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My fuel warning light is always on. It has been since I bought this 2002 R which now has 20000 miles.

So, I figured with the tank off the bike and the pump/sending unit assembly out, I'd do a little investigating. Here is what I got:

electrical connector connected, turn on bike ---> indicator on, regardless of orientation of fuel level sensor float (which I can hear moving so it is not seized up).

leave bike on, unplug unit ---> indicator stays lit

turn bike off, then on again (still unplugged) ---> indicator off

leaving bike on, plug in assembly ---> indicator on, stays on even if then unplugged

I know that somewhere in this circuit there is some sort of damping unit. Don't know if that plays a role.

This is not a big deal to me but it irks me so....

And, yes, I know that if it really bothers me I should pull out the indicator bulb.

I just want to understand.

The stay-on-when-not-connected circuit appears to be "designed in" so the low fuel light does not flicker on-off-on-off with up/down road inclines when the fuel reaches the low fuel actuation level. That means it is most likely NOT a wire harness short and it is most likely the low fuel sending unit itself, even though you hear the internal mechanism working with positional re-orientation. The 12 year old internal switching system is at fault.
Is that correct . . . 20,000 miles for a 2002 model? If so, there is a huge opportunity for old fuel in the tank to form gummy deposits, which could easily give you an always-on low fuel light. It sounds like you need to do another pump removal for an aggressive, carb cleaner spray-n-soak of the fuel level sensor. It's no good the way it is now.

Yes, 20000 miles. So soak in carb cleaner. Sounds simple enough.

Thanks a lot for the response. I will be psyched if this works.

Yet again another excellent and logical diagnosis of your symptoms by CycleRob!

If I remember correctly, there is a mini-magnetic reed-switch that looks like a small glass capsule mounted inside the plastic fuel-level sensor barrel. Maybe this has come loose and is always actuated by the magnetized float, or maybe the reeds are stuck making a closed-circuit. You could check continuity between the leads as you tip the sensor over to see if it is switching properly. If not, you can replace the reed-switch it if you have a soldering iron.

From what I remember when looking at the white/grey plastic low fuel sensor, it was a non serviceable sealed unit. Having no problem with mine in 9 years of riding, I find it odd now today that I was not more curious about its' internal mechanism at the time the filter change job had it apart. Checking the connected wiring for crush/crimp/wear-thru ground-outs should be Job-1.

Job-2 is as Sunbeemer mentioned, to inspect and Ohmmeter test the sealed glass reed's functionality with tilting the pump assembly to gravity activate the internal float. That test would tell you if it's good or not.

The bottom of my fuel sensor unit popped off while I was trying to extricate the fuel pump assembly from the tank, and that's how I noticed how it worked. I didn't know it was missing a bottom cover at first until I found the ~1" dia. plastic disc in the residual fuel in the bottom of the tank and wondered, "Hmm, where did that come from?" and soon realized that it popped right into the bottom of the fuel level sender
All good!

I am able to pretty easily pop the lid of the unit (which is kind of like the size of a film canister). there is a donut shaped freely moving piece inside that moves according to gravity - slides up and down the length of the unit on a central post. it is stopped at the end of that post by a small metal piece with 2 smalls ears that extend out. It appears to be soldered in place there. Due to it's orientation in the tank, when the tank is full the slider floats up and stays in contact with the metal tabs at the end of the post. I guess somehow this contact disables the fuel light.

As the fuel level drops, the float drops away from the contact and at that point I guess the light comes on. I don't really understand how that would work electrically speaking. Or maybe the float comes down the post and then makes a contact to complete the light circuit. I am sure I'm thinking about this in too simplistic terms.

I have a soldering iron and I plan to disconnect the float assembly and test it for continuity with its two wires.

Well, here is what I did.

I removed the fuel level sensor from the fuel flange assembly by heating the soldered connections. I then did continuity testing with the two wires (brown, yellow) from the unit with the sensor upright or upside down. Had continuity all the time, no change.

I then heated the soldered connection inside the unit and removed the two wires that enter the bottom of the unit. I retested this two-wire assembly and found that once removed from the unit there was no continuity. I was surprised to find that the two wires (brown, yellow) are soldered together at the end, however spliced into the yellow wire is glass-encased section which appears to have a metal contact. When a small magnet was held within about a centimeter of this glass encased contact, there was continuity in the two wire circuit. However when the magnet is moved near a certain segment of the glass encased contact, continuity was interrupted.

So, presumably the float contains a magnet which is brought into proximity with this specific segment of the metal which opens the circuit. As the fuel level drops, the float drops bringing the magnet to a different area, the circuit is closed and the light goes on.
So I replaced the wires into the unit and adjusted the glass-enclosed part slightly to get the "no-continuity" section right with the magnet.

This probably is poorly explained, but it works now.
I'll try to add some pics.

If I remember correctly, there is a mini-magnetic reed-switch that looks like a small glass capsule mounted inside the plastic fuel-level sensor barrel. Maybe this has come loose and is always actuated by the magnetized float, or maybe the reeds are stuck making a closed-circuit. You could check continuity between the leads as you tip the sensor over to see if it is switching properly. If not, you can replace the reed-switch it if you have a soldering iron.

Job-2 is as Sunbeemer mentioned, to inspect and Ohmmeter test the sealed glass reed's functionality with tilting the pump assembly to gravity activate the internal float. That test would tell you if it's good or not.

Looking back at this thread, you both had it right. I didn't understand what the "glass reed" part is. Now I get it.

Thanks a lot.

Here are some photos. You can see the sending unit, the glass reed connector, and the brass tubing I used in place of the fuel filter.

https://www.flickr.com/photos/85219855@N04/12870329495/
https://www.flickr.com/photos/85219855@N04/12870400023/
https://www.flickr.com/photos/85219855@N04/12870155405/
https://www.flickr.com/photos/85219855@N04/12869161663/

The other interesting note is that the fuel level sensor can be adjusted higher or lower on its mounting post. So that needs to be taken into account when discussing variation in fuel reserve when the light comes on. There are three different positions.

Nice job!
Manual says the low fuel light turns on when there are 1.3 gals left in the tank (of which only 1.0 gal useable), and it should take 4.1 gals to refill the tank when the light turns on. But however it's set, as long as you know how far you can go when it comes on, that's what counts!

I just picked up a 96 R1100R during the week as a project bike. One of the obvious faults is a continually on fuel warning light!

So thanks for this detailed analysis, I'm not brave enough to tackle this yet, so I'll just ride on the trip meter for now. Plenty of other less challenging things to be attended to. I will probably eventually have a go at fixing the fuel light issue myself. One thing I am getting a "404 not found" message when I try to view your flickr photos?

I know. I still have not gotten that skill down. Good luck with the bike.

Strange thing happened today, my fuel reached low for the first time since I got the bike last week and the warning light went out!

At least I now know I have a (reverse) fuel warning.

The PO told me that the PO before him had said this happened, but he had never seen it because he was paranoid about filling up every 150 Km by the trip. I kinda discounted it at the time.

Any thoughts on the likely cause for this?, my initial thought was reversed wiring somewhere, but I would imagine the connectors seem to preclude that, unless someone was messing around. I'll eventually have the tank off and have a good look around , but our unpredictable weather is unusually good right now, so riding takes priority!

Thinking about this what would make the light operate in reverse is the float arm being upside down ( flipped 180 in the wrong direction), would it be possible to do that)?

You could not mount the sending unit upside down. I wonder if the reed switch needs to be advanced or pulled out a bit from the inner tube of the sending unit. It could be that the magnet is not lining up with the correct part of the switch within. Or maybe the magnet is gunked and not sliding correctly. Do you mean that the light is on with a full tank but then goes off when the tank gets low?
By the way, I am a total newbie at wrenching so don't do anything based on my thought process alone!

sstein wrote: Do you mean that the light is on with a full tank but then goes off when the tank gets low?

Thanks for the reply, yes, exactly that. I had the tank off to replace all of the external hoses. I'm still assembling the parts to renew all of the internals so have not opened it yet. However after I put everything back the behaviour changed slightly. Now the light comes on on a full tank, and goes out at about 120 km (used to go out at about 180), I fill up at 210 - 230.
Here's a pic of the tank flange area outside.

latest update in R1100R here: viewtopic.php?f=21&t=31074&p=242301#p242301