An Obscure and Difficult to Diagnose Intermittent Air Conditioning Problem

• Model1987 Jeep Cherokee 4x4
• Symptoms
• Air conditioning starts to blow warm intermittently (especially when it is very hot outside)
• The myriad of other potential causes have been eliminated ... low charge, pressure switches, etc, etc

Discussion

There are many causes of automotive air conditioning problems. This one is interesting because it is obscure, hard to find and in this case, was introduced at the factory from day 1 on this vehicle. Several dealer shops worked on this, in and out of warranty. In the end, each one added some freon and sent the car on its way.

I finally fixed this myself. The root cause turned out to be that the magnetic clutch air-gap was over-shimmed from the factory. See #14 on the exploded parts diagram and the shim discussion.

Here are the clues that lead to this discovery:

• Diagnosis
• A/C would start blowing warm, usually after the engine was hot, and the outside ambient was very warm
• If you have a voltmeter on your instrument cluster you will see it drop slightly when the A/C kicks in due to the current draw from the magnetic clutch
• This is important. When the A/C starts blowing warm, turn the ac OFF then ON, observe the VOLT reading. If it jumps slightly going from ON to OFF and dips slightly going from OFF to ON this means the A/C clutch is being powered.
• On a hot day I was able to get the failure to occur in my driveway. I noticed that the compressor would quit spinning (you can easily tell by looking at the FRONT PLATE #13). And I verified that the clutch was drawing current.
• Measuring the clutch airgap showed that it was out of spec on the high side. There was an extra shim installed at the factory.
• Removing the extra shim centered the gap in the spec range.
• SUCCESS - no more a/c problems for 250K miles!

• Root Cause

• The extra shim caused a slightly large airgap. This means that the magnetic force required to engage the clutch is increased from its design goal. At high ambient temperatures, when the engine gets hot, the IR copper losses in the clutch winding increase, thereby reducing the clutch current and the magnetic force. This magnetic force drops low enough to release the clutch even though there is current in the winding. On cooler days, or more airflow (high speed), or before the engine reaches max temperature, there is enough magnetic force to engage the clutch and things work fine.

• The key to this was noticing that the clutch would quit spinning while current was still being applied.