Lights on pegs tank gauge & kills oil gauge?

[ulrichd]

Just started today on my 70T. When I pull the light switch my gas gauge pegs at max and the oil level gauge dies. When I shut the lights off both gauges return to normal. Headlights (hi and lo beam)work fine.

Any idea what's happening? Thanks.

Ulrich

[304065]

Short in wiring behind gauges. Disconnect battery, remove gauges, note where the wires have chafed/melted together, replace with factory wiring.

[ulrichd]

Thank you John.

Ulrich

[teenerted1]

might check your fuses too.

i know when the #9 fuse blows on a 914 all kinds of funky things happen to the gauges and lights till it is repaired/replaced

[Grady Clay]

This can also be the ground for one or more instruments. It reverses the current flow through the "Gauge" circuit. This can actually damage the oil pressure sender, fuel level sender and oil level sender.

Best,
Grady

[john walker's workshop]

i would think it's a ground issue also. at least that's what i would check first. see that all the brown ground wires are present and attached to the gauge bodies and on the proper ground terminal of all the gauges, for instance, an oil pressure gauge would have 12V, a ground and a sender signal wire in a group of three. each terminal would be marked as such.

[ulrichd]

Well I think it's time to come clean. I let the pos terminal of my battery accidently touch the chassis and it sparked. Stupid I know, but there it is.

I have checked the fuses and they are all ok. I found no charred wires on the back of the combo gauge. Here's situation:
- Headlights work (hi/low)
- Blinkers work
- parking lights out on passenger side
- Brake lights work
- License plate lights are out
- Gauges are lit except for the above mentioned combo gauge

Could have been worse. Any advice on how to start?

Ulrich

[Rich Lambert]

"parking lights out on passenger side"
- Check the bulbs. If they're good, check for power & ground in the sockets.

"License plate lights are out"
- Same as above.

"Gauges are lit except for the above mentioned combo gauge"
- Same as above.

"...gas gauge pegs at max and the oil level gauge dies..."
- Try disconnecting the sending units and see if the gauges are still pegged. You may have fried them or the gauges. Check for power & ground at the sending unit plugs. Sending units are easy to replace. The individual guage components in the combo guage are replaceable too.

Good luck...it seems like whenever I do something like that there's a screwdriver with a partially vaporized tip involved.

[ulrichd]

Thanks Rich. Good advice on the sending units,

Any idea on the lights. If the fuses and bulbs are ok what do I do?

Ulrich

[Rich Lambert]

Originally Posted by ulrichd

Thanks Rich. Good advice on the sending units,

Any idea on the lights. If the fuses and bulbs are ok what do I do?

Ulrich

If the lights and fuses are okay, try tracing the power and ground circuits as far as you can. It could be that some of the contacts in the light switch are fused together in the lights-out position. If that's the case, I think you're going to need a new light switch. If your feeling brave, disconnect the positive power wires from the light switch for the license plate lights and the passenger parking lights and run a jumper from the wires to a postive power source. If it's the switch, the lights should work.

I was looking at the schematic for the combo gauge and both gauges and the lights use the same ground wires. You might double check those too.

[ulrichd]

Thanks Rich, that gives me a path to follow.

Ulrich

[WolfeMacleod]

Originally Posted by ulrichd

Well I think it's time to come clean. I let the pos terminal of my battery accidently touch the chassis and it sparked. Stupid I know, but there it is.

That happened to me once, although it wasn't my fault. Completely killed my first Porsche.

[ischmitz]

One thing to mention: Shorting the battery in the way you described won't really damage anything but the battery or the ground wire to the chasis. The current flowed between the positive pole across the chasis to though the ground cable to the negative pole of the battery. If your car had modern semiconductor devices these could have been affected by the transient. However, your issue does not seem to be related.

Measure the voltage between a chasis point and connector labeled GND on the gauge in question while you turn the lights on. I agree with outhers that you have a spotty GND somewhere (lights, gauge cluster) that causes your issues.

Good luck,
Ingo