New AC compressor won't engage



Installed new compressor, condenser, drier… Local shop wanted to charge $140 just to evac. Bought vacuum pump and gauges/hoses. Evaced the system, held -30 for 45 minutes. Started car and proceeded to add freon… Compressor would not engage. Low pressure showed 90lbs but would not take any more and compressor wouldn’t engage…


The BCM is likely cancelling the AC system due to recognizing a problem. If so you must clear the codes and then charge the system. If you install a jumper wire at the low side pressure switch the compressor won’t be cycling on and off but you must be certain to remove the jumper and reconnect the switch when finished. A paer clip will work.


I assume the 90lbs are static pressure readings. In other words the engine was turned off. With the engine running it should have been more like 30PSI. That should have been enough to engage the clutch. I wonder, can you see the clutch wanting to engage?

I also assume that the new compressor came with the clutch kit assembled.

I installed a new clutch kit on my wife’s car a couple of weeks ago. Dumb as I am, I just installed the darn thing before I found out that the coil was dead…no continuity. Even “new” parts can be broke.

Rod_Knox has a good idea, but make sure you don’t run the a/c with the bypass for too long. Running a compressor without sufficient amounts of refrigerants can cause big troubles. 3-4 minutes should be sufficient for the system to suck in enough refrigerants to run on its own.


Like so many problems that posters here face, gaining a basic understanding of the system they are dealing with would sure be a great benefit to correcting their problems quickly, safely and economically.

If the gauge set is connected (both high and low gauges) with the system at 30" of vacuum and the engine off a can of refrigerant can be connected and both high and low valves fully opened and left open until it is apparent that line pressure and system pressure are equal. At that point fully close the high side valve, disconnect the low side valve, start the engine and turn the AC on MAX, HI FAN. Then install a jumper at the low switch and watch the gauges. The low gauge should quickly drop below 30#, possibly even going to a vacuum for a few seconds while the high side gauge climbs steadily above 100#. A second can of refrigerant will need to be connected when the low side again goes very low.

At this point is it critical to understand that if the high side jumps beyond 300# or the belt begins to screech due to slipping or there is any indication of a high pressure leak YANK the jumper to shut off the compressor.

But if the pressure slowly builds on high and low, when the low passes the 30# mark momentarily close the low valve and allow the pressure to stabilize. It should stabilize at about 25# and the high side under 200#. If so open the low valve and while watching the gauges continue to add until the low side gauge is at ~35# while the high side is less than 300#. At that point close the low valve and watch the pressures, monitor the engine temperature and if the low side pipe is sweating with the gauges within safe limits you’re a winner.


Good advice from @Rod_Knox. One more thing to remember is adding the proper amount of the proper oil to the system.


Yes, NYBo. Hopefully the compressor has a note explaining if it is pre-oiled and if so which oil. Some OE compressors were shipped totally dry and required pouring the correct type/amount of lubricant into the low side port and rotating the compressor shaft numerous times to distribute it… There are just so many details that can be overlooked whithout a thorough understanding of the system. So much that is done in a shop is just second nature. But McParts compressors for late model automobiles have normally come with nearly a full charge of the correct oil.


As a matter of fact, most of the factory replacement ac compressors I’ve installed were shipped dry

And there were documents stating that fact

So it was no mistake


Are you certain that 90 PSI you’re seeing on the low side is actual system pressure? What has been known to happen sometimes is that the low side Schrader valve is not being depressed by the gauge fitting and what you’;re actually looking at is can pressure while the system itself is still under a vacuum.

You might use a test light or VOM and verify power at the low side pressure switch. With the system pressured up even to a partial degree you should see power to and through the low side switch.


Sorry for not getting back to you guys. The problem was my fault. Upon the first try I pulled the AC relay and tested that it had power. Come to find out when I put the leads in the socket i spread the connectors so far they would not connect with the relay. So. when I twist the relay it worked. I bent the relay ends like you do to a lamp cord with worn out recepticles and it worked fine.


Thanks for the followup OP, good to know your AC system will be ready for you needs this summer.