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Help Diagnosing A/C issue - compressor engages but no cycling

dfwcarguy on Wed May 16, 2012 10:48 PM User is offline

Year: 1994
Make: Chevrolet
Model: K1500 4x4
Engine Size: 350
Refrigerant Type: 134a
Ambient Temp: 85
Pressure Low: 25
Pressure High: 255
Country of Origin: United States

After spending some hours reading here, I need to post a question.

I started this process trying to troubleshoot an idle air conditioning issue. I was not getting very good a/c performance at idle when temperatures were over 80. When temps are over 85 I don't seem to really get the a/c performance I want at idle or in motion.

The truck was purchased about 8 months ago. Previous owner provided receipts for a new Orifice tube and dryer. Dryer definitely is new. Also, a few of the hoses look new - mostly the input hose to the dryer coming from the compressor. The compressor has a Murray sticker on it, so I am assuming that may have been changed at some point.

The a/c seems to perform as desired when temps are below 80 - in the 72-77 range. Ambient humidity around 20 to 30 percent. The issue comes when the temp gets to 80 outside - the compressor will never cycle - just stays engaged.

Here is what I have done so far.

On the way to work in the morning, when temp is 76 or 77 outside, the a/c compressor will cycle all the way to work while driving, and even sometimes when idling at a stop light. And it seems to cool as it should. Verified w/ handheld scanner watching the A/C engagement parameter.

Static pressure @ 84 degrees ambient temp in the shade and the engine warm after driving home 13 miles non-highway from work. By static I assume not running.

75lbs on low and high side

I then started the truck and let it run for 3 minutes at idle. Vent temp was coming in at 49 degrees w/ a digital thermometer sitting outside the vent in the center. Pressure was:

37 low / 240 high. The compressor never cycled off.

I then increased idle w/ an assistant to 1900 rpm, and pressure went to:

25 low / 255 high. Held for a few minutes, and again the compressor never cycled off. A/C vent temp went down to about 44.

I then took the truck for a drive w/ my hand held scanner reading real time data about the a/c engagement. On a 15 mile trip around down - sometimes cruising on main roads going 50 mph for a few minutes, I never saw the compressor disengage. A/C vent temp is better when moving - coming in around 43 degrees or so, again w/ digital thermometer.

I came home, hooked the gauges back up. Then, I tried the hose showering on the condenser I read about here. When doing that and holding idle at 1500 ~ 1800 rpm, the low side worked it's way down to 20 psi, and the high side pulled all the way down to 150 psi - but after initial dousing of water it was at 200 psi. Only went to 150 after some time. The temp inside went down to 40.5 degrees at the vent w/ digital thermometer and was really fogging it out and felt "right". Ambient temp was 84 degrees per digital thermometer. BUT, the compressor STILL never cycled off.

What the heck is going on? The fan clutch seems to function as it should, it was pulling all sorts of water thru the condenser / radiator and throwing it around. Plus, it does sound like a tornado when I start it in the AM and up to about 15mph before it begins to free-wheel. I noticed my truck does not have a condenser fan, but when I look up on various auto parts sites I do see one listed. Did this thing get removed by the previous owner? Or am I battling a weak compressor and air movement / condensor issue?

After I shut off the truck, the pressure equalized at about 45 lbs low/high. Not sure about that one either.

Thanks for any assistance or ideas.

Thanks for the help.

GM Tech on Wed May 16, 2012 11:00 PM User is offline

Equalized at 45 psi??? That is way too low---should be about 100+ psi on a hot engine. So throughout all this, I never heard how much refrigerant you have in it...You need to recover and weigh the refrigerant, this is your number one diagnostic info....You may have 1 lb in a 2 lb system for all we know...If it were mine, I'd measure the refrigerant first- so find someone with a reclain machine- pull and weigh the charge and then vacuum and put the right amount back in...then report your pressures and temps...

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

iceman2555 on Wed May 16, 2012 11:05 PM User is offlineView users profile

Seriously doubt that the problem is with the compressor. The indicated pressures tend to dispel this problem.
There is no mention of a possible evac and recharge to OE spec's in this post, so it is assumed that you are attempting to determine a performance issue without knowing if the system is properly charged. A suggestion would be to have the vehicle serviced by someone that has the correct equipment to complete a system recovery, evacuation and recharge. The recharge should be completed utilizing the correct recharge equipment, not cans and pressures. Pressures are not an indication of a fully charged system. The first and most important aspect of a system diagnosis is to know that the system is charged properly. Have this procedure completed and then post results of your test.
Good luck!!!

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

dfwcarguy on Thu May 17, 2012 2:14 PM User is offline

I do not have the history on an OE evac and recharge by the previous owner. I will get that scheduled in the next few days and report back the new reading.


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