Intermittant Ac

Jcol · May 24, 2010

Last summer my trucks AC started to act up. It would blow nice and cold and work well until the outside temps got above 85 F. Above that temp it would work OK for about 20 minutes and then stop. The compressor would stop cycling. This occured sooner in stop and go traffic then on the highway. In fact going 65 to 70 mph down the road the AC would work all day reguardless of the temp however if you stopped for gas it wouldn't start up again. Thinking it may be low on freon I bought a 134 canistor with built in guage from Walmart. I waited till it got hot and stopped working and put on the cylinder. I was surprised to see that the guage read that the low side was over pressure. After letting the truck sit and cool down, the compressor started to cycle again and I checked the guage and discovered that the low side pressure had lowered and was now in the green zone. I watched as the truck heated up and noticed that the low side pressure also increased till it was high again. At this point the compressor stopped cycling. So I thought I knew the problem. For some reason pressure was building in the system. I did not add any gas from my Walmart can, but I did take it to a garage with a good rep for AC repair. I explained the symptons to them and they did a thorough check of the system. They changed out the old freon and replaced it with the proper amount along with lub. They saw the pressure build up on the low side and focused on the condenser unit. They removed it and cleaned it, they also changed the clutch on my fan. They basically said that my system wasn't getting enough air flow across the condenser to remove the heat from the system. They said that cleaning the condenser and improving the fan action should solve my problem. This was late last August and the tempartures got cooler so I never really got the truck back into the heat again. The AC worked fine all through September however this weekend I was on a bit of a trip and hit some warm weather and sure enough the AC cut out again after stopping for gas on an interstate. The temp was just under 90 F. It's just the same as it was before. I plan to take the truck back to the garage and work with them on this, however I'm thinking there must be something else going on. BTW the truck has no issues with it's coolent temps. The temp guage sits and just below 200 F reguardless of what I'm doing or what the outside temp is. Sorry about the long post. Any advise would be appreciated I already spent quite a bit last year on this problem and I would like to go back to the garage with a little more knowledge. Thanks.

dewfpo · May 24, 2010

From a general A/C standpoint : If pressure is getting higher on the low side with the compressor running there may be a problem with the Orifice Valve or more likely, your compressor. With the compressor running the low side should stay between ~25-45 psi no matter how hot it is outside. Some compressors (Sanden) misbehave when they get hot. Some have a high pressure and/or high temp cutout switch to protect the compressor. If the correct amount of R134a and PAG oil were installed, the compressor should not overheat or overpressure if there is sufficient airflow thru the condenser core and the engine compartment at 90F. How many miles are on the truck? Do you live in a hot climate where it is used 10 months of the year?

DEWFPO

Jcol · May 25, 2010

From a general A/C standpoint : If pressure is getting higher on the low side with the compressor running there may be a problem with the Orifice Valve or more likely, your compressor. With the compressor running the low side should stay between ~25-45 psi no matter how hot it is outside. Some compressors (Sanden) misbehave when they get hot. Some have a high pressure and/or high temp cutout switch to protect the compressor. If the correct amount of R134a and PAG oil were installed, the compressor should not overheat or overpressure if there is sufficient airflow thru the condenser core and the engine compartment at 90F. How many miles are on the truck? Do you live in a hot climate where it is used 10 months of the year?

DEWFPO

There are about 85.000 miles on the truck. I live in Ontario so we really only have about 5 months where the AC is used. Thats why when I had the work done late last August I really didn't get much 90 degree F weather to try it in. It does seem to be related to air flow in hat it will work going down the highway, but at slow speeds when it has to depend more on the fan pulling the air through is when the over pressure seems to build. On days below 80 or 85 it can keep up with it.