R1150/R1100 Fuel Gauge Reading Low -- A Fix

[Roger 04 RT]

Not sure if this will help anyone, but ...

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.

 

[tadhgocuilleain]

Nice bit of work, there. Thanks for taking the time to share.

 

[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.

 

[Jamieboy]

At the risk of sounding utterly useless.... where can I find this connector to clean it?

 

[alanbmwgs]

At the risk of sounding utterly useless.... where can I find this connector to clean it?

Me too Jamieboy, my gauge has been ONE bar short of a full tank for the past couple of Years....

Great work Roger.

 

[RedRick Esq.]

That is impressive work mate !




Is it just me that doesn't understand a word of what you have written ?

 

[newboy]

Good. But I'm clueless with this stuff


Sent from my iPhone using Tapatalk

 

[Green Boy]

Me too Jamieboy, my gauge has been ONE bar short of a full tank for the past couple of Years....

Great work Roger.

It's the large black connector roughly about where your right knee would be when you are riding the bike. It's behind the black plastic panel and is the one you need to pull apart before removing the tank. I had the same 'one bar missing' issue a year or so ago and 5 mins cleaning the contacts and a small dab of acf50 in each terminal has solved the problem.

 

[alanbmwgs]

Brilliant Greenboy, many thanks.

 

[Roger 04 RT]

That is impressive work mate !




Is it just me that doesn't understand a word of what you have written ?

You got me laughing.

The simple version goes like this. BMW saved about 50p when they designed the motorcycle and that causes the fuel pump to interfere with correct reading of the gas gauge and low fuel light unless the fuel tank electrical connector has spotlessly clean contacts.

You can fix it permanently but clipping the correct wire and then connecting it though a spade terminal to Battery (-). It will cost you the 50p BMW saved.

 

[haydw]

As per the photo, my ground wire has burnt and will no doubt explain why my fuel gauge doesn't work at all. I seem to recall a couple of months ago Mort had the same issue.

Would another solution be to link the earth that feeds the gauge (small wire) to the earth for the fuel pump INSIDE the tank instead of outside at the connector? It does the same thing?

I seem to recall a thread on advrider explaining this as a fix for the burnt small earth wire issue.

Is it safe to do this?

 

[Roger 04 RT]

As per the photo, my ground wire has burnt and will no doubt explain why my fuel gauge doesn't work at all. I seem to recall a couple of months ago Mort had the same issue.

Would another solution be to link the earth that feeds the gauge (small wire) to the earth for the fuel pump INSIDE the tank instead of outside at the connector? It does the same thing?

I seem to recall a thread on advrider explaining this as a fix for the burnt small earth wire issue.

Is it safe to do this?

That would get the gauge to function but would be the wrong thing to do as it would add more potential error to the gauge. The best thing to do is replace the burnt wire and connect it to ground through a new one pin connector.

I'm told that wire burns when B+ inside the tank gets shorted to ground while removing the fuel plate--not sure myself.

 

[Massive Lee]

Had that problem, starting last week on my way to Mississippi. It may have been caused by two days spent in torrential rains, thanks to hurricane Joaquim. I picked a small screwdriver and simply scratched the male and female pins. I am now back at home, and will use some DeOxit contact cleaner.

 

[Roger 04 RT]

I have been meaning to do this for over a year now, so it being a day off (Turkey day in Canada) I did the mod.

I used a 6 pin BMW connector set and swapped out the 4 pin one.
It should be noted one could use any 5 pin connector or combination of 3+2 or 4+1 etc.
I just happened to have all the BMW parts, and like to stay with OE if possible.
Extra pin and socket was salvaged from old harnesses.

The fuel sender & low fuel lamp now have their own dedicated ground pin/socket (position 5)
I did not run any further than the 4.0mm (11awg) ground on the bike side of the harness as the voltage drop from that point to X9223 (on the GS) is negligible to this issue IMO.
The whole mod took less the 1.5 hours, tank not removed.


I started the bike and measured the following across the connector (tank full, RID read full bars)
Fuel pump ground Pin 1 - 2.3mv (connector set was polished clean so this low reading will increase with time)
New RID/Level sensor ground Pin 5 - 0mv


I then measured from Pin 5 on the tank side of the connector to the engine block - 32mv
Then to B- of the battery - 19mv
This confirms my belief that B- is a better ground point than the engine block.
Ideally I would tie the new ground to X9223 but that is a lot of extra work with little benefit.



Adding the ground (small brown) to the bike side of the harness



New heat shrink cover



Small Brown sender ground (tank side) to be cut



New socket connector (pin 5) soldered in



Ready to insert into the connector block



Completed tank side assembly



Finished mod - Full bars on the RID

Here's a good write up on how to mod the connector.

 

[Grim Rider]

The fuel gauge on my 1150GS used to never reach the top bar when filled - taking the file cap off and just bending the fuel sender up a bit seemed to fix it

 

[Massive Lee]

The fuel gauge on my 1150GS used to never reach the top bar when filled - taking the file cap off and just bending the fuel sender up a bit seemed to fix it

I think it would have been easier and more accurate to simply clean-up the ground contact. It takes less than a couple of minutes and doesn't even require to remove the gaz tank. ;-)

 

[alanbmwgs]

The fuel gauge on my 1150GS used to never reach the top bar when filled - taking the file cap off and just bending the fuel sender up a bit seemed to fix it

For we half wits what the feck is a FILE CAP !!! And where can I locate said FILE CAP ??

 

[Roger 04 RT]

I'm guessing he meant Filler Cap.