W140 no heater fan blower

[fdm_alex]

93 600SEC with the old push button unit in the dash.

Vehicle was brought to me because the blower quit. Opened it up and found that there was an open circuit across the resistor, easy enough. Replaced the resistor and the motor burned out quickly. (the AAC filter had been severely clogged before, which is probably what burned up the resistor to begin with)

Replaced the blower motor and it started to work again, except that I only had high speed and no control over anything at the dash, including not being able to shut the thing off. The guy took his car back and rescheduled to have it looked at again.

When I got it back it had begun to go intermittent, and by the time I got it in the shop, nothing worked at all. I hooked up my SDS-DAS, 27 pages of faults in AAC, the first several being for the module itself! I checked and found that the resistor was still good, and that I had 12v supply to the motor, and that the control signal voltage was correct. I checked all of my grounds and they were good, so I ordered another AAC module.

Put in the new module, still nothing, except now I have no voltage at the motor on any setting. I have 12v at the pigtail in the blower box, but none of it makes it through the resistor to the motor, and I also no longer have signal voltage, even to the pigtail. I put in the old unit and have exactly the same condition as the new one.

This whole issue is a mystery to me, according to the wiring diagrams I have been through everything related to the blower, all of it is good, and it still doesn't work.

[fdm_alex]

I've just remembered, the hot/cold air selector in the center vent does not work, or at least the LED's don't. I know that can shut down the center vents, but will it make the whole system go down?

[fdm_alex]

FIXED!

Through some more digging into the wiring diagram I found that all affected functions, including several things I didn't mention that were setting codes in the AAC module through SDS-DAS, ran on the Z3/1 splice, which gets power from relay K30 in the main fuse box. I opened it up and founf that the relay had come fully unplugged and was lying on its side, it's a wonder anything had worked at all at any point during this project. All functions are now normal.