Fuel gauge issues

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I had been fixing up a no start issue on my 1980 Spider 2000 and after getting frustrated and out of ideas I let it sit for a while. Now the car starts but the fuel gauge isn't working. The gauge was always a bit finicky, needing me to ground out the wires at the sending unit every once in a while to get it to work. For context sometimes the gauge wouldn't read anything until I temporarily grounded each wire, then it would function as normal for a while until ceasing to work after the ignition was cut. Now trying to ground the wires doesn't work, even for the low fuel light. I have a running theory that the needle is stuck in some way, and giving it that temp grounding got it unstuck and moving again, but that doesn't explain why the low fuel light doesn't come on. I think the pink wire is for the needle from the sending unit, and I have +7 volts on it with ignition on, nothing with it off.

If anyone could tell me how the sending unit works and how the gauge interprets it that'd be great as well!

[spider2081]

Yes it sounds like either the gauge in the dash is defective or the cars wiring. I think the fuel sender fail more often than t he gauges do however. So you may have multiple issues.
I believe 7 volts measured on the gray/red wire at the sender is too low. Fuse 1 ( closest to the steering column) powers the gauge and both the top and bottom terminals of the fuse should measure battery voltage with the ignition key in the "run" position. with the gray/red wire disconnected at the sender measuring voltage at the end of the wire should also be battery voltage. At 7 volts the light could be lit too dim to see.
The sender has 2 sections. One is a variable resistance with exposed wire and a wiper that moves on that wire relative to the fuel level in the tank. when the float reaches a point below 1/4 tank it makes contact with the light connection grounding it gray/red wire to light the low level light. So it is safe to ground the gray/red wire and look for the light to be lit.
The blue/yellow wire is the wire that moves the needle. One should only strike that wire to ground momentarily. Connecting it to ground for too long a time could damage the gauge.
Things I would check:
Voltage on the top and bottom of fuse 1
The instrument cluster ground at the pod above the ignition switch behind the dash
the fuel gauge connection (connector) unplug and plug in a few times to clean the contacts.
Remove fuel sender and visually inspect. It can be functionally tested with an ohm meter.
use a jumper wire to ground the sender metat ring to the battery ground.

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Update: So the test lead I was using to try and ground out the gauge ended up being extremely high resistance for some reason when measured with my multimeter, so I confirmed with another test lead that the gauge does work. The sending unit bolts are also grounded (confirmed with a continuity meter) but there is an open circuit between ground and the gauge terminal (not the indicator light terminal). Obviously now I need a new sending unit but I'd like to know if when these units stop working, does the low fuel light continue to work? I think I could end up working with the light alone pretty well.

[spider2081]

I woould make an index mark on the sender housing to the fuel tank so it can be replaced in its original position. Then I would remove the sender and inspect it. The sender can be operationally tested out of the tank . Connect a jumper to the sender's metal plate the mounting studs pass through. connect the other end of the jumper to a known good ground. Turn on the key and holding the sender by the cover plate move the float arm up and down. full down the low fuel light should be lit. Moving it slowly from full down to full up should cause the gauge needle to move toward full.
This procedure tests both the gauge and the sender. It tests the gauge needle and the gauge low fuel light. It tests the senders low fuel light switch as well as the variable resistance of the sender.

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Thanks for the suggestion I'll be sure to try it out as soon as I can, side note do you have a quick way to drain the fuel tank? Is there somewhere in the engine bay I can disconnect a fuel line and have it drain into an external tank? I know there's the cold start valve but the hose there is a little fragile and I'd prefer not taking it off, could I actuate the cold start valve manually and just keep it open to drain?

[spider2081]

Harbor Freight sells a fuel transfer pump that uses 2 "D" cells I remove the sender and put the pump in there . I bought mine for $10 but the last price I saw was closer to $14. I think they are well worth the money to have around.

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My issue is that the sender is a little bit below the top of the tank, and when I try to take it out the fuel is higher than the sender hole so it starts spilling out.

[spider2081]

If fuel is coming out when you try and remove the sender I think the tank is full and possible over filled with fuel in the filler neck. Can you drive the car about 100 miles That should burn off close to 4 gallons of gas and then you can remove the sender with out the gas comming out the sender opening.