Expert: Roland Finston - 7/1/2011

Question
I have had my mini van servied for ac issues, the auto mechanic has told us that, it needs a new "Main Computer" which would need to be programmed at the dealer???  All components of the AC unit are charged and function properly if he jumpers across a relay,
I have researched and see that this van has two control modules, one and ECM the other a BCM...I am considering getting a new BCM as I think tghis is what ultimately controls the AC unit...All other electrics in the van function properly...Can you please advise???

Answer
Hi John
I would advise against replacing any major module/controller/computer without first getting a fault code readout of both the pcm and bcm memories using a good quality readout box that can access both. You could try an Autozone or similar nationwide parts store for a free readout, or an independent shop should be able to get the codes for under $40. Did the mechanic try for those codes and if so what were the code numbers? They are 4-digit and are preceded by a P or a B depending upon whether the powertrain or the body computer is sending out the fault code. Once you get them let me know what they are and we can go from there.
The a/c 'on' switch is in the control module (tell me if you have the conventional or the automatic temp control when you write back). It communicates to the body computer your request and then the body computer communicates that to the powertrain unit. The powertrain unit activates the compressor clutch relay's control coil based on getting the request from the bcm and also having a voltage reading from the reftrigerant pressure sensor that there is the proper amount of refrigerant in the system.
There are therefore several possible causues for the 'no compressor' action (which I assume is the issue) and which might be the problem. We just have to find out which it is and correct it. Do you have a volt-ohm meter in case we need to check some wiring?
Roland
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