The Fuel Gauge Needle in my '75 1.8 is jumping around when I rev the engine and turn on the wipers and lights, but is stable when I turn on the heater fan, radio, and emergency flashers, and push on the brake.
In 2018, the tank was removed and cleaned, the sender, screen, gasket and fuel lines were all replaced. The last year or 2, this problem began, and has become worse now. I keep the fuel nearly full with non-eth, and this past year the gauge is reading 1/4 +/- and then falls to empty. Sometimes it begins at near full and then drops after minute or two. Routinely I turn the ignition on to heat the coil before starting, and the gauge will drop erratically.
I presume to begin by testing the wire connections before pulling the relatively new sender unit.
Any other suggestions?
Thanks World.
See a brief video here: https://youtu.be/NDds6T5DvEo

Chasing dirty connections is so much fun. (That's what it is.)

or just the grounds. You can try using a direct wire for a ground from your battery negative or good body ground to your gauge ground. Check if it will fix it.

That sounds like a ground issue.
Check and clean all the main ones, especially the one in the drivers footwell, inner fender, and the one in the engine bay under the relay board.
Every 40 years these things need to be cleaned up.

OK, seems you all point to check the ground connections to begin.
I didn't expect there are multiple grounds (i.e., the one in the drivers footwell).
Damn,.. yes, I hate chasing electrical problems
Thanks you guys...
I'll report on my findings.

Good idea....

Better idea would be use a DMM, and a schematic and avoid guess work.

I can’t seem to understand why those two thing are overlooked as the first rational step.

Faulty fuel level sender? My car goes from 1/4 tank to empty and dead on the side of the road in an instant. I fill it up, drive 250 miles (max), and find a gas pump.

I found the the connector on the gas tank didn’t always make solid contact; while sitting in the driver seat with ignition on, have your helper grasp that connector and see whether the needle moves,
Easy test.

Reading schematics are like looking at a birds nest. They don't help much showing a wire from an instrument that ends with a number instead of a description of where the wire goes. They frustrate me. I can wire a house and troubleshoot my '59 Ford 100, but my 914 is a serious challenge for me.
I use a DMM in most cases, but also I'm old school, preferring to use a long speaker wire with alligator clips on the ends, and simply bypass the circuit.
I found photos of where to find the grounds. Intend to fill the tank first so I know what it should read, and almost sure it is near full right now.
Thanks everyone...

I would not fill up the tank with fuel. Unless you have 3 or 4 5gallon gas cans in case you have to drain it.

That means the wire is continued on the other page. Look for that same number on the other page of the diagram; double-check that the wire color is the same.

--DD