1970 Plymouth Fury III

Fuel gauge sits at empty at all times which leads me to 2 questions, can i test the fuel gauge itself without tearing apart the dash and how would i accomplish testing it? and secondly do i need to pull the gas tank to test/replace a sending unit?

also radio, interior dash lights, and cig lighter do not work if these have a common issue or are somehow interlinked. Also I have ohm tested every fuse and all of them are fine.

Thanks for any help

 

For testing the gauge, start HERE

 

If the temp gauge works, then the fuel gauge is getting power. Both get power from the instrument panel voltage limiter.

The quick test is go around to the tank, disconnect the sender wire, and MAKE SURE it is grounded with a clip lead. Turn on the key, the gauge should peg to full.

If this happens the gauge at least works, and either you have a bad sender or the sender is not grounded.

for years, Chrysler used a strap which bonded the sender unit to the metal fuel tube going up to front. This tube may or may not be grounded, as it's only clipped against the body underfloor. Add a known good ground to the sender, to the body.

 

the temp gauge does work, i know where the sending unit wire hooks up to the tank where do i ground it to? the frame? how does the sender ground to determine if its the sender not grounded or a bad sender, if that's the case?

 

OK, it's a simple series circuit. The battery comes through the ignition switch, is dropped down by the "voltage limiter" to nominal 5V which supplies one end of the temp gauge, the oil gauge if you have one, and the fuel gauge.

The other terminal of the gauge runs back to the fuel tank, where the sender is a variable resistor. Less resistance = more full= more current= gauge goes towards full

So if you short the sender wire to ground, the gauge will go to full scale and beyond. DO NOT LEAVE THIS this way for long. You should be able to short the wire to ground, go up and turn on the key, and in about 1 second or 2, the gauge should REALLY head for full.

If that part works OK, then you probably have a sender problem, or the sender/ tank is not grounded. Remember, the tank is held up against the rusty/ undercoated bottom of the car with rusty/ undercoated straps, so it may NOT be grounded. This is why you want to run a separate ground wire to the body.

One way to do this is use one of the smallest size fuel clamps, clamp a wire right to the sender fuel line, and hook the wire to the trunk bottom with a small screw.

 

Ok thank you for the detailed explanation, ill try it this weekend and get this all sorted out.

 

Has anyone ever experienced a fuel gauge working opposite what it normally does?
My gauge reads empty when it is empty however when I put gas in the tank the needle moves to the left past empty and and is buried, but as the gas depletes, the needle moves back toward the empty mark. Any ideas? Thanks for the help.

 

B -

Sounds like somebody mixed in a GM sensor/sender.

Archer

 

Do you mean the Sender with the float which is in the tank? Hey anything is possible but is that even possible??? How can I check?

My grounding strap is in good condition but not trusting it I put an additional jumper to ground and nothing changed. I will however be trying the advise above by grounding the Sender wire to see if the gauge goes to Full.
Now that I think about it, I have a new Sender which I bought a while back when I was last motivated to figure it out. Any thoughts on just hanging the new sender on the sender wire with proper grounds to see what it does without installing it in the tank?

Thanks again

 

neat thread, gonna follow. 440, you know your stuff!

 

B -

I would just replace the sender with a known Chrysler/Mopar unit and see what happens.
You never replaced the fuel gauge, right?

Archer

 

Gauge seems original but hasn't worked since I got the car 15 years ago. All other gauges work.

All the input is very much appreciated, thank you