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Erratic Fuel gauge 900

On many ocassions, the fuel gauge always show less than the actual fuel. Worst now, while driving the fuel gauge will show half tank. After a while it is between half and 3/4 tank and moment later back to half tank. Notice the needle is unsteady when the car is at stationary. Can someone advise what is wrong and how to fix.








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    Erratic Fuel gauge 900

    You should do a search on '900 fuel gauge'. The 900's have had a number of fuel gauge problems which have been discussed here. The problem sometimes has been with the sender in the tank and sometimes with the wiring in the dash cluster.

    I believe the 700/900 FAQs give a procedure for determining if the problem is in the sender or the dash. Once you have done that, you can proceed. If you haven't looked at the FAQs, go to 'select link' in the upper right corner of this page and select 700/900 FAQ from the pull down menu. A really great resource!

    In my '93 945, the problem was in the dash cluster. When the gauge was reading low, I'd thump the dash above the cluster and often the gauge would start reading correctly. I pulled the cluster out and traced the fuel gauge circuit on the flexible printed circuit. I resoldered all the connections as I came to them . I finally found a solder connection which was cracked but you could only see the crack with a 10X magnifier. Once I resoldered that connection, the gauge has worked fine for over a year.

    I think I remember other's have has a dirty or defective sender in the tank, a more difficult problem to work on but there were a number of good posts, some with pictures, on how to deal with this.








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      Erratic Fuel gauge 900

      Hi Jim,
      I am writing to thank you. Did as advised by simply banging on the dash and later it works giving the right fuel level. I am no mechanics so I will be sending it to the mechanics telling exactly what happens. At least it helps me knowing what is the cause.








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        Erratic Fuel gauge 900

        I hope your mechanic will be reasonable. Tracing down a break in that printed circuit and then repairing it doesn't cost anything for parts but takes time ($$$$). I've had to repair mine twice, once for the fuel gauge and once for the ABS warning light. In my experience mechanics are not as likely to dive in and do electrical troubleshooting as I am. I used to be an electronic technician.

        Perhaps the least expensive repair a mechanic could do would be to replace the instrument cluster with a used one. This would not take much time.








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          Erratic Fuel gauge 900

          Dear Jim,

          Since I last posted the fuel gauge is still functioning OK. So, I tried my luck to delay fixing until the problem appears again.








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            Erratic Fuel gauge 900

            Dear Larry,

            You can probably go a long time before you 'have' to fix the gauge. The gauge in my 940 just slowly got more unreliable over a period of a year or two. At first I would whack the top of the dash and the gauge would work for several weeks. Gradually the time shortened and eventually the 'whack repair' failed. Then I operated by the mileage in the trip odometer and eventually got inspired to fix the darn thing.

            You can operate a car without that gauge if you have to. I remember the first VW beetles had no gas gauge.








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          Erratic Fuel gauge 900

          Dear Jim,

          Good a.m. and may this find you well. When you wrote you reply, you must have been reading my mind.

          On my '93 940, the ABS and Coolant Level Warning lights suddenly stopped coming on at start-up. The ABS works; there is adequate coolant. The bulbs are fine (I put them in other locations, and they came on).

          To take care of any micro-cracks, I re-flowed the solder on almost all of the flexible circuit card connections. This did not help.

          For what - and where - should I look, to find a break in the circuit card itself? Is the fold at the bottom a "stress point"?

          I look forward to having the benefit of your guidance.

          Thanks for your help.

          Yours faithfully,

          spook








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            Erratic Fuel gauge 900

            Good morning Spook,

            In trying to find the fault in the ABS warning light, I, too, flowed all the solder joints to no avail. I then connected my ohmmeter to the input connection for the ABS light and traced point by point through the printed circuit to the light until I found I had crossed the break in the circuit. I had to scrape off some of the protective 'varnish' in places in order to test the circuit.

            I found a break in the circuit at the bottom of the cluster where the flex PC folds under, as I recall. The break was under one of those plastic 'fingers' which hold the flex PC down. I scraped off the PC trace on each side of that finger and soldered a jumper wire across the two sides of the finger. That took care of the problem.

            Hope this helps.








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      Erratic Fuel gauge 900

      Mine was a crack in the ribbon board on the back of the IP cluster. I just used some metallic paint to repair the area.







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