Strange Electrical Issues

[Bulldog9]

This is for a 76 912E (basically a 76 911).

Just finished a full restoration, all electricals completed 2 years ago and working fine with NO issues.

Issue 1. Without making any changes, suddenly the headlights would come on when the switch was pulled out all the way. Before it only came on with key on.

Issue 2. Happened today, while driving the whole gauge cluster died. Tach, Speedo, Oil Pressure and Temp, clock. Seemed to be bump induced all on or all off. I pull in to park, shut my lights off and bam, all the gauges fire up. Out and all are off.

Issue 3. Turning the dimmer switch will trigger the gauges whether the switch is on or off.

It's all very strange...... I want to restate that the electricals have not been touched or changed in 2 years, so this is not a result of that.

Is the Headlight switch that integral to the electricals? I could see a bad ignition switch causing this but not headlight.

Thanks for any input!

[E Sully]

All I can suggest is making sure all the ground wires are making good connection. If the area the ground wires contact has been painted, then you need to clean it to bare metal.
Poor grounds can cause strange behavior by back feeding.

[gomezoneill]

Like E sully said check grounds. Definitely sounds like a ground problem.

[steely]

Steve,
This is just an opinion as I don’t have a 76 schematic but I do have a 74 schematic courtesy of Dennis (Timmy2). The headlight switch is pretty busy and integral, but I will stop short of saying it is the cause because my experience is limited to my 87, and this might need more testing.

1) According to the 74 schematic, the headlight switch gets unswitched power for things like the instrument panel, parking and license lights. But the power for the headlights is switched from the ignition. The ignition switch could be providing unswitched headlight power to the headlight switch when it shouldn’t, or the headlight switch may be internally shorting unswitched power to the headlight portion.

2) Are you referring to just the gauge illumination here, or are you including their readings as well? If just the illumination, yes, they get their power from the headlight switch. Are you saying that they operated with the headlight switch off, but not when the switch was on? If so, I am thinking HL sw internal contacts – a short between unswitched and switched power source.

3) Yeah, power there when it shouldn’t be. They should be expected to work with no ignition key ON, but not when the headlight switch is OFF. Am thinking more of the headlight switch alone here.

4) Gremlins – is everybody plugged in under the dashboard (round connectors from steering column) above the footwell?

[Bulldog9]

Thanks Dan,

Out of the blue while driving the gauges went dead. When I pulled off and shut the lights off the came back on, and when I pulled the switch out (parking or headlights) the gauges died (oil temp/pressure, tach, speedo, and clock. So something is either shorted in the switch, or a loose ground.

I'm leaning towards the switch because when I turned the rheostat (dimmer) to the right the tach came back on but the oil gauges were still dead. I was driving on the highway in the daytime, so I'm not sure on the gauge back lighting.

Today, I Pulled and cleaned all the grounds, and checked all the connections on the back of the headlight switch. I noticed several of the connectors in the back of the switch have some play and slide in and out. The headlights still come on with key off, but everything else is working, went for a 2 hour drive today, and problem did not repeat. I really dont want to drop $450+ on a headlight switch....

Originally Posted by steely

Steve,
This is just an opinion as I don’t have a 76 schematic but I do have a 74 schematic courtesy of Dennis (Timmy2). The headlight switch is pretty busy and integral, but I will stop short of saying it is the cause because my experience is limited to my 87, and this might need more testing.

1) According to the 74 schematic, the headlight switch gets unswitched power for things like the instrument panel, parking and license lights. But the power for the headlights is switched from the ignition. The ignition switch could be providing unswitched headlight power to the headlight switch when it shouldn’t, or the headlight switch may be internally shorting unswitched power to the headlight portion.

2) Are you referring to just the gauge illumination here, or are you including their readings as well? If just the illumination, yes, they get their power from the headlight switch. Are you saying that they operated with the headlight switch off, but not when the switch was on? If so, I am thinking HL sw internal contacts – a short between unswitched and switched power source.

3) Yeah, power there when it shouldn’t be. They should be expected to work with no ignition key ON, but not when the headlight switch is OFF. Am thinking more of the headlight switch alone here.

4) Gremlins – is everybody plugged in under the dashboard (round connectors from steering column) above the footwell?