AC Compressor Cycling On&Off

[jes1789]

2008 550i
Noticed my AC started barely cooling. Checked out compressor to see if it was running. The compressor is turning on and running for about 2 seconds, then off for a few seconds, back on for 2 seconds, continuously. Put gauges on to check refrigerant level, low side is oscillating between 40-50 psi continuously, at the same rate as the compressor cycling. High side reads about 100 psi steadily.
I evacuated the systems and changed the low side sensor. Refilled refrigerant, and no change, still cycling on and off. Anyone know what may be going or had a similar issue?

[Juandyzimo]

Food for thought.

From my experience, cycling on and off is a sign of low refrigerant, which usually means a leak, but I see you refilled it and still got the same cycling, correct? Then, the cycling may possibly be caused be the incorrect system pressures, in this case on the high side. From what I have personally experience if the system is low on refrigerant and is cycling like it is in your case then the low side readings also fluctuate. For example, if the low side reads low or zero and the compressor comes on then the low side reading increases while the compressor is running but then drops again when the compressor stops running because as the low pressure switch (sensor) kills power to it because it ran out of refrigerant to pull from the evaporator(low side), once the system pressure somewhat equalizes and raises the pressure enough to "trick" the low pressure switch into powering the compressor again, rinse lather repeat... As far as your pressure readings, your low side seem to be within a normal range especially if its steady, however your high side is way too low as it should be reading around 200psi to 250psi +-20 (usually about 4 to 5 times higher than the low side depending on ambient temperature and sea level etc...). This is usually signs of a bad compressor. If you had low refrigerant due to a leak you can also loose oil which can damage the compressor internals and seals thus loosing pressure on that side.

Have you tried jumping the low pressure sensor to see if it continues to cycle or if it stops? And if you jump it and the cycling stops, is the a/c getting cooler?
Is your cooling fan coming on when the a/c is on?
Is the compressor making any noises?
Assuming you used a recovery machine, how many ounces/pounds of refrigerant did you recover? Was it the recommended amount, or was it low?

On another note, check for any obstructions at the condenser which will affect the performance of the a/c. Look specifically between the condenser and the radiator. I say this because the front of the condenser may look clean and unobstructed, but my son's 2007 530i was half way blocked with leaves that somehow managed to find their way between the condenser and radiator. This will cause the system to underperform, but I don't think this should cause the rapid cycling. Asking for a friend here.

[jclausen]

Get a proper ac service then recheck