Hi @habibmohammed
Is the fuel gauge the only gauge affected in the instrument cluster or are there problems with the other gauges e.g. tacho, temp speedo or warning lights as well, when the lights are turned on?
If nothing else is affected it seems as though there may be a problem with the power supply to that particular gauge in the instrument cluster.
If other gauges and/or warning lights are also affected when the lights are turned on, I'm wondering if there is a problem with the light rheostat (dimmer) that is affecting the power supply in the cluster somehow.
Here's an image from the service manual, showing the wiring for the fuel gauge and the rheostat, that may help.
[image|3066670]
(click on image)