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]

Question is would a dead or shorted diode cause the up/down observed?

Yes

Let’s say the UP diode (D4) on the driver is shorted internally - allowing current to back flow though the driver side UP diode. That current will backflow on the shared wiring over to the passenger side through D2 (becoming forward biased) and then through the passenger side relay. Now on the passenger side we have forward biased diodes on both the UP (D2) and Down (D1) wires.

When the passenger side motor hits the down position and the rotary switch opens the down contacts it doesn’t matter. The UP contacts are simultaneously closed and there is 12v sitting there ready to go due to the backfeed through D4. This goes to the relay, and the motor cycles up. When the up contacts open, we still have 12v sitting there ready to go on the down contacts. Now the motor cycles down and opens the down contacts and closes the up contacts. But … 12v is still sitting there on the Up circuit (backfeed from shorted D4) and now the motor cycles up again. This will continue until the headlamp is moved to the UP position. At that point the D4 is no longer in play, the backfeed through D4 is removed, and the motor will cycle up, the UP rotary contact will open and the motor will stop and will stay up on both driver and passenger side.

See your modified schematic below:

Red is current flow through the down headlamp switch and through the passenger and driver relay.

Green is the reverse current flow though an internally shorted diode D4 or shorted wiring that bypasses D4. That current can flow over to the passenger side and will forward bias D2.




I don’t believe OP has verified his driver side diodes. This should be done but I know it’s a pain. I was asking him to pull relays one at a time so I could verify the current source to the motor to make sure the short to the motor wasn’t somewhere else other than the driver side diode.