70 chevelle gas gauge problem

[ls6crate]

70 chevelle ss....i have just finnished converting my idiot light dash to gauges. i modified the harness to the gauge harness specs, but now i think i have a problem. everything seems to work fine but when the key is off the gas gauge goes to way overfull. when the key is on it reads to the full mark, which i think may be OK because i do have a full tank of gas. shouldn't it be reading below empty when the key is off? thanks in advance. http://www.yenko.net/ubbthreads/images/graemlins/confused.gif

[Mr70]

Sounds like you are sharing a common ground.That gauge should be an isolated ground.See if it is,and if indeed it is grounded properly.

[ls6crate]

thanks for the qwick reply.
i don't understand, i thought ground is ground no matter where on the body it's located. i have three black wires coming out of the harness and i thought each one went to each metal caseing on the gauge cluster. the wiring diagram doesn't seem to be much help with the gauges. by the way, realy nice car you have there mr70.