Gas gauge help

[use2teech]

Hoping someone can help me ID the root cause of my non working fuel gauge, 65 coronet 500. I think I have it narrowed down to the cluster or the plug at the back but have a couple questions. Since it doesn't appear to work off straight twelve volt, can anyone tell me what voltage should be present at the pin for the sending unit wire. I'm talking about the pin that is part of the round harness plug that goes directly into the back of the cluster. I just want to verify what it should read before I assume the plug/pins are my problem. The gauge itself seems fine as it does move once voltage is applied at the junction down in the footwell but doesn't move at all once everything is plugged back in and powered up.

 

[wannadrag]

Pull the wire from the sender on the tank,run a long jumper to it so you can watch the gauge.With the key on touch jumper to ground,if it goes to full the guage is good and the sender is bad.

 

[Beekeeper]

The reference voltage for the sending unit is 5 volts. You can check the gauge as stated above but 12 volts applied too long will smoke the gauge.

 

[nm9stheham]

5 volts as noted above. The same 5 volts runs the temp and oil pressure guages. So if those read good, then it is probably the sender or the wiring.

 

[ram250098]

Check the guage as above works..but it is not always true. My 73 Runner had this problem and was driving me nuts..checked eveything...twice..even the gauge....I swapped out the gauge and everything worked. I went as far as to try the old gauge again and nothing.
AL

 

[eric383]

This is for 1966 but I wouldn't think it changed much in 1 year.
http://www.mymopar.com/downloads/mtsc/227.pdf

 

[nm9stheham]

Check the guage as above works..but it is not always true. My 73 Runner had this problem and was driving me nuts..checked eveything...twice..even the gauge....I swapped out the gauge and everything worked. I went as far as to try the old gauge again and nothing.
AL

You are 100% correct; the temp guage working Ok only verifies that the 5v regulator is OK. For the fuel it could indeed be the guage, wiring or sender or ground from sender.

To answer the OP's question. looking at the fuel guage sedning wire out of the cluster connector with a voltmeter and the key in RUN, you should read approx 5V AVERAGE. The voltage reading will cycle up and down between approx 12v and 0V constantly, but spend a little more time at 0V than at 12V. This is the limiter (regulator) action. If you get no voltage, but the temp guage works OK, then the fuel guage or wiring in the dash cluster to the fuel guage is open. If the tmep (and oil pressure guage if so equipped) don't work, then it is the 5V limiter (regulator) or the wiring to/from it.

If OK but no reading on the guage, then try the ground from that pin as listed in post 2. If that does not work, then take out the guage, and see if the tiny movement bearings are corroded or the grease is dry or something like that which is freezing the movement. That is not uncommon for cars that have sat a bunch.

And one other possible problem is that the sender unit's ground through the gas tank is faulty. You should be able to put an ohmmeter on the lead into the sender and the meter - to the car chassis and read some resistance. I can't recall the ohms range but certianly under 1000 ohms with the fuel at any level. See if there is a ground strap from the sneder unit to the car's chassis; the sender's mount ring has to be grounded somehow for the gas guage to work.

 

[use2teech]

Thanks All ! Still havent resolved the problem but following the suggestions. Just got all the stuff from Radio Shak to eliminate the regulator and resistor and hoeing that's the answer.