AC compressor cycling every 7 secs but still cold

[Oz_850GLE]

I have an 94 850 GLE Australian spec Volvo, that has had one previous owner with full meticulous service history.

The a/c in the car seems to be working fine, it blows cold, but when the car is idling the compressor clicks on and off every 5-7 seconds. When driving the car for a long period in high temp weather (100degF or 38degC) the a/c works for a while but intermittently starts blowing warm for a minute or so before going cold again.

I'm quite familiar with a/c systems having replaced a full system on a BMW 325 incl the evap.

Where should I start looking

Thanks

Steve
Sydney, Australia

[JRL]

It may seem cold but it's either low on freon but most probably the compressor CLUTCH is failing (common)
Search how to shim it here in the repair database.

[jblackburn]

You may have both of the above issues. Pick up a can of R-134a at an auto parts store, and fill the system according to the ambient temperature and the pressures indicated on the back of the can.

The clutch gap may be what's causing it to drop out when it gets too hot outside.
http://volvospeed.com/vs_forum/topic/80 ... h-gap-fix/

(You will need a Volvospeed account in order to see the picture at the bottom of that post).

[precopster]

My wife's 850 has the same issue, it is original with factory refrigerant still inside. What have you tried so far?

The overpressure switch located near the firewall controls over & under pressure. These often go out of tolerance, so if the switch believes there is more pressure than it's higher limit allows it will cut out. You can test this by running a volt meter to the non-powered side of the switch and seeing if the voltage drops to zero when the compressor switches off.

Unfortunately refrigerant sales ( R134a) are restricted to those with a refrigeration mechanic's license in Australia since the late eighties.

Best to replace this first before a regas as it is almost impossible to swap one out while the system is pressurised due to the seal blowing out. FCP Groton stock these at around $30.

[misha]

Isn't that normal for ac compressor to cycle every few seconds when engine is idling(in these cars with electronic ac)?

When i bought my car it was almost new and ac compressor did that from beginning.

When it is low on freon it cycles more frequently or no cycling at all and no more cold air.

It is simple...no freon=no pressure in the system=no cold air.

I know that because i had to refill freon on mine 2 years ago after 12 years...car is '97.I needed to refill ac because compressor was cycling every second or two and there was no more cold air from vents.

After refilling i went for a leak check after a week of driving and everything was ok...no leaks,and ac compressor still cycle in about few seconds when engine is idling.Ac works like a charm.

[precopster]

OK but is it normal for the A/C to go warm for up to one minute & then cool down again??

Me thinks not.

[jblackburn]

Nope. Look at the clutch gap fix for that issue - when they get real warm, the clutch no longer makes contact with the compressor wheel.

On a hot day, when it's holding a proper charge, my S70's compressor stays engaged about 95% of the time at idle. It may click out for a few seconds after running for up to a minute. On cooler days, it will cycle in and out more frequently to keep the evaporator from freezing up. At higher RPM (puttering along at 2000 where you can still hear it), it will also cycle in and out.

[misha]

OK but is it normal for the A/C to go warm for up to one minute & then cool down again??

Me thinks not.

Since you still have factory freon in ac compressor i think that it is just low on freon.

Maybe it's a bad ac clutch,like jablackburn said...but i think it's just low on freon.

[cn90]

I think "misha" is right.

My 1998 S70 with ECC seems to do the same thing (cycling every 7 sec).
Air from center vent is cold.
So I hooked up my gauge with engine running and AC "On".
- When Clutch is off (compressor not running), pressure climbs to about 37 psi, then clutch kicks in.
- With Clutch engaged, the pressure drops from 37 psi to about 22 psi, when the Clutch is disengaged.
- The system is off for about 7 sec, during that time, the pressure builds up to 37 psi and the cycle starts again.
- In other words, this on-off cycle is about 14 sec, or about 4x/minute.

- Thinking the system is low on R134a (I might have been wrong), I added 1 can (12 oz) at a time and measured the pressures. After a total of 2 cans (24 oz total), I stopped as it did not make a big difference in pressure readings and before I blew up the system lol.

Anyway, I am back to square one with the system cycling about 4x/minute. Interior Vent drop is about 20F: outside air was 60F, center vent was 40F.

I am giving up on this, maybe it was designed from the factory this way.

PS: I will do the AC Clutch shim with zip ties later.

[precopster]

My 2000 model's a/c held pressure (got to as low as 25psi) with a hole in the evaporator. The a/c would cycle about every 6 -8 seconds.

After replacing the evaporator and having the gases proffessionally evacuated and refilled pressures can be seen through Vida as 450kpa with compressor off and 1400kpa with compressor on. That's a lot higher than 50psi even when the compressor is off. I don't know which point in the a/c system provides feedback to the ECU.

My guess is that any system cycling the compressor often is low on pressure.

[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.