Ok now im lost, I was working on my 89 firebird formula when my fuel gauge pegged out full, so i did some testing and grounded out the sender while plugged in and also unplugged and it dropped the fuel level to empty. So im pertty sure the sending unit is bad so replaced the sending and put a new fuel pump in since was out of the tank, but before i placed the new sending unit in i wanted to make sure the the gauge would work. So i hooked it back up and moved the float and worked great. So i put it all back together and and took down to fill it up but will only read 3/4 full with a full tank, And i made sure the tank was full. So now im lost, WHAT COULD BE WRONG????

My son just replaced his chevy pick ups sending unit two weeks ago and had the same problem. Pulled it back out and the float had moved on the unit. I guess on install, he put it back in place and it works fine now.

When the fuel gauge connection is broken,... it will "peg" past full. ( good car, unplug connector, gauge pegs ) This broken connection can be at the ground source, anywhere along the wiring, at any plug/connection, on the cluster printed circuit, or on the sending unit itself.

Where & how you the circuit you tested will tell you what direction the problems is - forward or back. ( if you tested at the cluster - anything behind you could still be the problem. )

Found the problem the sending unit i replaced was the dorman brand and didnt think much of it before hand but after pulling it back out and compare to the stock i saw the problem. The float arm was much long there for was hit the top of the tank and would not let it top out. sent it back waiting on new one..

lol