Its hot in florida, and my damn AC sucks. After the ac runs for like 3 min It cycles on and off like it is low on freon but when I put my gauges on it it reads 35-40psi. Also when idling and it kicks on and off there is a loud clunk you can feel in the floor.

The system might be overcharged, did you add refrigerant yourself? R134 systems are more critical with charge levels and pressures than the old R12 systems were, and to do it properly requires a full set of manifold gauges.
The more noticeable cycling you are getting may be the high pressure cutout operating rather than the normal cycling. Have a shop check it out.

yea i know my ac stuff i have a full set of gauges and its running in the spec pressures. i just hate to start replacing switches and sensors if it doesnt need it. i didnt know if someone else had this problem?

compressor could be getting weak, those low side pressures are a tad bit high imo

Are your fans working properly?

What is the high side pressure when it starts cycling? if your condensor fan is not running it might be cycling due to over pressure. low side pressures will also be high if its overheating. you might want to recover and evacuate the system (good 15 min. vacuum) then recharge. I have found that after a few recharges with out pulling a vacuum you will get a fair amount of air in the system from the charge hoses. Air will make pressures like a prop. charged system, but will not cool at all. As always with a/c systems, it's almost always back to the basics. let me know if this helps.

thanks

Pulled the info below from my Service CD, low pressure side "normal" is between 18-28psi. If you are at 35-40psi this could be the problem ( overfilled like Mulder suggested ).
--------------------------------
A: OPERATION
1) Place the vehicle in the shade and draftless condition.
2) Connect the manifold gauge set.
3) Open the front windows and close all doors.
4) Open the hood.
5) Increase the engine to 1,500 rpm.
6) Turn ON the A/C switch.
7) Turn the temperature control switch to MAX COOL.
8) Put in RECIRC position.
9) Turn the blower control switch to HI.
10) Read the gauge.
Standard:
Low pressure: 127 - 196 kPa (1.3 - 2.0 kg/cn?, 18 - 28 psi)
High pressure: 1,471 - 1,667 kPa (15 - 17 kg/cn?, 213 - 242 psi)
Ambient temperature: 30 - 35 OC (86 - 95 OF)

back from the dead as the weather starts to warm up...

So I've read a few posts about my A/C. It was not blowing cold air and the compressor was not coming on until I put just a little bit of refridgerant in it. I guess enough to bring it up to a level surpassing the failsafe switch for low pressure. I noticed that with the gauge hooked up to the low side, the needle on the gauge moved a little bit while the compressor was running but as it cycles off and stops turning, the pressures increased dramatically. Everything I've read says to turn on the A/C to max, have it running, blah blah blah, so I assume that the pressures for all of these directions would be with the compressor spinning. However, when it is not spinning, what are the low side pressures supposed to be?

back from the dead as the weather starts to warm up...

So I've read a few posts about my A/C. It was not blowing cold air and the compressor was not coming on until I put just a little bit of refridgerant in it. I guess enough to bring it up to a level surpassing the failsafe switch for low pressure. I noticed that with the gauge hooked up to the low side, the needle on the gauge moved a little bit while the compressor was running but as it cycles off and stops turning, the pressures increased dramatically. Everything I've read says to turn on the A/C to max, have it running, blah blah blah, so I assume that the pressures for all of these directions would be with the compressor spinning. However, when it is not spinning, what are the low side pressures supposed to be?

Good question -- I'm wondering the same thing.

Does anyone have access to a shop manual online or hardcopy that can look up what the low side pressure is when the compressor is not spinning? Or can anyone out there with a proper pressure gauge for our A/C system go check what their low side pressure is when the compressor is not spinning?

Also, if the compressor cycles on and off in very short intervals (like 30 sec) thats indicative of too much of a charge in the system, correct? Is there anything else that would cycle the compressor on and off like that?

Does anyone have access to a shop manual online or hardcopy that can look up what the low side pressure is when the compressor is not spinning? Or can anyone out there with a proper pressure gauge for our A/C system go check what their low side pressure is when the compressor is not spinning?

Also, if the compressor cycles on and off in very short intervals (like 30 sec) thats indicative of too much of a charge in the system, correct? Is there anything else that would cycle the compressor on and off like that?

When I was charging my A/C system last night I saw low side pressures similar to what the OP reported -- about 35 to 40 psi when the compressor cycled off.

Also, I noticed that with the compressor running the pressures would vary quite a bit from about 15 to 25 psi (low) and 150 to 240 psi (high).

After turning the car off, the low&high pressures equalized at about 95 psi.

Unfortunately, the pressures quickly dropped to 60 psi and when I left for work this afternoon they were back down to zero.

Now I need to find out where the leak is. There is nothing obvious -- no signs of compressor oil around the fittings, no hissing sound.

I'm wondering whether to use the 134a with dye in it; buy a professional UV leak detection kit; or use gas leak detection liquid and pressurize the system with either more 134a or nitrogen.

Any suggestions?

When I was charging my A/C system last night I saw low side pressures similar to what the OP reported -- about 35 to 40 psi when the compressor cycled off.

Also, I noticed that with the compressor running the pressures would vary quite a bit from about 15 to 25 psi (low) and 150 to 240 psi (high).

After turning the car off, the low&high pressures equalized at about 95 psi.

Unfortunately, the pressures quickly dropped to 60 psi and when I left for work this afternoon they were back down to zero.

Now I need to find out where the leak is. There is nothing obvious -- no signs of compressor oil around the fittings, no hissing sound.

I'm wondering whether to use the 134a with dye in it; buy a professional UV leak detection kit; or use gas leak detection liquid and pressurize the system with either more 134a or nitrogen.

Any suggestions?

Let me get this straight:
Low side
Engine running, compressor spinning - 15-20 psi
Engine running, compressor not spinning - 35-40 psi
Engine not running, compressor not spinning - 95psi drop to 60psi, then after sitting for awhile, 0psi.

I guess what I'm looking for is what the normal pressure for the low side is with the engine running and compressor not spinning. I had charged the system a little bit so the compressor would kick on and I'd see pressures of about 20psi but it would cycle on and off quickly like it was overcharged. However when the compressor was not spinning during those brief on/off cycles the pressure would climb quickly to like 60ish psi and then gradually increase until the compressor kicked on again. I dont know ultimately what the pressure would've settled on since the compressor would start up but I didnt like the fact that it was that high. I never checked it with the engine not running either.

It sounds like you've got a pretty quick leak going. I'd probably have it serviced so they can pull all the moisture from the system out. If its leaking that quickly, I'm sure its been exposed to the outside environment. I'm sure a shop could easily fill it up with the UV dye and tell you where its coming from, then when you fix that leak, have them service it.

Let me get this straight:
Low side
Engine running, compressor spinning - 15-20 psi
Engine running, compressor not spinning - 35-40 psi
Engine not running, compressor not spinning - 95psi drop to 60psi, then after sitting for awhile, 0psi.

I guess what I'm looking for is what the normal pressure for the low side is with the engine running and compressor not spinning. I had charged the system a little bit so the compressor would kick on and I'd see pressures of about 20psi but it would cycle on and off quickly like it was overcharged. However when the compressor was not spinning during those brief on/off cycles the pressure would climb quickly to like 60ish psi and then gradually increase until the compressor kicked on again. I dont know ultimately what the pressure would've settled on since the compressor would start up but I didnt like the fact that it was that high. I never checked it with the engine not running either.

It sounds like you've got a pretty quick leak going. I'd probably have it serviced so they can pull all the moisture from the system out. If its leaking that quickly, I'm sure its been exposed to the outside environment. I'm sure a shop could easily fill it up with the UV dye and tell you where its coming from, then when you fix that leak, have them service it.

Thanks for the reminder maengelito -- I've been so busy with other stuff I temporarily forgot about the A/C. I would have remembered when I get into a hot car though!

I talked to an A/C tech at work. He works on the HVAC systems on the trains and does side work on residential A/C (not much on cars) but he said it's normal for the low and high side pressures to begin to equalize when the compressor cycles off. That explains the low pressure increase to 35-40 psi I saw.

I spoke with a local gas station owner the other day and he said that on a Subaru with high miles like mine (~145K) that it's almost certain that the compressor seal is leaking. Either around the outside, or around the inside between the seal and the shaft. The former is a cheap fix (new seal), the latter means new compressor.:(

I need to take the compressor clutch off to check for signs of oil leakage.

I'm afraid he may be right because I can't see _any_ signs of leakage anywhere else.

I'm going to try and fix the leak if I can find it and then take your advice and take it to a shop for a vacuum and fill.

Someone here at nasioc posted the normal low side pressures as 18 to 28 psi. IIRC, those numbers came straight from the SOA service manual but you should double check. FWIW, the HVAC guy at work said those numbers seemed a little low for R134a. He showed my on the low side A/C gauge that a certain pressure = a specific temp. He said we should shoot for 32 degrees F, which corresponded to a higher pressure (maybe ~ 32 psi, IIRC).

Please double check the above as I'm not certain it's accurate. Let me know if you come up with something different.