Diagnosis of Inoperative Mercedes 380 SL Blower Motor AC/Heater -- Problem Solved!

 During the summer I did not use my AC/Heater Blower Motor. However, a few weeks ago it finally turned colder for a day and behold -- the blower motor was no longer operative. What to do?

1) I followed Ken’s advice at MercedesSource.com and began by examining the fuzes. On my SL the relevant fuzes are 6 and 7. They looked good, although I did remove and also rotated them in their settings. Nothing changed.

2) OK, then Ken suggests that more than likely the problem must be with the motor itself. Rather than replace the motor brushes, I decided to buy a new URO motor from Rockauto for $88. Replacing the motor is very straightforward, using directions from a pages that one can buy from MercedesSource.com. Well done descriptions and photos. My only problem was that my blower motor was in its place for the past 40 years. It was glued to the seal, and thus I had to take a heat gun to free it up to pull it out. I replaced the motor, cleaned everything up, replaced it after having to deal with the plastic blower case that has polygonal parts that demand your careful attention. The numerous metal clips that hold this case together are tight and you must take care not to over stress old plastic that can be brittle.

Result: nothing changed -- probably I wasted $88.

3) Now what? There were two other potential trouble spots -- the control panel and a black box accessible by going through the glovebox. I had alternative used parts for each that I acquired at a pick and pull yard from a 1983 380SEC. I substituted my salvage black box and no change resulted. I could not properly substitute  the control unit, but I did resolder a few suspicious contacts on the control box. Again, no change!

Using advice published on the Program Internet page under R 107 Control unit. I made several chucks using a VOM better on left and right connectors of the control unit once they were unplugged. With key on, checked amperage on two contacts for the possible failure of the auxiliary water pump. The reading needed to be below 1.3 amps, and indeed it was 0 amps. So Aux water pump OK. Also didsimilar checks and confirmed that the A/c relay was OK and that the Monovalve was OK. You need to do these if you are replacing the Control Unit with a Reman from Programa.

4) Took my car to Brock at Specialty Motorwerks in Moraine. He thought it was a black box issue, ordered a reman unit, substituted it, and again no change. One of the other mechanics kept on saying “ignition switch,” "ignition switch.” Sure enough, that is what it was. It was recently replaced, too. But the part I bought from Rockauto came from India. It had failed miserably.

Moral of the story -- only use MB parts if available.