Low fuel light

I have added an aux fuel tank to my R1150RT. I purchased the setup used from a rider who also purchased it used and never hooked it up. It is gravity feed to a bulkhead fitting in the fuel filtefilter/fuel pump plate. The fuel seems to be flowing from the aux tank to the main tank ok. Unfortunately the low fuel light is staying on. The fuel plate that came with the setup came from either an 1100GS or an 1150GS.. MAX BMWs fisch shows different part numbers for the GS & RT plates but there appear to look the same. Could the wiring for the fuel light be the problem? What else would cause the light to stay on?

Thanks for any assistace.

On the R1100RT there are two fuel sensors, a tube that controls the fuel gauge and a float that controls the low fuel light. I believe that the 1150 is set up the same way. If your float is caught and unable to lift, then the low fuel warning will be lit up.

I was afraid of that. I really hate the thought of opening it back up again. Thanks for the advice.

Has anybody successfully unstuck the float thru the fuel filler opening? I really don't want to open the plate again. I was hoping to use the light to tell me when to open the valve for the aux tank.

Has anybody successfully unstuck the float thru the fuel filler opening? I really don't want to open the plate again. I was hoping to use the light to tell me when to open the valve for the aux tank.

Morning Bill

The GS & the RT fuel tank have different internal shapes so the low-fuel float arm is a different length & shape on the GS than the RT. I doubt the GS low fuel float will work in the RT tank as it probably interferes with the tank shape.

If you don't need the low fuel light (with extra fuel on board probably not) then you might try disconnecting (unplugging) the white wire in the fuel pump 4 pin connector.

Or just go back to your original RT fuel pump pass through & plumb your aux tank into that (either add a new fitting or just plumb it into the fuel return line).

Thanks DR. The float arm looked to be the same on both with the arm on the GS being closer to the fuel pump. The one thing that we did not think to check was the lengtlength of travel of the float arm. I have 3 trips planned in the next 6 weeks and will try to fix it after that.

Doing this type of fiddly work is difficult has I have essential tremors and need help with the small stuff and my buddy lives over an hour away. Couldn't do any of this without him.

He has to bikes with aux tanks, an 1100 and a k12lt, that T into the fuel return line but he uses a pump to transfer the fuel. Would it work on a gravity systesystem? Why wouldn't the fuel just go into the aux tank from the return line?

Thanks DR. The float arm looked to be the same on both with the arm on the GS being closer to the fuel pump. The one thing that we did not think to check was the lengtlength of travel of the float arm. I have 3 trips planned in the next 6 weeks and will try to fix it after that.

Doing this type of fiddly work is difficult has I have essential tremors and need help with the small stuff and my buddy lives over an hour away. Couldn't do any of this without him.

He has to bikes with aux tanks, an 1100 and a k12lt, that T into the fuel return line but he uses a pump to transfer the fuel. Would it work on a gravity systesystem? Why wouldn't the fuel just go into the aux tank from the return line?

Afternoon RoanokeRider

For most of your riding you will have the aux tank valve turned off so obviously nothing can flow back into it with the valve shut off.

With the valve open between the aux tank & the main tank return-line-- as long as the aux tank is much higher than the main tank both the return flow & aux tank flow-out will want to go to the lowest possible place (that is the lower main tank).

About the only time you would have a problem is if your main tank vent plugs up & you have an open to atmosphere vent on the aux tank but that usually ends up being a low pressure (re vacuum) problem not a pressure problem. I haven't done many BMW RT/GS aux tank installs but on the few that I have I always combined the vents & vented to atmosphere as one vent line.

On the float arm difference?? I haven't ever measured the difference but the bottom of the float when all the way down shouldn't be any lower than the bottom of the pump pick up screen (as bottom of tank is bottom of tank & that is where you want top pump pick up. If the pivot is higher then the float arm would have to be longer.

Are you sure the GS pump pick up screen (sock) is at the bottom of your RT tank??????-- If it doesn't sit as low in the tank as your RT pick-up did then you won't be able to use all the fuel in the tank. (before your trip you might run the pump (use a jumper wire in pump relay socket) to see if it will suck ALL the fuel out, or if it leaves a gallon or more in the bottom that it can't get to.

DR, great ideas. The aux tank outlet is just above the level of the rear rack. I would not say that it is "a lot higher" then the bulkhead fitting but it is higher hence the aux tank draining into the main tank. This has only been tested in my garage. Before working on the tank, I rode 20 miles with the fuel light on. After reassembly we put 1.5 gallons in the aux tank and let it flow. Later the aux tank was empty. I really prefer the simplicity of a gravity feed system even if it is being a pain to hook up.

I am not able to redo the tank before our Fl trip next week so will likely be riding with the fuel light taped over. Do you think the bulb will get to hot and melt the lens or socket?

Thanks again for your time and expertise.

DR, great ideas. The aux tank outlet is just above the level of the rear rack. I would not say that it is "a lot higher" then the bulkhead fitting but it is higher hence the aux tank draining into the main tank. This has only been tested in my garage. Before working on the tank, I rode 20 miles with the fuel light on. After reassembly we put 1.5 gallons in the aux tank and let it flow. Later the aux tank was empty. I really prefer the simplicity of a gravity feed system even if it is being a pain to hook up.

 

I am not able to redo the tank before our Fl trip next week so will likely be riding with the fuel light taped over. Do you think the bulb will get to hot and melt the lens or socket?

 

Thanks again for your time and expertise.

Afternoon RoanokeRider

You need to be careful in how you use this set-up. If the lower tank (main tank) has a lower vent than the upper tank & the lower tank won't hold all the contents of the upper tank then the upper tank will completely fill the lower tank & the excess upper tank fuel will just flow out the lower tank vent to the ground.

With this set-up (personally) I would route the lower tank vent up to the upper tank vent then vent both tanks to atmosphere from the top of the upper tank.

If you need to use it as is then make sure the lower tank can hold all the fuel from the upper tank, or only open the valve between the tanks until the lower tank shows full on the gauge then turn the valve back off.

I should be ok. The aux (upper) holds 4 gallons, the main tank is 6.6 gallons. Even with a gallon left, I should be ok. If I run the main tank vent into the top of the aux, the valve could be left open all the time and it would function as a 10.6 gallon tank? I am only plan on having that aux tank on the bike for trips.

I should be ok. The aux (upper) holds 4 gallons, the main tank is 6.6 gallons. Even with a gallon left, I should be ok. If I run the main tank vent into the top of the aux, the valve could be left open all the time and it would function as a 10.6 gallon tank? I am only plan on having that aux tank on the bike for trips.

Afternoon RoanokeRider

Yes, as long as you don't open the valve until the lower tank has 4 gallons + of room then you will be just fine.

If I added an extra 4 gallon tank to my RT I would also have to add a remote vent to my bladder.

I have a sidecar on it and when traveling at 70mph+ on interstates it can drink some fuel. I have had the least range of the the others I generally ride with. One has an 05 R1150RT with a larger heavier sidecar and gets noticeably better has mileage. The other rides an K1200LT with a 2 gallon aux tank.

Extend mileage is not always a about distances between stops. Sometimes it is more about when and where to get gas. We had a homemade 8.5 gallon tank on our 2 wheel Airhead. We were riding cross country with friends on a K100Rt. I remember one fuel stop near Edmunton where it was pouring down rain. No canary over the pumps. My buddies is out there leaning over the tank while filing it. I was inside sipping a cup of coffee. YMMV.