checking PCM
 

I have a 94 Dakota 3.9 and can't get power to the AC clutch. With ignition switch and AC switch on I get power through the cycling switch and through high pressure switch but none at the connector to clutch. I have replaced the clutch relay and fully tested the new relay. The only thing I can think of now is that the PCM is not outputting a signal to the relay, but I don't know how to test whats coming out of the PCM because I have to have the connector off to tell where the cavities are and then, of course the PCM can not function. Can anyone tell me how to test that?

Assuming the 1994 is like the 1995, then socket position "C" under the A/C relay socket is the ground controlled by the ECU; socket position "A" should be at 12V.

What you can do is see if a trouble light between C and A lights up when A/C is commanded.

If not, next step is to check position 27 on the ECU's connector, and see if it has 12V when you turn the A/C on inside (no, it won't be running, but at least you'll know if the 12V is GETTING to the ECU).

For a 1995, it's section 8W-42; should be the same for a 1994 in the factory service manual.

RwP

Thanks a lot Ralph. I believe that will be very helpful when I get time to check it out, which should be later today. I will try to remember to post the results.

Update for Ralph P:
With ignition on and AC switch on socket C has no continuity to ground and socket 27 of PCM connector has no voltage. So somewhere between the high pressure switch and the PCM connector power is lost. I have trouble reading all the fragmented diagrams I have. Having all the AC related circuits on one page would make it so much easier to track the pathways.
Do you have any more ideas on how to find the open?

I was looking at it backwards.

It goes from 27 --> High Pressure Switch --> Low Pressure Switch --> AC Control --> Ground, so you won't see voltage on pin 27 of the ECU with the connector disconnected.

HOWEVER, if you have voltage on the high and low pressure switches with the A/C commanded (i.e., motor running, A/C on) then the ECU is sending the voltage out; but it's not being pulled to ground with the control. So first step is to ground that line to test the relay and clutch.

If you ground either of the switches, it should see the ground, and say "Oh, good, we've got the right range of pressure!" and turn on the relay.

If that causes the compressor to kick on, then it's somewhere between the low pressure switch and the A/C control unit.

If you can pull the A/C control unit and ground the 20gauge Dark Blue/Orange Wire, and the A/C kicks on, the problem is the head unit. If it doesn't time to chase that wire back to the low pressure switch connector (the 1995 diagram shows two connections, C105 and C116. C116 is the bulk head connector on the driver's side, C105 is a 4x4 with only eight wires used (the outer two 4 wire runs). For 1995. Again, there may be differences for 1994, check your FSM to verify.

Now, getting back to the relay, referring to page 8W-42-7, terminals B and D are the NO contacts. If you take a 20ga or bigger wire with two quick connect males of the appropriate size on it, and shove it into the contacts where the relay came out (pull the relay, you're faking it with this), does the clutch engage? If not, then the problem is the clutch; check to make sure B has 12V from the 30A Fuse B. If there's 12V on terminal B, then pull the A/C clutch connector and make sure one side has 12V the other side is grounded with the relay bypassed.

That's how we troubleshoot where the wiring has failed, without swapping ECUs.

If all the external wiring is solid, and the ECU just isn't commanding A/C, well, there are programs and adapters to query the data stream; I've not used them, don't know how well they work on a 1994.

RwP