Hi all, I'm at a loss as to the problem of not getting power to the AC compressor clutch. The truck is a 98 Ram Ext Cab Dually w/Cummins 12v with over 250K miles. The AC hasn't worked since we bought the truck about 6 or 7 years ago. Got tired of the Texas heat, so we bit the bullet and replaced the entire AC system except for the evaporator. There was no freon in it originally, so it had a leak. Now we can't get power to the compressor when the AC is turned on. It does have freon (r134a), but I need to double check the values as it's been a few days since I last checked, but it has held steady for the last few weeks. We've checked the AC relay, and it gets power. Relay itself has been replaced. The fuses all have power as well and the fuses themselves check good with a meter. We just replaced the HVAC controls inside the cab hoping that was the problem, but no dice. We have 12 volts to the low and high pressure switches. I've even jumped the low pressure switch to remove it from the equation. The high pressure switch should be closed at normal operating pressures, and it shows continuity on my meter, so that probably isn't the issue either. Am I looking at a PCM problem? Is there any way to test the PCM for AC output? The only other issue we have that may be PCM related is the oil pressure gauge keeps rising and falling like it's loosing power. It's really annoying. New sensor didn't help that either, so we figured PCM maybe. Any other ideas before an expensive PCM replacement? Thanks in advance.
Ted C.

 

Make sure fuse 9 in the junction block is good. (side of the dash) I would swap the fuse just for giggles.

If the fuse is good, at the PCM, connector 3, pin 1, is the control side for the a/c compressor relay. If you ground that, the compressor *should* run. Does it?

 

My father has the truck right now, but he should be over in a while, and I'll recheck that fuse.

I've been working on cars for over 40 years now, so its a blessing, but can be a curse when it comes time to working on the computers. So I don't fry anything, are you meaning to unplug connector 3, and jump pin 1 from the harness to ground?

Thanks,
Ted C.

 

Connector needs to stay plugged in, some of the a/c circuits are powered thru it.....

Do you have a service manual for the truck?? Is not, get one here. (free download)

 

Thanks, I did have the FSM, but didn't realize that the PCM was making a ground to complete the circuit, I thought it was the other way round with power coming out of the PCM. Anyway, it was raining for much of the day, so not much of an opportunity to work on it. Maybe the weather will be more agreeable tomorrow or on Friday. I'll follow up on what I find. Thank you so much for the information.
Ted C.

 

That's pretty common because it doesn't require a high amp power feed to the PCM, just good grounds. The 12V at the pressure switches is not "powering" anything either, it is a sense voltage. The climate control switch inside the cabin grounds two different pins at the PCM: A Select Sense telling the PCM that a AC setting is selected, and a Request Sense which goes through the two pressure switches. If both switches are closed the AC relay should close and run the compressor. In normal operation the low pressure switch opens after the compressor has created enough of a pressure differential, and the compressor stops.

Things to check:
12V at pin 30 of the relay (I think you did that already)
Jump pin 30 and 87 in the relay socket and see if the AC clutch engages
12V at pin 86 of the relay with the ignition on
The 12V at either pressure switch should drop to 0V with the ignition on and the climate control switch in an AC setting
Pin 85 in the relay socket should connect to ground with the engine running and AC requested

 

I'd jumper the safety switch on the receiver/dryer to make sure that is good. It's designed to shut the compressor off if you have low pressure but bad switch will cause it too.

 

Ok, sorry for the long delay. Between the weather, work, and my twin boys' 18th birthday, it's been hectic around here. We recently found an issue with the wiring harness to the compressor. We now have 12 volts at the connector, but we're not sure if it's making it through the connector. I also need to confirm that we have power to the correct side of the connector as well, and the previous owner made a mess of the wiring harness. Is there any way to test the ac clutch?

Thanks, Ted C.

 

A/C clutch doesn't care about polarity, it's just an electromagnet. So long as it gets power, and has a good ground, it should work.

You would use some jumper wires to apply power/ground to the clutch, see if it engages....

 

I'll give that a try when I get home from work this evening. Thanks again.