Air conditioning compressor doesn't engage

[BPH]

Now that the crank sensor is behind me...I hope, my 1999 C280 2.8 electronic climate control unit is permanently in the "EC" mode which can not be switched off as indicated by the red light. The air conditioning has been evacuated and recharged, fuses and wiring checked, and I have been told that the cc unit "ec" switch is deffective. Further, my independent shop says a replacement unit must be purchased from MB and programmed? Can anybody help me through this problem and the logic of the MB only programming thing for the climate control? Thanks!

[forp]

First of all, read this thread, especially the last post:
210 chassis climate control problem

I had a similar problem with my '96 C280. After talking to the dealer and my local indy mechanic, like you, I was looking at replacing the Climate Control module. After searching here for "climate control" I found instructions for resetting the CC module using the front panel buttons (See the above thread). Note that this is different from using the CC to access real time performance numbers. It was not for my year/model MB but I Figured I had nothing to lose so I went for it. The procedure involved entering a code. There were only 15 combinations possible for my unit so I tried them all. I found one that turned off the EC light. Everything appeared normal. Yeaaaa! That is until I tried to get some cold air out of the AC! To make a long story short, using the schematics in my MB CD manual, I found the problem was the CC unit was biasing the AC pressure sensor with 8V instead of 5V causing the sensor to constantly output too high a value which the CC interpreted as a fault and would not turn on the compressor. I don't know if I caused this by playing with the resetting, but it seems very unlikely such a basic 5V output could be changed at the front panel. More likely my original problem with EC light and this voltage flaw indicate a “challenged” CC module. Hmmmm…What to do? The pressure sensor wants 5V, so by George, it’s going to get 5V. My vocation is in electronics so I had no trouble cutting the “5V” line from the CC at the pressure sensor and splicing in a 7805 5V voltage regulator using a fused line I installed from a switched 12V terminal I found in the engine compartment. So…$3 in parts + some of my precious time = no problems in the 6 weeks since the repair. I hope something in all of this helps you.

Dan