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Discussion Starter #1
After my second fill up today, I got to thinking about my flaky fuel gauge. I believe that if I place a properly sized resistor or rheostat across the plug connections to the sending unit I could prove the wiring and gauge. I just want the da*n thing to work. Anyone have the resistance values?

Robert
 

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you might of bent the tube inside the filler neck, this stops the float from moving up and down, do you hold the nozzle when filling the tank or let it hang?, unfortunately the best way to check the sender unit it is to take it out of the tank and bench test it with an ohm meter, if the tube is damaged a broom handle inserted is close to the right size to straighten it
Linton
 

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Discussion Starter #3
you might of bent the tube inside the filler neck, this stops the float from moving up and down, do you hold the nozzle when filling the tank or let it hang?, unfortunately the best way to check the sender unit it is to take it out of the tank and bench test it with an ohm meter, if the tube is damaged a broom handle inserted is close to the right size to straighten it
Linton
Thanks, Linton. Been there, done that. No help. I want to be able to prove that the dash gauge is good or bad. The sending unit appears to work but I don't know what resistances I should be reading.

Robert
 

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2005 K1200LT
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I went and checked my spare unit but I could not get a reading with the ohm meter - it was all over the place. I'll take it apart today and see what I find. I thought I had measured it once before but for some reason I can't now.
 

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I went and checked my spare unit but I could not get a reading with the ohm meter - it was all over the place. I'll take it apart today and see what I find. I thought I had measured it once before but for some reason I can't now.
Thanks John. All I need is a starting point. Is a dead short equal to a full tank or empty? Or is there some BMW logic requiring a set resistance to equal full or empty?

Robert
 

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OK I had a flaky alligator clip. Empty is 77 ohms and full is 6 ohms.
 

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Discussion Starter #7
Re: Stoopid fuel gauges

OK I had a flaky alligator clip. Empty is 77 ohms and full is 6 ohms.
Great! Thanks John. I will "liberate" a 10 ohm resistor from work for testing purposes.

Robert
 

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Discussion Starter #8
I just had another thought. Since my fuel gauge shows zero, what triggers the low fuel light? Apparently not the sending unit.

Robert
 

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I just had another thought. Since my fuel gauge shows zero, what triggers the low fuel light? Apparently not the sending unit.

Robert
Hi Robert.

I think it does activate the fuel low light and that might imply that the problem might be somewhere other than the sending unit.

Ron
 

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I just had another thought. Since my fuel gauge shows zero, what triggers the low fuel light? Apparently not the sending unit.

Robert
Inside the relays electrical junction box (under fuel tank) there is a "Fuel-Warning-Control-Unit". Although it sitting near all other relays, this one is a lot smarter than an ordinary relay (and a lot more $$$).

I never fully opened this fuel-warning-unit, but from the BMW electrical-diagrams, you get these hints (available on CD from BMW, not from CLYMER)
(1) Fuel-Warning-Control-Unit has 5 pins being used by BMW
(2) Fuel level sender (float inside tank) has 1 wire for ground (brown wire) and another wire signal being fed to gauge and also to Fuel-Warning-Control-Unit (yellow-violet wire)
(3) Another pin of the Fuel-Warning-Control-Unit is for ground (brown wire)
(4) Another pin of the Fuel-Warning-Control-Unit is to light up the Low-fuel warning on dash (White-Green wire)
(5) Another pin of the Fuel-Warning-Control-Unit is for power and is fed from FUSE #1 which also feed instruments. Fuse #1 is only valid for 1999-2004 models (Green-black wire)
(6) Finally, the last pin of the Fuel-Warning-Control-Unit goes into pin #3 of the diagnostic connector for dealer's computer or GS911 (Brown-Black wire). You will understand better this part if you read this from the GS911 folks: http://www.hexcode.co.za/techinfo/tank-calibration-on-ma2.4-models

So based on this info, my assumption is that:
(a) The Fuel-Warning-Control-Unit is the one controlling the dash "low-fuel" warning lamp.
(b) The Fuel-Warning-Control-Unit has some kind of static memory as it can remember the fuel setting value for "low-fuel" even if Battery is unplugged for weeks.
(c) Electrical diagram shows than none of these input-outputs goes to BOSH-Motronic unit. This makes the assumption in (b) above very plausible.

I hope this helps...
 

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Discussion Starter #11
Inside the relays electrical junction box (under fuel tank) there is a "Fuel-Warning-Control-Unit". Although it sitting near all other relays, this one is a lot smarter than an ordinary relay (and a lot more $$$).

I never fully opened this fuel-warning-unit, but from the BMW electrical-diagrams, you get these hints (available on CD from BMW, not from CLYMER)
(1) Fuel-Warning-Control-Unit has 5 pins being used by BMW
(2) Fuel level sender (float inside tank) has 1 wire for ground (brown wire) and another wire signal being fed to gauge and also to Fuel-Warning-Control-Unit (yellow-violet wire)
(3) Another pin of the Fuel-Warning-Control-Unit is for ground (brown wire)
(4) Another pin of the Fuel-Warning-Control-Unit is to light up the Low-fuel warning on dash (White-Green wire)
(5) Another pin of the Fuel-Warning-Control-Unit is for power and is fed from FUSE #1 which also feed instruments. Fuse #1 is only valid for 1999-2004 models (Green-black wire)
(6) Finally, the last pin of the Fuel-Warning-Control-Unit goes into pin #3 of the diagnostic connector for dealer's computer or GS911 (Brown-Black wire). You will understand better this part if you read this from the GS911 folks: http://www.hexcode.co.za/techinfo/tank-calibration-on-ma2.4-models

So based on this info, my assumption is that:
(a) The Fuel-Warning-Control-Unit is the one controlling the dash "low-fuel" warning lamp.
(b) The Fuel-Warning-Control-Unit has some kind of static memory as it can remember the fuel setting value for "low-fuel" even if Battery is unplugged for weeks.
(c) Electrical diagram shows than none of these input-outputs goes to BOSH-Motronic unit. This makes the assumption in (b) above very plausible.

I hope this helps...

Thanks, John. The interconnecting of components sounds like BMW. I think I will try to burn enough fuel to make the light come on just to see if it works. Hopefully I don't get stranded. I remember someone saying the factory setting was at the one gallon mark. Hopefully they are right and the previous owner never reset it. Probably not, since he did not work on the bike and I bought it with less than 8K miles.

Robert
 

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Thanks, John. The interconnecting of components sounds like BMW. I think I will try to burn enough fuel to make the light come on just to see if it works. Hopefully I don't get stranded. I remember someone saying the factory setting was at the one gallon mark. Hopefully they are right and the previous owner never reset it. Probably not, since he did not work on the bike and I bought it with less than 8K miles.

Robert
Be careful with that, the LT is a heavy beast to push to the gas station when it runs dry - not an experience I ever want to repeat. You might want to carry a small can with a gallon in it when you start getting low just to save yourself some sweat.
 

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Discussion Starter #14
Update. I filled up today in South Dakota after stretching my range and put in 6.33 gallons. I had no lights or warnings. What gives?

Robert
 

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You were pretty luck as you were about on fumes. Maybe its time for a new (used) fuel sender. Where was the fuel gauge at the time of fill up?
 

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Update. I filled up today in South Dakota after stretching my range and put in 6.33 gallons. I had no lights or warnings. What gives?

Robert
Mine is doing the same thing, I just added 6.25 gallons the other day with ho low fuel light and the gauge was reading about 1/4 tank. It goes up to full when filled, I cannot decide if it is worth tearing my hair out trying to fix or not! Please let me know if you get to the root of the problem.
 

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Could be a bad ground
 

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Discussion Starter #18
You were pretty luck as you were about on fumes. Maybe its time for a new (used) fuel sender. Where was the fuel gauge at the time of fill up?
John, the fuel gauge was showing empty for most of that tank. When I fill up the gauge goes up to about 1/2-3/4 for about 10 miles or so then drops to zero for the rest of the tank.

Robert
 

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You have 2 separate devices, an indicator and a gauge, both controlled by a single wire from the fuel sending unit and both not indicating correctly. Symptoms point to a faulty sending unit. Assuming a resistance range of 6-77 ohm buy a $4 100 ohm pot and simulate the sending unit. You also have open and short for additional information, I assume an open will compare with 77 ohm and a short with 6.
 
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