I notice that my a.c. compressor runs for few seconds then stops. It goes on and on like that.

It's cold and seams to work properly.

Is it normal or is it an indication of a low freon level? Any ideas.
Thank you.

 

Total hypothetical WAG: Your A/C is fine, but the evaporator is gunked up with dust and crap, so airflow is poor. Perhaps this allows the evaporator to ice up, and the freeze switch is activating, cutting the compressor in and out?

How's your airflow?

 

I don't think the freeze switch would cause cycleing that fast due to the thermal inertia involved... but maybe the freon low pressure switch is initially sensitive to the compressor coming on... a bit counter intuitive but in a certain range maybe this happens... (I don't think you have a high pressure switch).

Alan

 

This to happen to another vehicle I used to have. When the Freon level was getting low it would cycle on and off. The car still put out cold air but once I topped it off, you could notice the difference.

I could be way off too.

 

Some vehicles (Ford and GM) have suction pressure switches that roughly take the place of both the pressure safety switch and the anti-freeze switch. Thay also did away with th thermostatic expansion valve and replaced it with the simpler and less-precise orifice tube liquid metering device. Those systems cycle based on cooling load. But I digress...

The 928's in all flavors, up to the first 1993 models with R-134a installed from the factory, have a low pressure safety switch installed in the high-pressure piping. If there's enough partial pressure from refrigerant in the system to allow the compressor to start, it will likely keep running as the compressor builds pressure at the switch. So it's quite unlikely that the pressure switch is causing the cycling.

The anti-freeze switch, OTOH, will cause that cycling. Especially if the ambient isn't high and/or the fan speed is low.


Diagnosis:

If you have a set of gauges and a charging manifold, break it out to watch the pressures as the system cycles on and off. If you see a pattern of decreasing high-side pressure before the compressor drops out, it might be the pressure switch. If you can see the suction side pressure dropping hard prior to the compressor drop-out, it's quite possible the anti-freeze switch is causing the cycling. You'll want to get a good thermometer to see where the air temps are, then decide if it's working correctly or not.

No gauges handy? You can get to the anti-freeze switch under the plastic tray at the base of the windscreen, behind the air filter box. There are two wires attached to that switch. Mate the two wires together somehow. They are female spade connections and will kind of connect if one is twisted 180º and they are pushed together, or make a male-to-male spade jumper to connect the two wires. Earlier in life, I wanted to be the male connecting two females, but figured out that it would crimp things... Anyway, with the swicth jumpered like this, you've removed the switch from the circuit. Cycling stopped? If so, you need a new anti-freeze switch. If it's still cycling, reconnect the wires as original and go looking elsewhere.


There are several write-ups on replacing the weak/underspec'd compressor relay that's buried in the AC control head. Erratic compressor operation is an early sign of impending failure.

 

It could be that your system is fine. What was the ambient temperature outside? What were the vent temps? What was your fan speed set to? What were the exact durations when the compressor was off vs. when it was on?

When it's cold outside, my all cars do a similar thing. I set the fan speed to "2" and the compressor turns on and because of the low ambient temps, very quickly brings the evaporator temperature down to freezing. The freeze switch then cuts the compressor off. The evaporator temperature begins to increase, and at a certain point the compressor kicks back in, then the cycle repeats. In fact, my wife's 968 that has the best A/C system ever ('93 model, still with factory R134A fill and system untouched) will do the same in a minute or so even when it's 80 deg F outside; the temp out the vents is low 30's F.

Dan
'91 928GT S/C

 

The air flow is good. From "0" to "4".

 

Interesting.
The outside temp. was about 65.
The fan was set to 2.
It was on for about 10 sec. only and off for about 30 to 45 sec. if I remember well.
The vent temperature was at about 60. The car was in the garage.

 

The outside temp, was not very high, about 65.
I will check the anti-freeze switch for sure.
Thank you.

 

+1

 

Sounds like your system is running fine. You had the temp lever set to somewhere around room temp, correct? So you had the climate control system mixing hot air with cold. 10 seconds on and 40 seconds off is not a problem.

Dan
'91 928GT S/C