Common issue with first gens. Easy troubleshooting to isolate if it is the gauge or the sender or the harness.
Unplug the sender lead in the trunk - in a 69 it is the brown single wire connection.
Turn key to on position.
Gauge should read past full.
Now ground the end of the harness leading back to the gauge - NOT THE LOOSE LEAD GOING UNDER THE FLOOR TO THE SENDER.
Turn key to on position.
Gauge should read past empty.
If it does both of these the gauge is in working condition and harness back to instrument panel is in working order. If not you can test the ohms being sent to the gauge as described below for the sender test, or have a buddy loan you his gauge for a second and plug theirs in and test as above.
If both gauges fail - and your description of your situation sounds as if the gauge is fine to me - the harness may have cracked or broken wire. You can also alligator clip the brown lead of the back of the gauge to a ground to do the above trunk testing to isolate wiring.
But if the gauge moves to past full then past empty with the trunk connector test, then problem is most likely the sender unit in the tank, or its ground.
The sender uses a ground fixed to the back passengers side tank strap and that is where I would start if you can get under the rear you should be able to visually inspect if the wire lead is corroded or loose. If it looks ok and is clean of corrosion it still may not be ok, and you can hook a test lead alligator clip to the connector at the tank strap and then the other end to a new clean ground you have made on the chassis and see if your gauge now reads correct with the key in the run position.
If you have a good ground at the tank or with your new temp ground, you reconnected the lead in the trunk and you still have a past full gauge reading, then maybe the sender unit itself has lost the will to live.
To test the sender you need to test the ohms (resistance) with a simple multimeter.
You measure across the loose lead in the trunk to the connector to the gauge. You
should read anything from 0 to 90 (empty to full). If you are getting something close to what you think is in the tank, the sender maybe ok. You can manually manipulate the float valve with a bamboo rod or wooden dowel through the filler neck. DO NOT USE A METAL ROD. Last thing you want to do is set your tank of gas and your ride, garage and house on fire with a spark. You can also affect change by removing or adding fuel to the tank as you check the ohms.
If you get a change in resistance the sender is most likely ok and the connector in the trunk may have been loose, or the float may have been stuck. if it is flaky better to replace then rebuild in my humble opinion. Dropping the tank is not an easy project without help or a lift and storing gas may not be easy where you live (condo owners take notice your neighbors may rat you out). And if you rebuild, how long would it last before it found its own groove again and left you high and dry.
Hope this helps you find your issue.
New member here. Great informative site by the way!
I too have the 3:00 fuel gauge thing going on and here's what I found so far:
At the fuel gauge, I have 12 volts on one side and meter shows 8 volts on the other side. I ground the 8 volt side and the gauge goes to empty. Unground and it goes back to 3:00.
In the trunk, at the sender connector, wire shows 8 volts, showing me the same as the back of the gauge so the sender wire is intact from the back to the gauge. If I ohm the sender wire to ground, it shows 44 ohms.
I ohm'ed the fuel tank to a body ground and it shows continuity. I can't get to the sender without dropping the tank, which at 44 ohms, is probably half full or better. I am stumped at this point, unless the sender itself has to have a separate ground. I don't see a grounding strap going to the tank.
Any ideas from you gurus?