Recently bought a 67 el camino, first time driving at night and when you turn on the lights the gas gauge goes to empty, turn the lights off and it comes back up, thought I was seeing things, any ideas ?

sounds like you may just have a bad ground to the back of the instrument housing ? But does 67 have a printed circuit on the back of the dash or all seperate plugs for lighting etc.?
dave

Sounds like an easy way to fill up.:D

Seperate plugs for all the lights, I am planning to pull out instrument cluster so possibly i can get to groun strap, any idea which side or where it might be?

You could just have a shorted gauge wire.
Lets see uh your gauge wire should be a ground connection and the gauge in the tank will change the amount of ohms to ground according to how much gas is in the tank.
You could be sending voltage into the tank lets hope not.
I have seen a few gas lines shorted under the seats and carpet.
One thought is the body grounded good? the tank ground may be the only one between the two.

Seperate plugs for all the lights, I am planning to pull out instrument cluster so possibly i can get to groun strap, any idea which side or where it might be?
just try any peice of wire from the column to the back of the dash first and see if it's just that, before you yank the cluster out, it dosent matter where.
If that's not it ? then turn the key to on, unplug the sender wire at the tail light loom (tan) or (light brown) the gage should go to full, then ground that wire and the gage should go to empty, that means gage and wiring is good, if not check ground wire attached to the sender on the tank, means pulling the tank down, but I reckon it's just a bad ground at the dash hopefully ?
dave

Thanks for all the help and suggestions,getting late tonight will try tomorow

Tried the wire from back of gsuges to steering column and gas gauge came back up so I guess it is bad ground, what would be the best way to ground permantly?

use a ground strap, bolt it to one of speedo housing bolts, or there may even be a terminal on the back for that very purpose ? it's been a while, ground the other end to any good ground source you can't see easily, like one of the under dash support brackets.
dave

Thanks for all the help, still got Bandaids and beer!

Thanks for all the help, still got Bandaids and beer!
Your welcome and you'll always need them !
dave