'92 190E Push Button Climate Control Problem
 

I'm having my 92 190E 2.6 A/C serviced and the mechanic is stumped. He said it's a problem he hasn't seen before, so I'd thought I'd post it here to see if I can find an answer.

It all started with a seized compressor. It turns out the compressor was hot wired to be on all the time and bypass the push button control. My mechanic replaced the compressor, clutch, drier, and expansion valve and flushed the system for a few days. The A/C works nice and cold with the hot wire to the compressor. But we can't leave it that way since it led us to this problem in the first place. (I don't know what this hot wire is doing, except it seems to bypass any attept to turn the compressor off in the climate control system).

He removed the hot wire and installed a new push control unit since the current one is fried. Within minutes, the new control unit fried. That's pretty much where it stands. The mechanic is trouble shooting a few things, but hasn't found out exactly what is causing this problem. He mentioned a possible aux fan in the heating system as the culprit but has now ruled that out. Right now, he also mentioned some potentiometer that's burried deep within the dash that could be causing the problem.

Any idea what could be causing a short in the system? Or why someone would even hot wire the compressor in the first place?

Thanks!

From what you are saying the circuit was hot wired because the PBC (pushbutton controller) was fried. A common source of frying is the auxilary water pump.

I seriously doubt that the mode door poteniometer could cause circuit overloading.

Thanks for the response.

And I see what you mean. A bad auxilary water pump caused the push button control to short out. Then, some joker hot wired the compressor instead of replacing the water pump and push button control unit.

That's the first thing my mechanic suspected and I think he checked that out and it still has the same problem. Could anything else be causing this shorting out of the push button control?

Thanks,

Control units control. They use inputs to make a decision. Typically overloaded circuits occur on the outputs. Outputs are these things: auxilary water pump, aux fan control, switchover valves, and clutch control. In the first three if the device is shorted the controller either protects itself (as the engine controllers will do) or will burn out. In the case of the clutch control; this is a ground circuit (to a separate relay)so a short would be (and I've seen it numerous times) where someone thinks they can hot wire the compressor control with battery voltage. The ground circuit in the controller is designed to pull down a transistor not a battery.

Well, the shop still has the car and can't figure out why the PBC fries. The latest news is they think the car is missing the Electronic Accelerator Control Unit and this is why the Climate Control system's PBC goes out because of some altered wiring. They came to this conclusion after obtaining a socket box tester from the dealer and hooking it up to the PBC harnesses.

I checked the schematic of the Climate Control system and do not see the EACU in the system. I really don't see how they could be related. After looking at some electronic accelerator system diagrams, and if the EACU was missing, it looks like there would be quite a few other systems (cruise control, backup lights, and more) that would not function.

Does any of this make sense? Is it time to pull the plug on these guys and take it to the MB dealer?

Thanks.