thank you for the info 3b! i reread your instructions just now and i guess i need to drop the tank down to take brown wire loose going to sending unit. i installed the aftermarket under the bed tank.
i did check the voltage comming out of the gage and it was around 5 volts with brown wire unplugged, around 1/2 that 2.5 volts with it plugged in. then i checked the voltage wher ethe brown wire goes from fuse box back to sending unit and it was the same 2.5 volts.
the odd thing is when i check it with a probe the needle would jump down to 1/2 tank or jump all over the place back and fowarth. then with the voltmeter it would go down a little it but not all the way to 1/2 tank. the wire going back to the sending unit is a couple of years old i replaced it when i put on tank. i replaced the tank on my 64 and had no trouble with the gage.
do you think i should go ahead and drop the tank and check there? thanks for any info.