Fix Headlight Dimmer on 90 vert with airbag
 

I am looking for some help with a headlight dimmer (actually the high beam) for an S5 convertible.

The headlights, on a 90 vert with airbag, fail to go to bright when the dimmer switch is pulled.

The flash-to-pass works, while the headlights are down and not on, when one pulls on the dimmer switch.

With the headlights up and turned on, the dimmer relay is grounded; the high beams come on.

This tells me the problem is not the headlights or the relay.

Next, back from the relay towards the ignition switch, is the cpu. How does one test the cpu?

I have access to a 91 FSM but the car in question is a 90 vert with an airbag. Is there a difference between the 90 vert with airbag and the 91 vert with airbag?

I suppose if one finds the wire in to the cpu, from the dimmer switch on the steering column, and grounds it, thus emulating what the dimmer switch send into the cpu. If the cpu is good, wouldn't it send a ground signal to the relay (which works) and that should dim the headlights?

If the cpu is good then the next culprit might be the dimmer switch on the column. This is a relatively simple switch even though it does have the turn signal switch in it too.

Peace

Does the car have any after market headlights? H4s, Xenon, or HIDs or anything like that?

They look stock. The brights dim when the relay is grounded. Dealership price for new cpu is $307.

I have another cpu you could try if you would like. I had a similar problem and it turned out to be a short on the hot side to the ignition switch. Ground was fine, and part of the circuit was complete however the short was causing the turn signal not to work properly. Hazards would work fine (different circuit.)
The reason I asked about aftermarket lights is that some do not have an option for brights. Let me know if you would like to try a CPU.

Robert,

Thank you for the most gracious offer. I may very well take you up on that offer.

For now I would like to find out if one can send the same signal to the cpu that a working dimmer switch sends to the cpu, without a working dimmer switch.

Here is how I think it works:

With the headlights up and on...

1. Dimmer switch on column is pulled towards the driver by the driver ...

2. This causes the line from the dimmer switch to the cpu to be grounded.

3. The cpu causes the line from the cpu to the headlight dimmer relay is grounded.

4. The grounded relay does what it does and the headlights go from normal to bright.

If that is how it works then I've done #4 without the cpu when I grounded the dimmer relay and the headlights went from normal to bright and then from bright to normal when I broke the ground. So relay is good. Is the cpu?

I need to be able to emulate the input to the cpu; the same as a working dimmer switch would do in #2 above. But which wire going into the cpu is the correct one to ground?

Peace

I will pull out the wiring diagram and take a look this evening. Is the car here in Albany? Maybe I could take a look one of these days. :)

How about Thursday evening?
 

I'm sure you remember it; the one with the new housing that had half a spark plug hole. :)

Let me know when is a good time for you. I'm signing out now and won't check this until Wednesday.

Peace,

Bob

Ok.... So after looking over the FSM I found that the dimmer switch is a latch circuit from the headlight circuit. It does not go throught the ECU. The switch simiply latches a circuit to the dimmer relay in the engine bay. The switch may be bad or the relay may be stuck open.
Also I would not rule out a short in the ground somewhere.
If you have the FSM refer to page T-23 for testing the dimmer switch,
Refer to page T-51 for testing the relay.
If those check out try grounding just the dimmer latch circuit to a known working circuit, (the headlight switch should be grounded separately.)
I am off the rest of this week. If you would like, give me a call and I can check it out for ya.

In my vert, my latching circuit is in the CPU and not the switch. My latching circuit failed and so my high beams would flash with the stock switch, but wouldn't stay on.

I needed to pass inspection so I wired up a switch and made it look pretty.

According to page T-50 in the FSM, the latch circuit for the dimmer switch is in the cluster switch.

Thanks for the 411.

Where is a diagram/schematic of the cpu?

t-33

Recap: Grounding the dimmer relay with the lights on, causes the lights to go from normal to bright.

Ran a wire from the dimmer switch ground (white w/blue stripe) to the relay. Pulling the dimmer switch on the steering column causes the lights to go from normal to bright.

Problem is the lights do not stay bright when the dimmer switch is released.

Conclusion: The latch switch is bad.

The latch switch is in the cpu.

Solution: Replace with a cpu that has a working latch switch or repair the latch switch in the cpu.

Alternatives: ?

Could you send me the part number for this CPU? I have a similar high beam problem.