Intermittance... the bane of the computer guy's existence. This time around it was the dashboard and console backlights on my 91 350SDL. When the problem happened all the lights - except the ashtray light - would go dark. Twiddling with the rheostat would not bring them back. Then magically things would go back to working. Tonight I decided to replace the cluster backlight bulbs and the problem presented again, and I was able to solve it.

First things first, you need to look at the wiring diagram. Power from the headlamp switch comes in on the round 15 pin connector on the back of the cluster on pin 5. It then goes through the rheostat and back out of pin 8. From there it goes to the other dashboard lights, and to the square 8 pin connector on pin 8 for the cluster lights.

My late (and all post 1989 cars according to the diagram) has a different rheostat than the early cars: Gen 1 cars (85 and before) have a wire wound rheostat which feeds all the lamps directly. 86-88 cars have the same rheostat, but it feeds a solid state voltage regulator which lives on a bracket behind the instrument cluster. My late car has the voltage regulator built into the "rheostat unit" which is actually a potentiometer, voltage regulator and some support parts. You can see the regulator in the lower right of this picture:



It is held onto the cluster with one screw and simply pulls off. Here are some pictures of the regulator with the covers taken off:


The voltage regulator lives under the silver heatsink in the middle.



Inside I found several cracked solder joints, which I resoldered. However that didn't fix the problem

I took the cluster totally apart, got out my trusty test light, and hooked the boards up to the car harness. I found that if I wiggled the wiring harness I could make the lights flicker. Using the test light I found that the power going to pin 8 on the round plug was being interrupted, no surprise there. Pin 5 remained steady. Then I checked the power going to the regulator and sure enough, it would get interrupted when I wiggled the harness. Taking the board back inside I found more cracked solder joints, which I resoldered:





Great success! The lights work and wiggling the harness didn't make them flicker I originally thought the problem would be in the regulator itself, thankfully a few dabs of solder have fixed this for good!

Not sure why my car doesn't match the wiring diagram (the ashtray light being before the dimmer).

PS: Do you ever get the feeling that someone has been here before you?



I added today's date and mileage

Thanks,

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.