Need help with climate control short
 

Whenever I turn the climate control dial to max heat the aux water pump turns on and immediately shuts the blower/heater off. I have to turn off the ignition and turn back on for it to work again. If aux water pump is disconnected and wheel is set to max heat, there is no issue. I plugged in the connector to a 12v light source to see if maybe the aux pump was the problem, but it shut down again. I have traced the issue to be the white/brown wire on right hand connector of pin#7 of the push button unit. I have the factory service manual, but it does not show a relay or anything like that. Anyone have any ideas?

The aux pump is internally shorted, this is a common problem. There's an automatic circuit breaker inside the head unit that turns off everything when this is detected. Either replace the aux pump, or leave it disconnected.

I have disconnected the aux pump and put the connector on a 12v light source and the same problem occurs. If I use another ground wire that is constantly grounded then the aux pump works fine, but it is constantly running. I am unable to trace any short to the white/brown wire that connects to the aux pump and goes to pin#7 on the push button unit. Is there a relay or fuse for the aux pump because I can't find any. In fact, I have run other issues that aren't related to this one in particular, but I have the FSM for my 1990 190E 2.6 and I have noticed that my car has factory wires/connections that I can not even locate on the FSM. I have a friend with a 92 190E 2.6 and even noticed that there are very slight deviations in some of the factory wiring that I can see on the FSM for his car, but no mention of anything on my car. Ex: There is a white wire that is on the left connector on pin#9 I believe on the push button unit, but nowhere on any part of the FSM is that wire listed for any model from 1987 - 1992.

There is an automatic breaker built into the pushbutton unit. It can't be serviced, you would need to have the unit rebuit.