When i turn on headlights gas gauge drops

[goatman]

Hi, i have a 86 c20 pickup and when the headlights are off the gas gauge works fine but as soon as i turn them on, the gauge drops suddenly and stays there till i turn them off. Any help would be appreciated

[1500cheyenne96]

Sounds like you got a bad ground that is shorting out when you turn your lights on try checking the headlights ground and the gas gage ground also there is a main ground on the firewall i think that is held on by a nut and sometimes that nut comes loose

[leolkfrm]

check the harness for a rub thru point along the frame

[crabtruck]

I would say you have a GOOD ground that is shortig the wire.
I'm with leo on this one for concept, might be either side of firewall too.

Before you get high tech with meters or get out the flashlight and magnifying glass,
have someone watch the gauge with the lights on and problem showing.
Jostle by hand the wires for the run from tank to gauge.
That might narrow down the area for you and is real fast.

If the gauge is not just dropping, but going all the way to empty,
You are looking for a blatant short, not where wires are somewhat slack.

[BernieB]

As an additional possibility, what's the condition of the battery and charging circuit? Are you getting a significant voltage drop in the system when the headlamps are on? Could be a weak battery or weak alternator.

[goatman]

Thanks for the information gents, now i have a good idea on where to start.

[crabtruck]

Upon further reflection, I'm going to change my answer.
Check your system voltage as bernie suggested.
Check that all lights are working in all combinations.
If the above is ok, then something either in the cluster or
the mechanical operation of the switch is the problem.

Gauge would drop for one of two reasons: power loss to sender,
or grounding out of return from sender.
Ground wires are not traveling with the gauge wire for most distance
between the rear and dash/firewall.
Operation of switch will not change physical condition of wires to rear.
A short from a headlight switched hot wire will not provide a ground path.
This is true both under truck and in cluster.

Therefore I believe you are mechanically moving wires when using switch.
Look for the problem behind dash, quite likely find by "jostling" there.

[goatman]

You're probably right. I checked all the wires and grounds in the engine compartment and everything there seems ok. I also had my son watch the gauge while i was under the truck moving all the wires from the sending unit and that didn't correct it either. I did notice that one of the previous owners changed the steering column in the truck and was thinking maybe they might of screwed something up inside the column somewhere or like you said maybe the light switch. I was about one step away from yanking every wire outta there and starting from scratch with a new wiring harness........then i priced one. Yikes!! i think i'll try to fix this one.

[Faxcion]

Hahaha! Good ol' GM keeping the American people scratching their head!