Fuel Gauge Failure

[jeffrey prothero]

I know this is an e9 forum, but I think the problem I have is not unique to my e12. So, please consider my present challenge. I have an '81 528i: Recently, I noticed that my fuel gauge was off by about 1/4 of a tank. 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 thing is that when I turn the key off, the needle jumps down to the actual low level, nearly empty ("nearly empty" is accurate for sure). 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...

 

[dj_efk]

Faulty voltage regulator perhaps? Does the temp gauge behave?

 

[adawil2002]

Resistance wire in the sender is very fine, it is possible for them to break.

 

[jeffrey prothero]

I replaced the sending unit with Genuine BMW part, and have the same read on my fuel gauge. Also, the TEMP gauge is normal. and battery charging fine and other gauges do not seem to be affected, so dont think voltage reg is the culprit.

 

[Stevehose]

Can you test your gauge by putting 12v to it?

 

[jeffrey prothero]

I am going to fold my 6'4" frame up accordian style and mae my way under the dash to see how easy it is to reach up in there to possible remove it. I kind of think I need to remove entire cluster, if my memory serves me right... stay tuned!

 

[jeffrey prothero]

I removed the inst cluster and cleaned the connections on each of the ribbon male connectors. Then did same to female. Then did same to the female connector contacts at the cable attaching to the new sending unit. Whatever it was seems better now. I fueled up and now wait to see what happens. I did not get inside the cluster to see how to get power to the gauge itself, although I deduced the likely areas on the ribbon based on relative proximity to the back of the fuel gauge. Either way, it seems better, so I' ll test this out first, before opting to dig deeper into the cluster. Thanks for your help, folks!

 

[Honolulu]

Also make sure the cluster ground is clean and making good contact. IIRC from my 2002 days (I had fifteen of the little darlings at one time or another) cluster grounding was a not uncommon issue that usually manifested itself by gage needles ticking upwards in sync with the turn signals. Your E12 is not that far removed from those days and the cluster may be subject to similar extracurricular activity.

 

[jeffrey prothero]

Thanks Honolulu, Charlie. Now I just need to locate the ground! Are they generally brown or brown striped? Seems BMW would have a rather uniform scheme.

 

[Honolulu]

Jeffrey:

BMW uses solid brown sheathed wire for all grounds. I think it's DIN standard to do so, but have little experience with other Euro cars (I try stay out of the wife's C280).

There is also brown with a black tracer or stripe, that is for something similar but not the same. You're looking for a brown wire.

My 6'6" frame isn't much for going under the dash either as I can promise from experiences replacing a pedal cluster frame on an E30 or a clutch master cylinder on an E36, you'll get the picture painfully fast. 2002 seats come out pretty quickly, just four bolts with 10 mm heads, that's the way I did my '02s and other early cars. Later cars with electric seats get heavy and there's additional wires to disconnect, but the idea's the same.

 

[stphers]

One thing on these cars at the cluster are those printed circuits that are folded over where the connector goes in, carefully check that there is not a crack in the plastic sheet right where it bends, I use to run into issues with those. Also make sure that the contacts in the connector make good contact with the printed circuit.

Thanks, Rick

 

[jeffrey prothero]

Jeffrey:

BMW uses solid brown sheathed wire for all grounds. I think it's DIN standard to do so, but have little experience with other Euro cars (I try stay out of the wife's C280).

There is also brown with a black tracer or stripe, that is for something similar but not the same. You're looking for a brown wire.

My 6'6" frame isn't much for going under the dash either as I can promise from experiences replacing a pedal cluster frame on an E30 or a clutch master cylinder on an E36, you'll get the picture painfully fast. 2002 seats come out pretty quickly, just four bolts with 10 mm heads, that's the way I did my '02s and other early cars. Later cars with electric seats get heavy and there's additional wires to disconnect, but the idea's the same.

Thanks Charlie. I kinda figured it was a brown lead. This is the last thing for me to check, as I have already cleaned the male connectors going into the cluster and the corresponding female laminated circuits they come in contact with. So, Im sure we'll be fine! thanks again for all your help here. You guys are really terrific.

 

[jeffrey prothero]

One thing on these cars at the cluster are those printed circuits that are folded over where the connector goes in, carefully check that there is not a crack in the plastic sheet right where it bends, I use to run into issues with those. Also make sure that the contacts in the connector make good contact with the printed circuit.

Thanks, Rick

Good suggestion Rick. I just posted to Charlie that I did all that on Saturday and all is nice and clean now. I even CRC'ed the connection contacts. Just have to trace the sender ground to inspect grounding point. Because all tail lights ground at the same point and the fact that all lights work fine, I expect that to be OK once I get int there. I kind of think the culprit will turn out to be a less than clean contact in the ribbon laminate of the instrument cluster or connector thereto. Thanks for piping up. I really appreciate all the help you guys at E9 Forum have been to an E12'er like me. Happy Thanksgiving!