new questions on fuel sending unit

[hikersteve]

so i replaced the fuel sending unit on my 84, the one i took out was very gunked up and in bad shape, the one i replaced it with was clean and seemed to be in good shape, i put it in with an almost full tank so it was hard to tell if it was working.
After about 180 miles and the gauge still being over half i figured it was a no go. However at about 250 (trip) the gas light came on (the prior gas light didnt even work).
When i was installing the newer one i turned car on with it unplugged to verify that it wasn't sending signal and the gauge read E, so i knew gauge wasnt just stuck.
After installing new one and plugging in, when i turned on car it registered half tank, and after it was on it gradually made its way up to full, so i thought we were good to go. But then it stuck again at 3/4 and been there since.

My question is : Is it possible for gauge to go bad in cluster, and is there a way to test that?
I would figure that it just works off of resistance, and maybe could just use ohm meter on two of the wire leads at fuel sending unit.

i guess its possible that both sending units are bad, but something tells me it is the gauge itself, i know i can pull dash cluster but would rather wait until i have a gauge to replace it. so i would like to test via plug in the back.

[Petros]

the sender is just an old fashioned variable resistor (a contact on the arm that slides over a coiled wire with the movement of the float). the fuel gauge test is simple, you unplug it and ground it with the key in the "run" position. with it fully grounded it should go to full, when you unground it, it goes to Empty. if that works than pull out the sender and clean the coil wire (you have to pull of the metal cover), and verify the contact is touching the coil over the full range of swing. If not it is easy to carefully bend the arm on the contract so it touches the full range of the swing of the float arm.

There is a detailed procedure for cleaning the sender contact over in the repair guilds sections. Usually an easy fix.

[hikersteve]

okay thanks,
i was starting on cleaning the old one and the two small screws that hold the cover plate on are pretty seized up, spray with nutbuster and soaked in cleaner and still tough to get out. i remember in last fuel post you had said something about tapping out rusted screws with drill bit, thought you were talking about access panel. Is it feasible to tap these 2 small screws out without damaging sending unit.?
This is probably covered in repair guides i will check it out
thanks again steve