Fuel gage always says full so I’m guessing fuel send unit is broken?

 

Maybe, but there's a lot of work involved to get to it just on a guess.

A forum search pulled up a dozen threads similar to yours; here's one such thread: https://www.thirdgen.org/forums/elec...tuck-full.html

 

High reading on gauge = high resistance looking toward the fuel sending unit

Check for a broken / disconnected / damaged wire

See my signature

 

Honestly, both the sending units and gauges for our cars were horrible...
They both crap-out on the regular.

 

I'm having a similar issue. It's strange. The tank was pretty much empty (had less than 1 gallon in it) for 20 years. The gauge read just slightly above the Empty hash mark. I pumped out that remaining gas and poured 3 gallons of fresh gas in. Turned the key and the needle went to slightly over 1/4, which is a bit inaccurate since 3 gallons is slightly LESS than a quarter tank. Here's the mystery: yesterday, I turned the key and the gauge went past Full. I bounced the back of the car so the gas in the tank would slosh around, turned the key again and the needle went back to slightly over 1/4. Added one gallon today and the SAME exact thing happened. Bounced the back end a few times and the needle went back to almost 1/2. What's up with THAT?? And 4 gallons is NOT a half tank. 8 gallons would be half...

 

Which size fuel tank do you have? 12 Gal, 15 Gal, or 18 Gal?

That could be the gauge or the sending unit...
However, I believe that coincidences rarely happen... it is more than likely the sending unit.

The electrical contacts of the sending unit often corrode, erode, break, or fall off, when the tank sits idle for a long time.

The electrical contacts are very small copper or tinned pads, that look similar to those in the photo below:




Look at the very small copper colored pads that are in the shape of part of a dimmer switch... they are very delicate.

 

Should be a 16 gallon tank. The problem seems to be intermittent, hence the confusion.

By the way, is the needle supposed to go back to Empty when the key is shut off? This one stays right where it read while key was on...

 

Not really.

As I've explained elsewhere, a gas gauge has 2 coils; one of which is always pulling the pointer toward Full whenever ignition is on, and one pulling toward Empty whose strength is determined by the sending unit. When power is turned off there's nothing pulling it anywhere, so it drifts to wherever it feels like.

The tach is somewhat similar; it has 2 coils that are set at 90° angles, such that one pulls the pointer toward 0 or directly opposite 0 on the scale (whatever value that happens to be) depending on whether it's energized in the positive or the negative direction; and one that pulls it 90° from 0 or directly opposite that, depending on the polarity of its applied voltage. Thus the tach can point to any point anywhere on the scale by the proper application of voltages. Similar to the gas gauge, all the pulling forces go away when power is removed, so the pointer just goes to wherever the spirit moves it to.

Very different from "modern" ones.