HEDALIGHT Disperate faulty, Unexpected fault Options

[searunner]

yesterday coming into my garage, I hear one ritming song- under the cover of my 914 2.0 1974, one strange movement, the LEFT Headlamp go UP and DOWN
The RIGHT Headlamp stays in the closed position.
Normally, I detach the battery of the car when in the garage, but I forgot this time.
The car was without a KEY IN - the Light switch was off -
The last trip with the car ALL was regular, and the headlamp was in working condition.

I can STOP the movement, detaching the battery- but this NO was the solution,
Then I searched for a better solution.
Extract completely the Light switch -(with the BATTERY connected and KEY ON) I have the two headlamps is up and in stable condition - also when I START the engine and move the car, and make a long trip

But if I return the light switch to the POSITION light, the LEFT Headlight return to UP_DOWN movement.

Is clear that I have one short- Or one Relay issue - BUT 914 was full of relays
Can someone help me, to return to a normal 914 ??
I am not a native English so overlook my errors

[Spoke]

@emerygt350
@Superhawk996

Here is the basic schematic for the headlight motors. This is from a Haynes manual. Not sure I understand how the switches in the motor are configured. Are they switches riding on a cam or slide contacts on a PCB wheel?

The headlight switch is shown in the PARKING position where both UP and DOWN contacts are open. The headlight position is 1/2 up/down and both switches are closed. Both motors are off.

If in the DOWN position, the headlight switch provides power to the DOWN switch which is closed and the motor cycles until the cam/PCB wheel opens the DOWN switch.

If in the UP position, the same thing happens except the motor stops in the UP position.

Notice the headlight switch is not fused according to Haynes. Yikes!

For the life of me I can't figure out what the diodes do. Assuming they minimize interaction between the 2 motors but I can't figure it out.

For the issue for @searunner where the motor stops at UP but cycles up/down on DOWN, the switch in the motor for DOWN is not opening up. If the relay was sticking on, it would stick on for UP too.

[Superhawk996]

For the life of me I can't figure out what the diodes do. Assuming they minimize interaction between the 2 motors but I can't figure it out.

Not sure what you mean. You have it correct.

Since the headlamp motors cannot be synchronized, the diodes block current flow between the UP & Down circuits during the period when the rotary switch would otherwise allow continuity between those circuits when they are between fully up or fully down.

The way the Haynes manual shows the rotary switch matches its physical layout.

Picture below ( @georgekopf ) photos

[Spoke]

For the life of me I can't figure out what the diodes do. Assuming they minimize interaction between the 2 motors but I can't figure it out.

Not sure what you mean. You have it correct.

Since the headlamp motors cannot be synchronized, the diodes block current flow between the UP & Down circuits during the period when the rotary switch would otherwise allow continuity between those circuits when they are between fully up or fully down.

OK I see what the diodes do. Question is would a dead or shorted diode cause the up/down observed?

Thanks for the pictures. I didn't think there were switches in there. Back to the original up/down issue, what are the chances that there's metallic build up on the insulator for the down wiper? What else could cause the down to skip right through the stop to cycle again?

[Superhawk996]

What else could cause the down to skip right through the stop to cycle again?

Shorts in the wiring itself.

I suspect this is the more likely scenario for the OP.

Two problems with my internally shorted diode scenario.

1) Diodes usually fail open. In that case, the up or down function just doesn’t work.

2) When the driver side motor goes to the down position it should open the down rotary switch which itself would keep the current from reverse flowing through to D4. So although the wonky up/down on the passenger side can occur when lights are between fully up/down, it would eventually resolve itself after a few cycles and should not continue indefinitely. For the behavior to continue indefinitely, there would have to be a dual failure of both D4 and the passenger side rotary down switch. Not impossible but not likely either.