Aircon Problem...
 

I have a '92 500 SL with an aircon problem. ie it doesn't work! Won't blow cold or hot, fans don't operate! Recently had the problem diagnosed as needing a "main control unit" but with no guarantees of a cure. Cost me about £350 for unit and fitting and, voila, it worked perfectly! About 3 weeks later, (yesterday), it stops working again I'm thinking that maybe there's some other fault which has maybe caused the new control unit to blow. Don't want the expense of another unit without tracking down whatever other problems there might be first. Any ideas anyone? Many thanks... kenny

Is the display dead? The only problem I have seen cause repeat failures are shorted auxilary water pumps. The fan and compressor are actually powered by other units on command from the AC pushbutton controller and shouldn't cause a repeat if they were shorted.

In most cases MB does a good job of protecting controllers. They current limit large outputs such that even shorting them out won't hurt them. They either continue supplying the signal controlled to the highest output or they shut down.

Hi thanks for the reply. The system on my car doesn't have a display. Just temperature rollers and fan control dial. I think the system with display came out the year after mine.

The problem you describe could be related to a worn ignition switch. Try moving the ignition back to the left a little next time you start it with the fan switch on high and see if it comes on. If so, replace the ignition switch and your problem will be fixed.
Not a sure thing, but I have seen enough about this being the problem, it would be worth a try before you spend any more on parts or diagnostics.

Thanks for the ignition switch idea. I've tried what you suggest and it doesn't seem to make any difference. Any other suggestions will be most appreciated. kenny

Just discovered today that the unit replaced was actually the "heater blower speed resistor unit" not the main controller. Maybe this puts a different light on things. Still hoping... :confused:... kenny

Check your fuses - fuse box under the hood. If you lost the fans, you may have simply lost a fuse. Won't hurt to look.

Thanks Mike, yep I've checked fuses, no problems there... kenny