Fuel Gauge Problem

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Jcat
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Joined: Thu Aug 02, 2012 8:01 pm
Location: Roswell, GA

Fuel Gauge Problem

Post by Jcat » Fri Nov 09, 2018 6:56 pm

'81 528i: Recently, I noticed that my fuel gauge was off by about 1/4 of a tan. It now shows a level higher by 1/4 than it actually is. I replaced the sender unit, as I thought it was surely the culprit. No so. Currently the tank is very low. But the needle shows 1/4 tank when the engine is running. Odd think is that when I turn the key off the needle jumps down to the actual low level, nearly empty. The needle pauses there for about a second before dropping all the way off to the left of the AMBER reserve lens.This all started, as I recall, the last time I filled it up about a couple of months ago (not my daily driver). I just cant seem to figure out why. Any suggestions you have, I'm all ears...
J Prothero
Roswell, GA
Former FF Ambassador to Montvale
1981 528i stock
Jprothero600@gmail.com

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528i-1981
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Re: Fuel Gauge Problem

Post by 528i-1981 » Sat Nov 10, 2018 8:11 am

Hey JCat - You have angered the German car gods. Repent. As you know, the sender is a variable resistor that works through the ground. IIRC there are three brown wires at the sender, two with stripes (black and yellow I think) for the gauge itself and the reserve light, and the plain brown to the ground point. I would check the ground wire (plain brown) attachment to the body first if you haven't already. If that doesn't work then isolate the gauge and test it through 0-12 volts to verify its behavior. This is a pain to access but easy to diagnose. If everything checks out normal I think you're only left with the striped brown wires, which could have bad connections or cracked insulation. That would stink. I'll throw this out there, if either of the striped wires pass through the well in the left quarterpanel (I can't recall), check to see your jack hasn't dropped and pinched one of them against the body metal.
(oo=00=oo) Eric
1981 528i 5-speed

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Jcat
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Location: Roswell, GA

Re: Fuel Gauge Problem

Post by Jcat » Sat Nov 10, 2018 10:03 am

Eric. i need a sanity check. (Stop laughing now). Go to your car. Look at your fuel gauge. Turn the engine on so the needle registers your fuel level (must have engine on). then turn engine off. watch carefully to see if your needle pauses on its way to the final stop which is below the amber lens in the fuel gauge. Or does your needle go directly past the amber lens without any pause whatsoever? Mine pauses just before dropping all the way down to the amber lens. I dont recall that ever being the case before and since you have a 81 model, you have the perfect 'sanity check" for me to keep trouble shooting this.
J Prothero
Roswell, GA
Former FF Ambassador to Montvale
1981 528i stock
Jprothero600@gmail.com

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528i-1981
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Location: Boston North Shore, MA

Re: Fuel Gauge Problem

Post by 528i-1981 » Tue Nov 13, 2018 5:38 pm

JCat, Haynes shows BR/YE and BR/BK wires from the sender to the cluster. BR ground heads aft to the ground point shared with the tail lights and license plate lights. I can post a video of the fuel gauge after the new radiator is in and I can start her. Probably be after Thanksgiving.
(oo=00=oo) Eric
1981 528i 5-speed

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528i-1981
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Re: Fuel Gauge Problem

Post by 528i-1981 » Tue Nov 27, 2018 3:55 pm

Be it hereby noted for posterity that a normally functioning fuel gauge in a 1981 US spec E12 will, upon the key being moved from the run position to off, initially descend, pause, and subsequently nosedive to the below-empty rest position. This behavior is exhibited when stopping a running engine but not when the key is turned with a still engine. Attested to this 27th day of November 2018 under penalty of perjury.
(oo=00=oo) Eric
1981 528i 5-speed

brotherhay
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Re: Fuel Gauge Problem

Post by brotherhay » Wed Nov 28, 2018 8:59 pm

If youve already checked the gauge wiring and such mentioned above and everything seems fine, try this:

remove the 4 nuts that hold the sender in place
Pull it up but not all the way out of the hanger pump, and let the gas drain out of the sender
you can turn the key to the run position in the car and with the sender still connected to its wiring, lift it out and turn the sender from its natural resting position position, to upside down, and back again.
You should see the gauge fluidly move from F to E, or in our vehicles case, from F to R

if for some reason this doesnt work, while the sender is still plugged in, then my guess is the float has come free of the guide wires, or one is or both is broke.

If you think thats the case, just remove the small 4mm nut at the bottom of the sender, un-crimp the egdes at the top that are holding the aluminum sleeve in place, and push from the bottom where the nut is, up and through the outer housing. Youll expose the sender guts. Theres a Tech FAQ about this, describes it a little better there.

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