I'd say you are missing a ground or a wire is loose.

 

Yes. Full means the sender wire is grounded, no you probably have it hooked up wrong. The sender normally has three wires. Level, low fuel warning and ground. With a 1991, you just hook up ground and level. The low fuel light is controlled by an electronic module.

 

Ok thanks guys- I apologize in advance as electronics are not my strong suit- so you think maybe I have the wires hooked up in the wrong slots?

 

Yes.

 

Just replaced the sending unit on my 91 110 200Tdi. Had 3 wires on the sender as shown. Hooked up the new one the same way and now it works properly. Replaced the sender because when below about a third to quarter tank the gage generally dropped to zero and the low fuel warning light came on. Now it reads properly to the bottom, with the warning light coming on when fuel level is in the red.

I know all this because I ran the tank to empty because I thought I’d have to drop it to get the sender out, although turns out I didn’t. When refilling put in only a couple of gallons to ensure the gage read at the low end, which it did. Then watched as I added fuel to see where the low warning light went out, etc.

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