Air Conditioning Theory & Practice

CN90 » I just hooked my pressure gauge to the low-pressure port and made an interesting observation:

1. With AC button ON and Fan Speed at Low (@1/4 of the slide button): the AC Clutch cycles, and the pressure fluctuates between 22-35 psi. When the pressure is below 25 psi, the AC Clutch cuts off, and the cycles repeats itself.

2. With AC button ON and Fan Speed at High (@3/4 —> Max of the slide button): the pressure hold steady at 32 psi, which is above the cut-out point and the AC compressor runs continuously.

My theory is when the Fan Speed is on 3/4 —> Max, there is more heat removed the Evaporator, therefore more evaporation from liquid to gas states (simple Physics 101). Therefore more gas reaches the low-pressure side —> keeping the pressure steady on the low-pressure side.

Air Conditioning Theory & Practice

 SHOULD A/C COMPRESSOR CYCLE ON/OFF WHILE DRIVING?

wheelsup » I finished the evaporator repair today. Replaced it and the expansion valve/core and drier. Sucked it down with a not great but useful vacuum attachment from Harbor Freight, got to around 29″, so not super low. But I did it a lot, at first vacuum would rise as the moisture was still in the system but after doing it about 8 times the level was solid. Attached the 134a and vented a bit to ensure the line was full of refrigerant and added roughly 22oz per sticker.

Pressure when running is around 38-39 psi on the low side.

When sitting in the garage with a/c on the compressor would stay on, solidly. Never cycled.

Got it out up to 55 mph and I noticed the compressor will cycle on/off, with the selection on “full cold” on the electronic control system. I can see the thing cycling if it was set to a temp, but I would think for “full cold” it should command it to run all the time. I noticed output temp while driving was around 48-49F and would stay at that level. Temp while not driving was a “warm-ish” 62F-63F which seemed higher than what I was expecting, although temp on dash indicated 43C, so it was probably pretty warm in the enclosed area. Not too concerned because of the nice cool temp while driving.

Is something amiss? I don’t think there is a leak that bad yet where it’s cycling due to pressure loss as it did when first refilling (seemed to cut in at 20 psi and cut out at around 40 psi). My only experience with a cycling compressor is when the refrigerant level is low, but I don’t believe that to be the case here.

P.S. Here is my evaporator…smells MUCH better now!!

JDS60R » Its a standard fitting and line you can get them at most any auto store. Or back to HF for a good price on one.
In TN I charge after full vacuum pull down. After a full evacuate the system charge gives me roughly 32 low side ( with charge bottle off and system stabilized) and 200 high side. It cycles at idle and at highway speeds and air comes out center vents at 42 degrees when still and 38 degrees when moving.

wheelsup » I felt that the system might be overcharged as well when first putting it in. I bought two cans @ 12 oz each so total of 24 oz. I did not put all of the 2nd can in, left an once or two. Placard on strut tower indicates 1.63 lbs of 134a which is around 22 oz. I can’t imagine even if I had used the two cans completely, that 2 oz over would overcharge.

However my low side pressure is much higher than yours, at 38-39 psi. So it’s possible it is overcharged. 

Of interest. Before doing this repair I simply tried to charge the a/c. Not the best way to fix the issue but wanted to get through another summer with the least amount of work. I added one can and the compressor was running but no cold air was coming out. I added the second can and as I was doing that the accumulator blew out of it’s quick release. Low pressure was obviously too high. This has never happened to me before. 

I then decided to change the evap. I took the system apart and also changed out the orifice tube in the low pressure line – it was not plugged up what so ever, very little debris. I replaced the evap and accumulator, sucked it down as best as I could multiple times (29″ or so, give or take over the course of 30 mins). Attached the can and tap and bled the line to ensure no air in the charging hose and opened the service port. Charged up and the system does blow cool, not freezing cold, but cool air 48* or so. 

From what I am reading I do not believe I am dealing with a blockage, as that would indicate as low pressure (although hard to tell without getting the high side reading) in addition to very little debris on orifice tube showing no signs of internal damage (yet). The over pressure while charging before replacement is a new one for me, in the past I’ve just added 134a and gone the whole summer without issue.

I’m not home under Tuesday night, I will search out that adapter. It looks like I unscew the high pressure switch and some sort of adapter screws on (or does the hose screw to it directly? Looks to be too tall for that. The quick connect does not fit on it). I cannot connect my hose to that so will need an adapter.

I’ve had good luck with repairing a/c systems in previous cars, flushing the system and charging etc. so I’m not a total newbie at this. Really frustrated this time though.

Air Conditioning Theory & Practice

Last Updated on June 11, 2024