Fuel Sending Unit 1963

I need some help. My 1963 fuel sending unit has not worked since I replace it 12 years ago. I hooked up the wires and tested it out of the car and
it worked. After I installed it to the tank the fuel gauge registered full all the time.
I finally got fed up with always having to visually check the gas tank every time I drove the car. I purchased a new unit and tested it out of the car and the fuel gauge works. After I install it the fuel gauge now registers empty all the time. Just for kicks I tested the old unit outside of the car and it works.
Any ideas?
Thanks in advance for your help.
Mike
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Mike wrote:

sender out of the car, though with 2 of them apparently checking OK, that's probably not the problem.
Sounds like a wiring problem, but it's strange that you had an opposite reading on the gauge between the two senders, unless you connected the non-ground wires at the sender differently each time (ie, switched the two non-ground wires on the sender posts). You could try switching them now to see if that changes the gauge reading.
Here's some suggestions for electrical checks. You'll need an inexpensive multimeter for these; turn the ignition off between test setup changes, then back on for readings.
Check the fuse panel for corrosion (wouldn't hurt to change the fuse, too... 10 amp, marked on the fuse block). The emergency brake warning light is also on this circuit, so it may be possible for that circuit to cause issues with the gas gauge circuit if it malfunctions.
The dash panel gets ground from a spade terminal on the backside of the instrument cluster, high up near the center top of the cluster panel; you might check to make sure that connector is there, with a harness connector on it, and that you measure zero-ohm connectivity (not infinity) between the cluster panel and a good frame ground (or a long wire from the battery negative post) with the multimeter on the "ohm" scale.
Check for zero-ohm connectivity resistance from a good frame ground to the black (w/white stripe) harness wire connector (removed from tank sender) to make sure that harness wire has a good ground. Reconnect that black (w/white stripe) ground wire to the sending unit and now there should be zero-ohm connectivity resistance between the metal surface of the tank sender and the frame.
With the ignition switched on, make sure you have 12v at the sending unit by using the multimeter to check for 12v at the connector at the tank sender end of the light green wire. That connector normally goes to the center post on the tank sender, although it may not matter, since the purpose of the light green and the tan (brown) wire pair is simply to pass current through the tank sender, which is essentially a variable resistor (zero to 90 ohms).
Disconnect just the tan wire at the fuel sender, then turn the ignition on.... the fuel guage should read up past the full mark. Now do the test again, but this time with that same tan wire connected to a frame ground with a test lead... the gauge should read empty. If the gauge only goes partway down with the tan wire grounded, then the fuel gauge MAY have a problem... it's still possible the wiring harness has a problem.
With the ignition off, disconnect the tan (brown) wire at the guage and at the sending unit, then use a multimeter and a long test wire to check for zero-ohm resistance through that wire. Likewise for the light green wire. Both those wires travel through 2 harness connectors on their way to the tank, so corrosion at those joints is possible, which could give a non-zero reading. Before reconnecting them, check resistance of each end of both wires to a good frame ground.... both should read infinite resistance, meaning there are no shorts to ground in those wires.
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WayneC wrote:

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Paul H. wrote:

If the bolt/screw pattern on the sensor is symmetrical, there's the possibility that it was installed 180 deg rotated. I did this once and the float stuck against the side of the tank.
Paul H.
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WayneC wrote:

Does the mounting flange have a symmetrical bolt pattern? I installed a sensor backwards (rotated 180 deg.) and the float wedged against the side of the tank.
The '63 didn't have a size restrictor in the filler neck. How about a rag or other object in the tank?
Paul H.
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