Fuel gauge doesn't move

[redlion]

What a great forum !!
Here is my problem, 99 suburb.
Fuel gauge will read normally as long as I do not turn running lights on.
On trips it will descend to 3/4 ( or less if turning lights on at, say 1/2) and stay if lights are turned on. Knowing we have used alot of fuel when stopping to fuel if you shut the engine off and wait 2-5 min. and restart, the gauge will come down to read accurate. As said, the gauge is only effected while the head and taillights are on. With daytime running lights on only, it seems to read normal.
The good news is we haven't run out yet, but have come very close having approx. 3 gal left while gauge was showing near full( 620 miles later) !!
Thank you all in advance for your input.

[obadran]

The only thing I can think of is some type of short in the instrument cluster itself or the wiring harness that plugs into the instrument cluster. With the head and taillights on, the backlighting for the instrument cluster is on as is all backlighting for everything else on the dash. It seems that some how the wire that feeds power for the backlighting could be affecting the wire that provides the signal to the fuel gauge.

I hope this helps.