Im new thanks to google I found this site. I am working on my first dodge so its a learning process. What I have is a 97 3500 2wd cummins 5 speed. I changed the ac yesterday and am having no luck at all getting it to run. I changed everything except the evaporator, pulled it down and had to bypass the relay to get the clutch to engage. We filled it using this method using gauges and it cools great. When we put the relay back in the clutch will not engage. We have check low and high pressure switches and they seem to be working. I am lost for what I need to do next.Also the previous owner hooked up an external voltage regulator on the alternator, so I am wondering if I am not getting signal from the computer. I dont know if it has any effect on the ac or not.I am in Tx and its getting hot fast so I sure would appreciate any help getting this thing going.

The high and low should have continuity through them to get it to run as you probably know. It goes through the PCM for some reason but I don't think there is much problem there. Have you checked to see if the relay still works? You can pull it and hook 12 volts to the 86 and 85 terminals with the relay pulled and see if it clicks, though checking for continuity between the other 2 contacts would be better.

  ISX said:

The high and low should have continuity through them to get it to run as you probably know. It goes through the PCM for some reason but I don't think there is much problem there. Have you checked to see if the relay still works? You can pull it and hook 12 volts to the 86 and 85 terminals with the relay pulled and see if it clicks, though checking for continuity between the other 2 contacts would be better.

I swapped relays with a known good one and with the truck running and ac turned to on I have continuity on the hp and lp switches. I can unplug the relay and plug it back in and the clutch will engage for just a second then go off.

  mjames76 said:

I swapped relays with a known good one and with the truck running and ac turned to on I have continuity on the hp and lp switches. I can unplug the relay and plug it back in and the clutch will engage for just a second then go off.

Now check to see if you have 12 volts at the clutch even after it clicks off. You might have a dying coil pack that 'opens' soon after energized. Or you might have too much or not enough pressure in the system, and as soon as the compressor turns on, one of your pressure switches is 'doing its' job'

It sounds like pressures are incorrect when compressor kicks on. When I charge systems i pull vacuum and then put one can in. after that first can i start the vehicle and turn on a/c and blower as high as they will go. then i put freon in til i have about 25-30 psi on low side and no more than 250 on high side. I never get any complaints from people that run the tractors. Charged a few trucks the same way with problems.

Guys have a point it would be a good idea to drop a set of manifold gauges on and check the pressures of the system and see what happening it might be at 40-50 PSI when you start. Then as the compressor runs a second or 2 then drops below 25 PSI and shuts down.

  Mopar1973Man said:

Guys have a point it would be a good idea to drop a set of manifold gauges on and check the pressures of the system and see what happening it might be at 40-50 PSI when you start. Then as the compressor runs a second or 2 then drops below 25 PSI and shuts down.

Shouldnt that cause the lp switch to break the circuit. I can put my meter on both wires coming out of the switch with everything running and get continuity. Does anyone know which leg of the relay comes from the switch so I can check that.

Yes it will constantly keep breaking the circuit. Any pressure below about 30 PSI will cause the system to freeze up hence why the pressure tables for the A/C system. Also I typically charge to a solid 40 PSI on most vehicles.

If I remember right my pressures while the compressor was running were 45/270 and it was probably 85-90*

Then if its still cutting out then I would highly consider changing the low pressure switch. It can be done without losing any freon being its screws on a schrader valve on the accumulator can.

Well I finally got it figured out. It was the PCM. I swapped it with a friends and it came right on and is working like it should. Now to find a new PCM.

If the PCM is damaged I would test the alternator for AC noise just to be sure your not going to cook the new PCM or other electronics.