Has any one ever heard of the gas gauge going to empty when the lights come on?

[collector]

My friend has a 79 Seville and the gauge only shows 1/2 full when full but then when the lights come on the gauge goes straight to empty and the little red like comes on. What is this? When I sold the car 7 years ago it was perfect. I has gone through 4 owners and now it has come back and one of my friends bought it and now I'm trying to fix this issue. None of the wires look to be spliced and none what could it be? Any ideas??

[Glen]

I would think this is a ground problem on the gauge.  I’m thinking the gauge shares a ground with one of the dash lights.  The gauge and the light are still connected but the path to ground is broken.  With the lights off the gauge is grounded through the lights.  With the lights on the wire that is supposed to connect the gauge to ground is now at 12 volts so the gauge does not work. 

[beastly beauties]

Try to run a jumper wire from the ground wire on the tank sending unit to a clean grounding point on the frame. Bad grounding any place can cause weird experiences throughout an electrical system. Also check the gauge terminals and dash panel grounds. These would be first places to start and the least costly. Next check your shop manual for testing the gauge and finally testing the sending unit which may require removal. There are also tests that can check your circuitry on the dash panel and light switch. No matter what, try grounding anywhere you can first. Bad grounding can come from anywhere. I've been through some nightmares myself with $350k motorhomes that resulted in grounding issues from the factory and never surfaced for years. Then one day things started to go haywire and these issues  "reared their ugly heads" in equipment that was back feed from another grounding point having nothing to do with that equipment 40 feet away. Sometimes the simplest things can drive you crazy.  Good Luck,   David Symonds

[Julien Abrahams]

I would also look for a broken ground somewhere. Just to be sure: check them all, and try to run a jumper wire for example straight to the battery, then you'll at least know for sure that it is a grounding problem. And x2 on what David said: its the little things that drive your Really crazy .

[cadillacmike68]

Not unusual; on the older cars, an open caused the gauge to jump to full. Most likely it is back by the sender, less likely - but not impossible to be up by the gauge.

[Scot Minesinger]

Also verify that no modifications have been completed.  Whenever there is a modification (replaced radio/or sound system) often an undesirable side issue develops.  Even any recent service work, cruise control repairs, climate (dash) controls work, or other area of this type can create problems.  Look also at rear end work, new shocks, brake work, e-brake work, differential and etc. 

For example someone hired a company to work on a 1966 Eldorado sound system, and after that the climate control only worked in full a/c.  I was hired to fix it.  Turns out unbeknownst to me that they removed the top dash pad, and upon re-installation pierced one of the wires connected to the sensor behind the small grille above center of steering wheel with an attachment screw-sloppy I know.  Took a while to find that. 

The gas gauge is a fairly simple circuit, so it should not be too difficult to trace and find.

[35-709]

I have mentioned this before but one of the connections at the gauge (there are three) in my '73 CDV apparently had just enough corrosion to break the contact and the gauge did not work.  Just tightening the screws brought the gauge back to life so I removed each screw, cleaned the contacts, and reassembled.  Works great --- one guy's experience.

[mgbeda]

If there's power to the gauge and an open between the gauge and the sender the gauge jumps to full.  If there's no power to the gauge the gauge jumps to empty.  A ground issue at the gauge seems likely.

-mB

[collector]

Guys thank you, Imp going to unplug the ground from the sending unit at the tank. I was told it should go straight to full. If not then the sending unit is not good. Then I did see a wire that was jumped from inside the door to the back of one of the lights to the dash cluster. I'm going to disconnect that wire as see if that solves the issue. If it solved the light on issue then Ill tackle the sending unit. The guy that I sold the car too took out the tank and had it cleaned, I think they messed something up or maybe has the wires on backwards. Ill see once I get the wires disconnected from the under dash and then check to see if the wires are crossed on the sending unit. I really appreciate all the answers. You all make very good point and have a good sense of knowledge. Not like on the LCOC where I can never get anyone to answer. Either they are so old they have no idea or they just don't care. Either way, GREAT advice from knowledgeable people.

[mgbeda]

(Apologies if this got posted twice)

Disconnecting the ground (or feed) on a GOOD tank sensor will still cause the gauge to read full.

Also you can't be sure if you disconnect the ground wire that the tank doesn't ground through the body. You need a ground wire because you can't be sure that the tank DOES ground through the body, and because running current through gasoline is not nice.  But disconnecting the ground might not have an effect.

I think the others here are talking about adding an additional jumper ground, not disconnecting one.

-mB