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  #1  
Old 02-12-2008, 10:05 AM
latebraker latebraker is offline
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Low Fuel Light/ Fuel Guage ??

After following the threads concerning the low fuel light function and the threads that involved the fuel guage, I was stumped by the issue that I was having.

The fuel gauge was indicating properly ( Low fuel indicated on guage = low fuel in tank..High fuel level indicated on guage= a high level of fuel in tank.) The low fuel indicator light remained on. The next phase of investigation was based on my admittedly flawed logic: "if the fuel level guage is working and the low fuel level warning light is constantly on, there must be an open circuit that allows the low fuel level control unit to trigger the low fuel warning light" Yeah, thats got to be it!!! After all, there are only two wires coming out of the fuel sending unit.
Out comes the faboulous wiring diagrams courtesy of Clymer and the chase for the elusive fault ensued. Two days later... after pulling all connectors loose and checking for bent pins, faulty grounding, relay functions,etc.etc,etc, I was baffled, bruised and beaten. How could something as simple as a float controlled device be sooo difficult.
I called in reinforcements...sending unit removed, Check! Float clunked fore and aft.Check! Disasembled float unit, Check! wiped off the wires inside the unit.Check! Sanded off some flashing on the plastic float that might have impeded movement. Check! Wiped out the float tube and noted some abrasion on the inside diameter of the tube.Check!
Reassembled and installed unit back in the tank..EUREKA I've found it!! Everything is functioning properly .
My question is...How does this thing work?? The float with it's little metal probes reduce and increase resistance with it's movement along the wires,and cause the fuel gauge to indicate,correct? How does the low fuel circuit operate independently of this device? Do the two wires somehow detect a microvoltage resistance using the actual gasoline as a resistor??? MY BRAIN HURTS!!!! Any theorists out there??
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  #2  
Old 02-12-2008, 10:12 AM
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Re: Low Fuel Light/ Fuel Guage ??

Quote:
Originally Posted by latebraker
MY BRAIN HURTS!!!! Any theorists out there??
Bob (RFW) should be by any minute now.
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  #3  
Old 02-12-2008, 10:22 AM
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Re: Low Fuel Light/ Fuel Guage ??

Quote:
Originally Posted by latebraker
How does the low fuel circuit operate independently of this device?
According to the BMW repair manual, the level at which the low fuel warning light illuminates is programmable.
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Old 02-12-2008, 10:39 AM
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Re: Low Fuel Light/ Fuel Guage ??

After the process described, I did not have the B.M.W. moditec available. This was not a programmable fault. What information and what type of information does the motronic recieve from the sending unit? Thanks for the input as I am slow of study and I need all the help I can get!! It's fixed, but I am curious as to exactly what fixed the bavarian vixen!!!
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  #5  
Old 02-12-2008, 11:35 AM
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Re: Low Fuel Light/ Fuel Guage ??

You posted this in the krs/gt group.

Specifically what bike year, and model are you referring to??

This will help.
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  #6  
Old 02-12-2008, 11:40 AM
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Re: Low Fuel Light/ Fuel Guage ??

OOOps!! sorry for not including this bit of information: 2004 K1200 G.T. in the sublime grey green.
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Old 02-16-2008, 10:47 AM
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Re: Low Fuel Light/ Fuel Guage ??

Hi,
You asked: How does the low fuel circuit operate independently of this device?
It doesnít work independently. That low level circuit is a quite dummy device . You need that MoDiTeC unit to set the low fuel light. Follow this procedure :
Put 4 litres to an empty tank, if you want that itís suitable alarm level for light on or 6 litresÖ etc.
Then use the MoDiTeC unit to set that level. Only message from the sending unit is resistance.
The resistance values can be:
Ca. 0-5ohms for full tank
Ca. 12ohms for half tank
Ca. 45ohms for ľ tank
Etc.
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Old 02-16-2008, 11:16 AM
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Re: Low Fuel Light/ Fuel Guage ??

Thanks Silversmith, If the only value from the unit is resistance and it only provides one value at a time, I must have an issue with the low fuel light control unit. All ohm values received from the fuel tank are within the correct parameters. Again, Thanks!!
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Old 02-16-2008, 11:43 AM
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Re: Low Fuel Light/ Fuel Guage ??


By the way.
The fuel lever sensor isnít serviceable. How did you disassemble it? Did you remove the spring on the bottom as well as the small nut, which in turn released the float from the housing?
Was it easy to reassemble that?
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Old 02-16-2008, 12:29 PM
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Re: Low Fuel Light/ Fuel Guage ??

The sensing unit was easily dissassembled by first scribing the location of the aluminum tube in relation to the plastic base, then undoing the crimps on the spring retaining collar to release the spring. Then undo the small crimp on the nut retaining washer. After removing the small nut, CAREFULLY remove the spring retaining collar and CAREFULLY slide the aluminum tube off the central shaft. BEWARE of the two ni-chrome wires which parallel the central shaft, they are very thin and look to be fragile.
As this unit was dissassembled, we wondered how the craftsman could make his production quota without a few Pilz to steady the nerves.. Oh, by the way Silversmith, do you do any hand engraving? (Gravermax here)
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Old 02-16-2008, 12:45 PM
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Re: Low Fuel Light/ Fuel Guage ??

Many Thanks for your advises. I have something trouble with my sensor and Iíll try to disassemble it. But first Iíll purchase some bottles of bear.

Itís only my nickname.
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  #12  
Old 02-17-2008, 06:14 AM
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Re: Low Fuel Light/ Fuel Guage ??

Quote:
Originally Posted by latebraker
After following the threads concerning the low fuel light function and the threads that involved the fuel guage, I was stumped by the issue that I was having.

The fuel gauge was indicating properly ( Low fuel indicated on guage = low fuel in tank..High fuel level indicated on guage= a high level of fuel in tank.) The low fuel indicator light remained on. The next phase of investigation was based on my admittedly flawed logic: "if the fuel level guage is working and the low fuel level warning light is constantly on, there must be an open circuit that allows the low fuel level control unit to trigger the low fuel warning light" Yeah, thats got to be it!!! After all, there are only two wires coming out of the fuel sending unit.
Out comes the faboulous wiring diagrams courtesy of Clymer and the chase for the elusive fault ensued. Two days later... after pulling all connectors loose and checking for bent pins, faulty grounding, relay functions,etc.etc,etc, I was baffled, bruised and beaten. How could something as simple as a float controlled device be sooo difficult.
I called in reinforcements...sending unit removed, Check! Float clunked fore and aft.Check! Disasembled float unit, Check! wiped off the wires inside the unit.Check! Sanded off some flashing on the plastic float that might have impeded movement. Check! Wiped out the float tube and noted some abrasion on the inside diameter of the tube.Check!
Reassembled and installed unit back in the tank..EUREKA I've found it!! Everything is functioning properly .
My question is...How does this thing work?? The float with it's little metal probes reduce and increase resistance with it's movement along the wires,and cause the fuel gauge to indicate,correct? How does the low fuel circuit operate independently of this device? Do the two wires somehow detect a microvoltage resistance using the actual gasoline as a resistor??? MY BRAIN HURTS!!!! Any theorists out there??


As others have said the fuel level sensor is just a variable resistance with a small voltage passing through it and it's the voltage level which is monitored by the ECU to switch on the low fuel warning.

The ECU has a stored table value which corresponds to the voltage at which it sends out the low fuel warning lamp signal.

The Moditech merely reprograms that voltage level with a known quantity of fuel in the tank to put on the light. Some have actually added series and parallel resistance to make small changes to the sensor voltage seen by the ECU to avoid a Moditec re-calibration. But then with an expensive BMW that seems a bit cheapskate. Once the calibration point is set, I can't see much affecting it unless the sensor is changed, sticking, going faulty or there is some issue with instrument voltage regulation.
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Old 02-17-2008, 08:54 AM
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Re: Low Fuel Light/ Fuel Guage ??

So if I understand correctly, it is possible to program the low fuel warning light to come on when the fuel guage indicates full. That was my issue in the beginning. Thanks for all the input. I think I'll just drill a hole in the tank and use a stick with incremental paint marks on it, maybe I'll get fancy and add a float and let the sharp end of the stick protrude from the tank and I'll just count how many marks show on the stick. Ahhh progress!!! Now let's talk about catalytic convertors (1200F-1600F) and how they cook seals within the bell housing.
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Old 02-17-2008, 10:37 AM
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Re: Low Fuel Light/ Fuel Guage ??

The easiest fix is to black-tape over the indicator light...done.

Plastics
99 K1200RS silver/blue
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Old 02-17-2008, 12:33 PM
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Re: Low Fuel Light/ Fuel Guage ??

That's exactly what I should have done in the first place!! Thanks!!
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  #16  
Old 02-19-2008, 05:16 AM
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Re: Low Fuel Light/ Fuel Guage ??

Quote:
Originally Posted by latebraker
So if I understand correctly, it is possible to program the low fuel warning light to come on when the fuel guage indicates full. That was my issue in the beginning. Thanks for all the input. I think I'll just drill a hole in the tank and use a stick with incremental paint marks on it, maybe I'll get fancy and add a float and let the sharp end of the stick protrude from the tank and I'll just count how many marks show on the stick. Ahhh progress!!! Now let's talk about catalytic convertors (1200F-1600F) and how they cook seals within the bell housing.

Ah, but the tank has 2 compartments at different levels left and right side with the fuel sender monitoring the right side lowest level. That's why the low fuel light may come on, then you go round a twisty and it goes off for a while.

You could use something simple like a piece of clear pvc tube connected at the bottom with a foam breather plug in the top so you can use it as a sight level. I designed a manometer fuel level sensor for a marine fuel tank based on a very sensitive low pressure sensor- that worked fine. I couldn't really care what my fuel guage is telling me, but the low fuel sensor is what I believe most consistently accurate and I checked it once by draining the tank.

If your CAT is getting too hot you are riding too slow!
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Old 02-19-2008, 09:25 AM
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Re: Low Fuel Light/ Fuel Guage ??

I just love this forum!! It seems that almost everyone that visits, is extremely curious and enjoys delving into the mysteries and logic that inspired the engineering involved in this Bavarian mount. Although, that peculiar logic sometimes escapes me!!
I have removed the toaster oven ( commonly call the catalytic convertor) and replaced it with a Remus, hopefully before the seals were baked into very expensive doughnuts!!
Just my personal theory...
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  #18  
Old 02-20-2008, 12:44 PM
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Re: Low Fuel Light/ Fuel Guage ??

.................I cannot yet get inside the head of Bavarian engineering logic, but Bavarian fudge and over complexity is pretty close. Is there such a thing? I think I went on a self teaching course the first time I encountered a BMW wiring diagram.

Perhaps my logic is still in Japan!
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Old 02-20-2008, 01:39 PM
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Re: Low Fuel Light/ Fuel Guage ??

Quote:
Originally Posted by voxmagna
.................I cannot yet get inside the head of Bavarian engineering logic, but Bavarian fudge and over complexity is pretty close.
It's just like the gear display for the S - apparently, four wires come from the switch. While the three wires using the BCD scheme was on the right track (remember the old Magna display that used six wires, one for each gear position?), I don't see why a fourth wire (a "common" as someone called it) was necessary. I would have opted to have the shell of the transmission-mounted switch be grounded, and the contacts be grounded when needed, and used a combination of pullups and inverters at the cluster. Matter of fact, I developed something along this line for an older Honda I own that's still in the project phase...
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Old 02-20-2008, 01:56 PM
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Re: Low Fuel Light/ Fuel Guage ??

I believe that the Logic that inspired the engineers at B.M.W. was developed by the antics of Max and Moritz. (German folk cartoons) And we, (the unwashed masses) are victims of their cruel jokes.
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Old 02-20-2008, 03:17 PM
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Re: Low Fuel Light/ Fuel Guage ??

Quote:
Originally Posted by chuba
It's just like the gear display for the S - apparently, four wires come from the switch. While the three wires using the BCD scheme was on the right track (remember the old Magna display that used six wires, one for each gear position?), I don't see why a fourth wire (a "common" as someone called it) was necessary. I would have opted to have the shell of the transmission-mounted switch be grounded, and the contacts be grounded when needed, and used a combination of pullups and inverters at the cluster. Matter of fact, I developed something along this line for an older Honda I own that's still in the project phase...

Yes, it took a few moments for me draw out and post the truth table for the KRS switch.

Now it's naughty to have this earthing and grounding logic in your head which comes from house wiring and attempts to save money with wire. As far as possible all battery negatives (we call grounds) should have their own wire and be separate from the frame until they are 'grounded' at the battery in one common place. The motor and frame is used to carry the heavy starter current and alternator charging currents. By putting in separate negatives they avoid interference from the alternator/starter and high current switch loads - this is common practice to avoid 'ground loops' in audio - here we have sensitive electronics and microprocessors so no different.

If you break this design philosopy and start tying accessory wires down to the frame, you call 'ground' then not only may they get interference but in the event of a ground problem through the starter or alternator (like you left off that battery -ve frame ground) you will get high current finding the quickest way through your new accessories.

That Bavarian wiring principle is a good one. However their implementation of the battery negative on the KRS using a separate wire from the battery terminal to ground (rather than combine in one terminal) is a bad one when you forget to connect this, crank the starter and starter current burns out the loom.
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Old 04-20-2008, 04:24 PM
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Re: Low Fuel Light/ Fuel Guage ??

I had the opposite problem that the OP had. My low fuel light did not come on at all and I subsequently found out what the fuel range is on my '99 K12RS. 213 miles on one particular tank with mixed use riding. The gauge was working correctly, so I knew I was low on fuel, just not exactly how low. The low fuel light does light up briefly along with the rest of the idiot lights when the ignition key is turned on. I incorrectly figured a 40mpg average and 5.8 gal capacity would work out to be about 225-230 miles. As Maxwell Smart used to say, "AAW!! Missed it by that much!!" The "usable" fuel capacity is probably less. Fortunately, I ran out of gas close to home, and my wife brought a gas can to me with a couple of gallons of fresh 93 octane. "DOOOHHHH!!!!" What kind of range do the K12RS/GT riders on the forums usually get before the low fuel light comes on? Inquiring minds want to know....
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Old 04-20-2008, 06:53 PM
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Re: Low Fuel Light/ Fuel Guage ??

150-180 miles I think. To be honest it's so few I worry about riding the bike into the great unknown where there may be no gas station!

BMW should have integrated the small tank/low fuel light warning with a gps data base of nearest gas stations.
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Old 03-30-2009, 02:44 PM
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Re: Low Fuel Light/ Fuel Guage ??

After reading entire forum and comments I am still confused. My low fuel light comes on just below the 1/2 indication on the fuel gauge. This usually in the range of around 140-150 miles on the odometer. The information as I understand it from the forum would indicate that I have only about 1 gallon US remaining. When I fill the tank from that point it only takes around 3 gallon US to fill it. With some basic math that would indicate that actually there are about 2 gallons left in the tank. I would like to have the warning light come on at 1 gallon left in tank. Should I drain tank then add 1 gallon of fuel to get correct programming where I would want it? This of course would be done while hooked to the MoMagic computer if in fact it is programmable. Are my assumptions correct?
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Old 03-30-2009, 05:07 PM
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Re: Low Fuel Light/ Fuel Guage ??

Quote:
Originally Posted by knappy
After reading entire forum and comments I am still confused. My low fuel light comes on just below the 1/2 indication on the fuel gauge. This usually in the range of around 140-150 miles on the odometer. The information as I understand it from the forum would indicate that I have only about 1 gallon US remaining. When I fill the tank from that point it only takes around 3 gallon US to fill it. With some basic math that would indicate that actually there are about 2 gallons left in the tank. I would like to have the warning light come on at 1 gallon left in tank. Should I drain tank then add 1 gallon of fuel to get correct programming where I would want it? This of course would be done while hooked to the MoMagic computer if in fact it is programmable. Are my assumptions correct?

Pretty much... Your light comes on too early. Mine comes on pretty much about the time the needle hits the "0" line if not just above it and it takes 4.5 gallons from that point, meaning i had 1 gallon left. Now, granted, the manual states it should take 5 gallons when the light is on, as the reserve is only .5 of a gallon (am i right?), but i'll take what i can get. My VFR (with an LED bar fuel gauge) would run out of LEDs and start flashing with ~1.6 gallons left in the tank...
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Old 03-30-2009, 05:36 PM
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Re: Low Fuel Light/ Fuel Guage ??

Ok, fine then. Yours works more like I want mine to work. Guess the next time that I visit my dealer I will that corrected. Shouldn't be much of a problem since I also have to have the quick-disconnects replaced, one homemade and one still plastic. That is another subject, kinda p*$$e$ me off about an otherwise fine machine.
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