A/C compressor cycling quickly, 1999 Tacoma V6

I'm looking for tips on trouble shooting my A/C problem.

I've had my 99' Tacoma since new in 99' and the A/C has been spectacular. It just started a new problem where the compressor runs for about 10 seconds, then turns off for about 10 seconds, then the cycle repeats. Cooling capacity is probably 50% of normal and as best I can tell the refrigerant level is OK. There are bubbles in the sight glass immediately after the compressor starts, and immediately after it stops, but no bubbles while the compressor is running at steady state (engine at 1500 rpm as per the Toyota service manual).

With the engine at 1500rpm and the A/C is on high and recirculate (this is the setting where it cycles quickly) both high and low side pressures drop down (still within spec), and when the low side hits about 22 psi the compressor turns off. With the A/C on fresh air high, the compressor doesn't cycle as quickly and the low side is within spec, between 30 and 35 psi, and the high side gets a little higher than spec, up to about 270 psi. But high side is affected by ambient temps (about 75F) so this might be normal high side pressure, I'm not sure. The low pressure line is very cold and the high pressure line is slightly warmer than ambient (normal).

At this point there are no bubbles in the sight glass while the compressor is running, and there are bubbles immediately after the compressor starts and immediately after it stops, so the refrigerant level seems normal to me judging by the sight glass. I'm stumped, please give me your suggestions!

 

Dude I hope thats a typo.
If not sounds like you have clogged expansion valve.

 

Not a typo, I had the same thought..

Though, by memory, I believe the manual said up to about 250 was the normal range, and that it was affected by ambient temps. The high side pressure went up considerably when I switched from recirculate to fresh air. On fresh air, 1500rpm, it slowly fluctuates between about 250 and 270psi.

Do you know what range is normal for this truck when ambient temps are about 75-80F?

 

Been a long long time since I did this sort of work, but I'll take a stab... faulty low pressure switch?

 

"Short cycling" is sometimes the result of an overcharged system.

 

Air bubbles are a big indication of a problem. You should never see air bubbles. You most likely have a loss of refrigerant, resulting in a low charge, but air has been drawn into the system, resulting in erratic, and excessively high pressures. Normal charge for your system is 20.8 oz.

First thing is to do a static pressure check. With the compressor off, and the engine not running, pressure should stabilize at around 70 PSI with an outside temp of 70º F.

Normal operating pressures should be 25-35 psi on the low side, and 120-150 on the high side, with an outside temp of 70º F.

Having a 250PSI high side is borderline hitting the compressor's pressure relief valve, which may have happened, and that is why you lost some refrigerant.

If your compressor keeps cycling fast like that, and the low side drops low really fast, and high side goes up really fast, then the compressor cycles off, you have air in the system, and a low charge.

 

OK guys, I took it to the pros and the consensus was it was about 8oz short on refrigerant. Apparently they sucked it out and added the right amount by weight. It still cycles faster than I remember, but it's blowing cold now so maybe my memory isn't good. The valves where you attach the gauges were leaking and that's how the refrigerant got out. I will find out if this was really the fix once summer gets here. I'm not sure what caused the higher than expected high side pressure, but when I switched from fresh air to recirculate the pressure came down quite a bit. I will follow up if this turns out to not be the fix. Thanks everyone for the tips and help!

 

Nailed it! As for the high pressure being excessive, I am sure what happened was some air got drawn in due to the leak, and that extra air will ramp up the pressures fast, and cycle fast as well. Seen it many many many times in my shop.

 

I posted too soon; the problem is not fixed. Today was the first warm day since I got it "fixed" and the symptoms returned. The compressor runs for about 10-15 seconds then just as the air starts to get cold it turns off and the air goes back to "cool". With the AC fan on high the compressor runs a little longer, but still turns off just as the air starts to get cold. At idle in the driveway the compressor will run for some time with the AC on high, but when driving the compressor starts cycling too quickly. I'll try to get time to put the gauges back on it this weekend, but I'm wondering if I do have a problem with the expansion valve somehow. Maybe debris in it. Are there any moving parts in the expansion valve, anything to actually go bad?

 

Yes there are moving parts in the expansion valve...

http://www.docstoc.com/docs/108624055/The-inner-workings-of-a-typical-thermostatic-expansion-valve

 

Check the fan clutch for the belt driven fan. Also check for any blockage in the condenser. Both of these can cause an issue, but is least common.

 

Thanks for the suggestion. The condenser is clean and the fan clutch is good. As I hold the motor at 1500 rpm when the radiator gets hot the fan clutch locks up and a LOT of air starts blowing. The high pressure line is close to ambient temp and the low pressure line is cool and sweating near the evaporator. I saw the high pressure gauge occasionally jump a few psi suddenly and go right back down, as if someone smacked the gauge. I'll get some video of the gauges when I get time to hook them back up. Cooling capacity is definitely still down, it's only about 82F this morning and I had the AC on high to stay cool, and the compressor was cycling on and off every 20 or 30 seconds as I was driving.

 

https://www.youtube.com/watch?v=RkKry31lk1E is an interesting diy of an expansion valve. If there is any sticking, the hi side pressure switch may shut off the compressor b/f proper cooling.