AC compressor cycling every 7 secs but still cold

[cn90]

Update 3 days later,

Somehow the AC compressor now runs fine, no more cycling. Whenever I push the AC button, the AC Clutch is engaged and the compressor runs WITHOUT any cycling at all. Maybe the 2 cans of R134a take a few days to "settle in".

I also did the zip tie ghetto trick to bring the AC Clutch gap from 0.8 mm down to 0.3 mm.

Anyway, everything is working fine for now...

[jblackburn]

It all depends on ambient temperature.

If it's cycling constantly at ~80, you need more R-134. It's normal to cut in/out every 15 seconds or so at around 60-70.

[cn90]

It was in fact the same ambient temp for both of the days.

Day 0: Apr 4, 2013....it was 66F.
Day 3: Apr 7, 2013...it was 67F.

[precopster]

I always thought the ambient temp sensor reports to the driver's display panel. The internal cabin temp sensor would be a bigger factor in controlling the frequency of cycles. For example if you leave the car sitting on a hot day and have a 100 degF cabin temp the compressor would stay on longer until the pre-determined pressure switch limits are reached. Then it would continue on a on/off cycle until the temperature the climate is set to is reached, then the CCM would turn the system off, but only until the temp goes up again.

[JRL]

TWO cans????!!!
The entire system uses almost exactly two cans (1.63 lbs, do the math).
One can is 12oz, 16 ounces to a pound
You're either very overfilled or you were dead empty!

[cn90]

Yeah I know, I looked up the spec beforehand and knew that the system holds some 1.63 lbs.

- Then ran the AC, it kept cycling every 6-7 sec. Pressure varies between 22-35 psi. So I thought I was low on R1341. A little loss of R134a/year is normal, so I figure the system was low, which is expected after some 16 years...

- Filled with 1 can (12 ounces), no change in pressure.

- Filled with 2nd can (12 ounces), a bit better and knowing that the system holds 1.63 lbs, I stopped after 2 cans. Immediately after that, the system still cycled every 6-7 sec.

But 3 days later, it seems to "settle in" and all is good now. Maybe I over-filled it, but the system runs fine, AC is nice and cold, so I would leave it alone for now.

PS: Should I release some R134a?

[jblackburn]

I always thought the ambient temp sensor reports to the driver's display panel. The internal cabin temp sensor would be a bigger factor in controlling the frequency of cycles. For example if you leave the car sitting on a hot day and have a 100 degF cabin temp the compressor would stay on longer until the pre-determined pressure switch limits are reached. Then it would continue on a on/off cycle until the temperature the climate is set to is reached, then the CCM would turn the system off, but only until the temp goes up again.

It depends on the temperature of the evaporator (so the in-car air is taken into account, especially if you run it on auto and the car defaults to recirculate).

The low-pressure switch is set at 25 PSI. The car reaches that only when a) the freon level is low or b) pressure is pretty stable and the refrigerant isn't removing much heat from the air (such as a low fan speed setting on a cool day).

There are no temperature switch settings on these cars - it's a very old-school AC system. There's a high temp cut-out for the compressor, and then just high and low pressure cut-out switches. That, with the nature of R-134a itself, keeps the AC working in a temperature range where it should.

[precopster]

Thanks for that extra detail , Justin. My blue 2000 V70's compressor started making what I would call scraping noises imediately after a regas which involved receiver-drier & evaporator replacement and recharge with Hy Chill ( the LPG based newer gas on the market)
Soon after it seized. Long story short I paid for a new compressor from FCP and THEY (the garage) paid for 2/3 of the labour charge to fit after they recovered most of the refrigerant. Works great now.

The proprietor there said that Hy Chill is fine for most older cars that use triple wall thickness hoses. It's molecular size is apparently smaller than R134A and seeps from the rubber hoses of many modern cars. I'm so glad I drive an older Volvo!!

[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.

So if you don't want the AC to cycle, turn the Fan Speed up a bit.

[abscate]

I thought the AC system on these cars ran pretty much steady state and the temperature is determined by mix inside the car. Of course, that would still change the thermal load on the evaporator so the above analysis could still be correct. I guess it would depend on how the air is routed in the climate control system. ECC might be different than manual too.