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A/C compressor not engaging once engine gets hot

[email protected] on Sun April 25, 2010 9:39 PM User is offline

Year: 1999
Make: Jeep
Model: Cherokee
Engine Size: 4.0L
Refrigerant Type: 132a
Ambient Temp: na
Pressure Low: na
Pressure High: na
Country of Origin: United States

Hey Guys, I have a 1999 Cherokee Sport with the 4.0L, and I seem to be having some a/c problems. When I first crank the Jeep, and the engine is cool, the a/c works flawlessly. The compressor stays engaged until you turn the system off, which is where my problem begins. I have noticed that when the system is running, the compressor does not cycle off, the fans and the compressor stay engaged until you turn the selector off. After the engine has warmed and you have ran the system for awhile, and turn it off, and then attempt to turn it back on, the fan starts running, but the compressor does not engage. I know that the system is fully charged due to buying a pressure gauge to test the pressure. Any thoughts on the compressor not reengaging after turning the system off? I am wondering if maybe one of the pressure sensors is going bad or is their a compressor cycling switch on this car?
I also have tried jumpering the high pressure switch and the low pressure switch when the engine is warm, not seems to change. The fan starts running when either switch is jumpered, but the compressor will not engage. I have tried jumpering the switches when the engine is cold, and the fan runs and the compressor engsges, so i am thinking it has nothing to do with the switches, any help is greatly appreciated!

GM Tech on Sun April 25, 2010 9:46 PM User is offline

Typical wide air gap scenario- a/c ok with cold engine. not okay once engine warms.

The test: With engine running and a/c commanded on, (but clutch is not engaged) tap on front of clutch hub with a wooden hammer handle- just bump it-- if clutch pulls in, then you know your air gap is too wide- and the coil cannot pull in the clutch (especially when hot) across such a wide gap. Normal fix is to remove clutch and remove a shim or too to lessen the air gap- then re-install clutch. Air gap widens due to wear of the clutch/pulley surfaces-

typical air gap should be about .020" -- yours is probably .050" or more is the above test is positive.

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Edited: Sun April 25, 2010 at 9:48 PM by GM Tech

NickD on Mon April 26, 2010 9:05 AM User is offline

Cause is, as temperature increases, so does the resistance of the copper magnet wire cutting down the on the drive current by as much as 20%. Doubling the gap also decreases the magnetic force required to pull in the hub by a factor of four! You have two things working against you.

Like logical reasons for problems, but seem to be having a fit today with logical reasons for some of our laws.

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