2nd Gen Non-Engine/Transmission Finicky Air Conditioning

[surfbeetle]

My AC on my 95 2500 is giving me grief. In June of 2013, I had the system rebuilt, new compressor, dryer, recharged, etc for about a grand. It worked fine that summer. By November, I bought a new car and the truck gets driven one a week or when towing the fifth wheel. I have only put three tanks of fuel in it since last November so that's around 2,000 miles or less. I'm in So Cal so the AC is on most of the time. This summer, when pulling the trailer, the AC would cut off when stopped at a light after getting off of the highway and the truck was hot. Once we were moving again, it would blow cold.

Last week, I brought it into a local Radiator/AC shop and we decided to change out the 19 year old radiator and fan clutch. Yesterday, it was 95 degrees out and I drove the truck empty about 7 miles on stop and go surface streets then 5 miles on the highway. The air was blowing extremely cold, I believe colder than before the recent work was done. But when I got off of the highway, the A/C warmed up. After idling for 10 minutes in a parking lot, I headed down the road and it was another couple of miles at 50 mph before the A/C started cooling off again. Any ideas? I was thinking that maybe there's a high pressure switch that has gotten weak and is cutting out too soon/low?

Has anyone else had this type of issue? We expected that since the fan was now actually pulling in air now that it would resolve my AC problem
Thanks in advance
Craig

 

[surfbeetle]

Wow, 19 views and no one has a comment? I took it in the shop that rebuilt the AC and did the radiator/fan clutch, they ran it today and could not get the AC to not blow cold. I don't think they drove it however, probably just let it idle. Anyone have this kinda thing happen and have it turn out to be the computer?

 

[JR]

Did you replace your expansion valve when all the other components were replaced? What pressures are you seeing on the high and low side? You need a good baseline on both when the system is operating correctly and then again when it starts blowing warm, this will help a lot in diagnosing. If your a/c repair shop can't figure this out, you might wanna look for another shop. I mean no offense by saying that, but a/c systems are relatively easy to diagnose PROVIDED you have the correct equipment.

 

[fest3er]

The PCM could be failing (it should disable A/C under full throttle--something diesels don't worry about. Temporarily jumper the input lead (+12V, PCM-side) of the low pressure switch to B+ to bypass the computer's control.

Yes, it's possible the high and/or low pressure switches are flaky. I know you can remove the low pressure switch, clean it with a very light solvent, thoroughly dry it and reinstall it. Or take it off and shake it; if it don't sound/feel right, replace it. The high pressure switch should be similar.

Next, ensure the clutch (even though it's new-ish) completely engages. If it's slipping, A/C might not work well at idle.

Ensure the fan clutch works right. Block off the sides of the radiator so that hot air reaches the fan's bi-metal control strip. Restrict the exhaust to make the engine work a little while idling. The fan should come on soon enough.

In all these years (17 with my '98), I've never touched the high pressure switch. I replaced the low pressure switch a time or two. Shop bypassed the PCM's A/C clutch control after that function failed completely. The evaporator leaked since the day I bought the truck (10/18/97). I finally replaced it a couple years ago along with the dryer; the A/C's worked fabulously since. I had the shop put 3 ounces extra R134 in it. I have no problems cooling while idling in traffic.

In sum, patience. And carefully think your way through the system.

 

[surfbeetle]

JR, I don't know off hand if the expansion valves were replaced, I'll have to check. fest3er, the fan clutch is new and I can actually hear the fan pulling now more than it has in a long time. I'll have to start messing with the PCM and bypass, seems like it's about time for it to start failing me. I wonder if anyone has come up with an aftermarket or Megasquirt option for it. Thank you both for your replies.

 

[surfbeetle]

After a lot of reading and searching, I'm heavily leaning towards it being the PCM (or SBEC or ECU, whatever you want to call the engine controller). One post said to try a bag of ice on the PCM when the A/C or other issue suspected to be cause by the PCM acts up. I'm going to pick up the truck this week and give that a try. If once the PCM is cooled back down and the A/C begins to work, then I'll look at getting a replacement. The next question will have to be, do I get a dealer replacment, find a rebuilt one at a local autoparts, or find a rebuilder online that can do this? Since I work on computers and have done some soldering, I also wonder if I could just open up the box and touch up the solder points in the PCM. I've never seen the inside of a 1995 PCM so I don't know if that's even an option.