[Transamric]

You can tell it's Summer time cruising weather in the great Northwest because the fuel gauge in my '72 Esprit has started acting up again. It reads just over 1/2 full at all times until the tank is nearly empty at which time its reading drops to only about 1/4 and runs out of fuel leaving me stranded.
I've run the tests in the '72 Service Manual with everything testing Ok. Recleaned that pesky sending unit ground at the right rear corner of the trunk pan,...no improvement.
What I don't see in the Service Manual is what the feed wire (tan) voltage should be. Last year when it was acting up, someone here said it should be 9 volts. Not sure why that would be true when the voltage at the fuse box for the tank circuit shows me battery voltage, about 12.6.
What do you guys believe the feed wire voltage should be? Again, this is the tan 18 gauge wire that runs down the left side rocker channel.

[John V.]

Ric, no idea if this could be true of a '72 sending unit, but had a '97 Safari that began to do a similar thing, would read full all the time and maybe drop to 3/4 or 7/8 full when empty. When the issue first developed, it seemed to act up intermittently, some days seemed to read the tank level accurately. As it got progressively worse, there were times when it would seem to give a broader range over a tankful and I would hope that it was coming back to life. But eventually it was only accurate when the tank was full. The gauge needle would move, but the sender just had stopped sending a proper signal.

Drove it that way using the trip odometer as the fuel gauge.

In that van, the sender was sold as an assembly with the in tank fuel pump. Was content to live without a fuel gauge rather than pay the price for a new pump. When the fuel pump eventually died, replaced it and had a working fuel gauge for the first time in years (and only ran out once!).

The way I understood it, the sender operated with a rheostat and pointer. Depending on where the pointer was along the rheostat determined the resistance and therefore the voltage sensed and sent to the gauge.

I seem to recall it was the pointer (rather than the rheostat coil) that would wear so that it no longer generated the electrical signal it needed to.

Not sure I'm describing the idea accurately but the net result was that there was never anything wrong with my gauge or my ground or the voltage to the sender. Only way to fix it was to replace the pointer and coil as part of the fuel pump assembly.

If the '72 sender operates off the same principle (sans the integral fuel pump) could be the issue you are seeing.

Seems you could prove the circuit is good by plugging in to a known working sender and run the gauge from empty to full with the ignition switched to on (not running).

Hope this helps in some way.

[77 Canamman]

GM gauges do not utilize a cluster voltage regulator, as Ford and Mopar do. The positive side should read battery voltage. As is typical in most of these older GM cars, the sending unit is faulty if the ground is clean, and the wiring/connections solid. The resistance range is only 0-90 ohms.

[Formulabruce]

Touching the tan wire at the trunk to ground for a Moment only, while you ( with a ground wire extension ) or a helper looks at the gauge with key "ON" may get you get a full deflection, Then the problem is in the tank, or the short harness to the nipple on the tank. I have squeezed that round part the goes on the nipples for better contact on many. Good luck !

[Transamric]

Originally Posted by Formulabruce

Touching the tan wire at the trunk to ground for a Moment only, while you ( with a ground wire extension ) or a helper looks at the gauge with key "ON" may get you get a full deflection, Then the problem is in the tank, or the short harness to the nipple on the tank. I have squeezed that round part the goes on the nipples for better contact on many. Good luck !

Yep, did do that, and the dash gauge went way full right, well past the full mark.

And someone suggested plugging in a different tank unit to the wiring in the trunk. I've done that too, with two used units and a NOS unit fresh right out of its box. They all pretty much acted just like the unit installed in the car. IE., just goes a bit to the right of the 1/2 full mark.

All testings have been done with key on, engine off and a small battery charger on the battery to keep the voltage up & constant.

[Held for Ransom]

Do you have a different gauge unit to install? It may be a bad shunt in the back of the guage.