This is an aftermarket gauge in an aftermarket dash. The gauge is an autometer listed for the Mopar sender, 73-12 ohms. When the key is first turned on, the gauge moves to full. I know it is at least 3 gallons short of a full tank but most gauges will read full after some fuel is used.

When I flip the switch to turn on the MS ECU, and the fuel pump comes on, the gauge drops down to about 1/3 of a tank. It seems the fuel pump is maybe not entirely isolated from the sending unit or some such thing. How could the fuel pump change the resistance of the sending unit. The pump is in the tank. Any ideas?

Thanks in advance for any help.

Sam :

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How is the fuel pump grounded? Raising the voltage at the tank would cause the fuel gauge to read low.

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Joshua

I will check. Thanks.

Sam

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The pump ground wire comes up through the same grommet as the sender wire and fastens to a ground terminal strip in the trunk which is grounded directly to the in-trunk battery ground terminal. I know there is a snap on "ground strap" of sorts that is supposed to ground the tank through the sending unit output tube. That thing is long gone, and the output tube has a fair amount of rust on it.

Should I be trying to get a good ground established for the tank here? Should I ground it to the same terminal as the fuel pump? I could clamp a wire to the output tube maybe, and run that wire to the ground terminal.
For what it is worth, this sending unit never showed more than 3/4 full even when full with the old carb set up.

Sam

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I would measure the voltage between the tank and chassis ground with the fuel pump off and with the pump running. In either case there should be only millivolts between the tank and chassis if anything. If the voltage at the tank goes up with the fuel pump running then the pump and tank grounds need to be evaluated and improved.

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Joshua

I checked out the tank, and it is grounded solidly. It shows 0 ohms resistance on all ranges of the meter. Not a speck of resistance there. I guess it is grounded through the support straps.

I noticed that the gauge starts to drop before the pump actually is activated, and then continues to stay low after the pump cuts off. So, the pump is not the culprit, but the wiring for the ecu. I do not understand how these gauges work enough to apply logic to the problem.

Here is what I think I know: The sender wire provides a variable source to ground. Two things could thus effect the gas gauge: a poor ground, or ground loop of some kind, or varying voltage to the gauge itself.

I checked the voltage source for the gauges, and it stays constant when the ECU switch is turned on and off. So this kind of leads back to a ground loop of some kind. I do not understand how turning on the ECU would effect the ground path of the gauges. The switch for the ECU has no ground. The ECU and the relay that turn on the fuse bank for it are both both grounded to the front inner fender. The tank is grounded through the frame of the car. The gauge itself is grounded to a ground terminal strip under the console. Is it possible this is a source of trouble? And yet, it seems if this were a problem, it would show up before the ECU is turned on.

If you are in a mood to scratch your head over this, here is a brief description of the wiring set up on the car.

There are three sources of switched 12+. One is a group of fuses on the passenger fender that is powered up by a relay, fed by the toggle on the dash. This toggle is isolated by a plastic bezel. The second is the original key-on source from the factory fuse block. It feed a terminal strip under the console. The radio, and gauges are fed by this. The third switched 12+ is on the driver's fender. It feeds just a few things on that side of the car. It is powered from the fuse bank on the other fender.

All of this is fed by a fuse bank on the pass side for Batt, unswitched 12+. This is powered by a pair of heavy, fused wires from the starter, which is where the trunk mounted battery cable terminates.

Any clues as to why powering the fender mounted switched 12+ makes the gas gauge drop? It just occurred to me I could unplug various taps on the fuse block to see if there was a specific application affecting it.

Sam

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