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Thread: Fuel gauge... Stuck on full?

  1. #1
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    Fuel gauge... Stuck on full?

    Yay for the E30 fuel gauge! I've been searching around a bit, but can't seem to find a straight answer. Just need advice on where to look!

    When my car is off, the fuel gauge returns to empty like it should. When I turn the car on, it springs to full...and stays there until the car is off. But on the flip side, the "oh snap, you're out of fuel!" light comes on when I'm down to about 2.5 gallons or so. So it is reading my fuel level, it's just not displaying it. My mileage is fine (run about 350 on a tank, 25 mpg average), and all of my other gauges run fine (got the wrong coding plug for the tach, but it still works).

    So what do you guys think? I've heard that there's apparently sensors under the back seat that may contribute to this (considering the E30 cluster is electronic), but not sure.

  2. #2
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    richardodn is offline Old Guy BMW CCA Member
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    The low fuel switch is independent of the fuel gauge. That explains that part. For the gauge to read full and assuming the gauge is not faulty, the resistance over the two senders (left and right) must be near zero. It's unlikely that both are stuck in the full position. You can test that by removing the access plate over the left sender and measuring the resistance between pin 1 and ground. If it is near zero than the fault is in the senders. Otherwise I would suspect the problem lies in the gauge and consider this more likely. It may simply be a loose nut on the back of the cluster or it might be the gauge itself.
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  3. #3
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    Reviving this old thread, I have a similar problem, but it stays at 3/4 full. When the senders are in the tank it reads 3/4 full, but when i pull the sensors out and flip them upside down and right side up it read empty or full. Once the senders are grounded out it starts to read 3/4 full again. Anyone experience this before?

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    Re: Incorrect full tank needle deflection.
    If you pull the lift pump/sender assembly (pull sender first), you may find that the ground tab is fractured and giving sporadic contact.
    Otherwise the sender float is binding due to corrosion.

    m

    m

  5. #5
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    do you mean the ground tab on the sender on the passenger side?

    - - - Updated - - -

    Okay I think I tracked it down. The brown green wire from pin 4 on the cluster gets 7v with key on and once connected to the senders the gauge jumps to 3/4 even with the other connectors to the other sender disconnected. Once the engine is running it reads a .4 volts and the gauge reads at empty, but once everything is connected it shoots up to 3/4 mark(this was done with the senders in the tank). And yes I did check the level of the gas its about 1/4 tank. Any help will be appreciated
    Last edited by frostedE30; 09-07-2014 at 01:34 PM.

  6. #6
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    Also tighten the 7mm nut behind the gauge. When the gauge gets a poor ground, it will show full hot or near hot. Almost every cluster that i've rebuilt has this problem. The connections for power and ground (at the gauge) are almost always oxidized leading to poor connection. Same can be said about the main cluster contact points for the fuel and temp gauges.
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  7. #7
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    Quote Originally Posted by Gregs///M View Post
    Also tighten the 7mm nut behind the gauge. When the gauge gets a poor ground, it will show full hot or near hot. Almost every cluster that i've rebuilt has this problem. The connections for power and ground (at the gauge) are almost always oxidized leading to poor connection. Same can be said about the main cluster contact points for the fuel and temp gauges.
    I've tightened the bolt behind the fuel gauge, I also went ahead and soldered the contact points on the board for the fuel and temp gauges, I still have the same problems.
    Thanks for your input any other input will be greatly appreciated.

  8. #8
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    bump

    So the problem is still on going. When the wire from pin b4 is grounded it shoots all the way to the 3/4 mark. Also is there suppose to be voltage coming from it? I get 7 volts coming from it. I've checked the ground nut on the gauge, checked if the floats in the sender are moving freely and they are, and checked the resistance, they all checked out okay.

    - - - Updated - - -

    Okay upon further diagnosing i've noticed the brown wire that's suppose to be ground is -4v and the brown green is 7volts when you jump those two together the gauge is half tank, I'm really confused with this.
    Last edited by frostedE30; 09-14-2014 at 01:09 PM.

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