Hello last year my brother found himself in a pretty good deal on a rolled 95 C4 with a good lt1 and zf6, along with other good parts. This C4 was completely totaled no body work was fixing it. So he found another C4 on a auction that had a minor rear end accident and was a exact copy of the rolled one but was a 94 and automatic. So we swapped the zf6 in the 94, we've worked out most the bugs we've came across but this one has us stumped and I cant find anyone else who has had this problem. The problem is the a/c will not turn on when any of the options are selected on the climate control, excepted when its selected 'OFF'. Obviously before we figured this out we figured it was low on freon so we fiddled with it and it turns out it had freon so we jump the a/c and it works just fine. So as I'm messing with it I turn the climate control off and I hear it turn on. I'm really lost in the situation cause to me it makes no sense... We have tried both ecm's and climate controls with no success. When you select 'OFF' on the climate control it grounds the a/c relay and sends power to the compressor and works but obviously it will not blow in the cab. Unfortunately I don't know if it worked normally before we did the swap because it was cold outside at the time and we didn't think of it. Anyone have suggestions or any idea on how to go about this?

 

I would pull out the FSM and look at the wiring for the relay and it’s connector and do some testing. My guess is wires are reversed or there is a short to ground or short to 12 somewhere…also test the relay in isolation.

 

Is this the auto C68 system, or the manual C60 system. Can't answer with out knowing.................

 

It’s the C68 system

 

In this system a lot has to happen to engage the AC clutch. The control head sends a signal to the programmer, then the programmer sends a signal to the PCM which then grounds the low side of the AC relay coil which closes the relay and sends power to the clutch. That the high lights as there is more circuits but that is the main signal flow.

I just can't see with the complexity of the system any wires crossed as most wires are in connectors and are almost non moveable.

It's so strange that the clutch would engage with the OFF button pushed and the fan not running. Seems like the PCM which is the switch for the relay coil is working, but the signal to it is inverted. My first guess would be the programmer might be bad.

I also think if the control head was bad and inverted a signal the fan would run also. But most of the signals are digital to the programmer from the control head and I can't see that happening. The programmer puts out a 5 volt signal to the PCM but for all intensives purposes it's an analog signal with it being either 0 or 5 volts thru the cycling switch which enables the PCM to close the AC relay coil grounding it.

I would still take a look at any connectors and wiring I could see but in the end substitution might be the only way to go and try things.