AC Not Cooling (Again!)
Senior Member
Joined: Apr 2019
Posts: 853
From: Albuquerque NM
Many have suggested a pressure check of the system and that's where you need to start, if the compressor is working hook up your AC gauges and look for the following pressure at idle.
85°F (29°C) 45-55 psi -Low side / 225-250 psi -High side. If you cannot get the compressor to run you can remove the clutch connector and jump it with a fused wire to 12v. This will engage clutch even if the system is shut off in the cab.
Notice this value is at an ambient air temp at 85F, if it hotter outside they will be a little higher, if its colder then they will be lower.
This is the starting point, If you don't see something close to these values and the compressor is turning then if its low you have a leak, if the readings are very high you might have a stuck or bad expansion valve or blockage in the system.
85°F (29°C) 45-55 psi -Low side / 225-250 psi -High side. If you cannot get the compressor to run you can remove the clutch connector and jump it with a fused wire to 12v. This will engage clutch even if the system is shut off in the cab.
Notice this value is at an ambient air temp at 85F, if it hotter outside they will be a little higher, if its colder then they will be lower.
This is the starting point, If you don't see something close to these values and the compressor is turning then if its low you have a leak, if the readings are very high you might have a stuck or bad expansion valve or blockage in the system.
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Junior Member
Joined: Oct 2014
Posts: 18
From: Pittsburgh
Here's what I have done to solve this problem. Had the system evacuated of whatever amount of R134a might still be in it (knowing I was going to open up the system). Drove the truck home and pulled a quick vacuum down to 30" to verify no leaks - none were found, the vacuum didn't drop even after holding for 60 minutes.
Since the accumulator/dehydrator, orifice, and compressor had just been replaced, I went to the next possible culprit - the condenser. I pulled out the condenser and it actually looked fine. Although I wanted to do a flow test or a pressure drop test to check for restrictions and confirm that the condenser was indeed the problem, I had to resort to a simple air flow test due to a lack of test equipment. The air flow test (with out test instruments) seemed to be ok. Nonetheless, I dropped in a new condenser, added 2 ounces of PAG46 to the system (for the new dehydrator installed a couple days ago), and installed a new HP pressure sensor (some people call these a switch, but it's actually a sensor). With everything reassembled and closed out, I pulled a vacuum on the system and like before, it dropped to just over 30" in just a few minutes. Continued to purge the system for 2 hours. Stopped the purge, locked in the vacuum and held for another hour.
Recharged 1.6 pounds of R134a. I did this over the course of about 2 hours - very slowly with the AC on max, all windows shut except driver's side which was open 5 inches, with the system in recirc mode and measuring the left center air discharge plenum in the dash - the one that blows on the driver (if you chose). After charging about the first 6 ounces of R134a, the compressor finally started and then begin cycling on and off quickly (like they do when low on refrigerant). At this point the ambient air temp was 86 degrees, humidity was 69%, and the air outlet temp in the cab was almost 90 degrees. Continued adding refrigerant and the compressor cycling slowed until it eventually continued running steady. At this point, about 14 ounces of refrigerant had been charged. LP pressure about 43 pounds, HP pressure about 190 - air outlet was 72 and condensate was starting to come from the condensate discharge hose under the vehicle. It was making cool air (sort of) but the system was still not where it needed to be and the low and high pressures were not in spec. Continued charging and eventually reached the 1.6 pounds.
Air outlet temp in the cab was now 65 - right where it should be. LP pressure was 58 psi, HP pressure was 250 psi. Note - the specs with ambient temp >85 and humidity is >60% are 53-74 psi (low) and 243-316 psi (high) - with these conditions, the inside cab air outlet temp max can be as high as 79 degrees. So, the conclusion is that somewhere between replacing the condenser and adding 2 ounces of PAG 46 plus throwing in a new HP pressure sensor, the AC is now running fine.
Thank you to everyone who contributed to this thread. Sometimes it just takes reading someone else's thoughts to get my own mind headed in the right direction.
Since the accumulator/dehydrator, orifice, and compressor had just been replaced, I went to the next possible culprit - the condenser. I pulled out the condenser and it actually looked fine. Although I wanted to do a flow test or a pressure drop test to check for restrictions and confirm that the condenser was indeed the problem, I had to resort to a simple air flow test due to a lack of test equipment. The air flow test (with out test instruments) seemed to be ok. Nonetheless, I dropped in a new condenser, added 2 ounces of PAG46 to the system (for the new dehydrator installed a couple days ago), and installed a new HP pressure sensor (some people call these a switch, but it's actually a sensor). With everything reassembled and closed out, I pulled a vacuum on the system and like before, it dropped to just over 30" in just a few minutes. Continued to purge the system for 2 hours. Stopped the purge, locked in the vacuum and held for another hour.
Recharged 1.6 pounds of R134a. I did this over the course of about 2 hours - very slowly with the AC on max, all windows shut except driver's side which was open 5 inches, with the system in recirc mode and measuring the left center air discharge plenum in the dash - the one that blows on the driver (if you chose). After charging about the first 6 ounces of R134a, the compressor finally started and then begin cycling on and off quickly (like they do when low on refrigerant). At this point the ambient air temp was 86 degrees, humidity was 69%, and the air outlet temp in the cab was almost 90 degrees. Continued adding refrigerant and the compressor cycling slowed until it eventually continued running steady. At this point, about 14 ounces of refrigerant had been charged. LP pressure about 43 pounds, HP pressure about 190 - air outlet was 72 and condensate was starting to come from the condensate discharge hose under the vehicle. It was making cool air (sort of) but the system was still not where it needed to be and the low and high pressures were not in spec. Continued charging and eventually reached the 1.6 pounds.
Air outlet temp in the cab was now 65 - right where it should be. LP pressure was 58 psi, HP pressure was 250 psi. Note - the specs with ambient temp >85 and humidity is >60% are 53-74 psi (low) and 243-316 psi (high) - with these conditions, the inside cab air outlet temp max can be as high as 79 degrees. So, the conclusion is that somewhere between replacing the condenser and adding 2 ounces of PAG 46 plus throwing in a new HP pressure sensor, the AC is now running fine.
Thank you to everyone who contributed to this thread. Sometimes it just takes reading someone else's thoughts to get my own mind headed in the right direction.
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