistance should be 4 ohms or less, voltage with key on about 80 mV.

[RPG]

Roger, I'll do that.

 

thanks.

 

btw, to reinforce that the BMW Gods are into pranks, today after leaving lunch, all 10 bars appeared.........only long enough for me to acknowledge, then back to 9 bars.

 

RPG