Q. I have a continuing battle with the air-conditioning unit in my pickup. I have had the low-pressure cut-off switch replaced. The unit is fully charged. However, it does not put out cold air very much. I had the unit recharged and when I drove away from the repair facility, the air was a comfortable 42 degrees. This didn’t last one night.

It seems that when the engine is started from a cold start, the A/C will put out cool air, but will take about one mile to do this. If the weather is hot and I pull into a grocery parking lot, when I return to my pickup, the A/C will not even turn on.

I have on some occasions been driving along and suddenly the A/C unit will click on, but again it will take a mile or two for it to become cool, not cold, air. Any solutions?

Elmer C. Williamson,

Lake Forest Park, Wash.

A. This one’s a challenge without having a closer look, Elmer. The first job is to determine if you’ve got an electrical or refrigeration problem.

Depending on the car or truck model, various operating strategies might be employed. On simpler systems the mode switch (A/C select) and a refrigerant pressure switch each vote “yes” or “no” to engage the compressor’s electric clutch.

On more sophisticated systems additional switches or the engine control computer might allow or deny compressor operation based on power steering effort, engine load or temperature. As one might imagine, the more chefs there are in the kitchen, the greater the chance of disagreement.

Based on your excellent description of the symptoms, my hunch is that an electrical demon is intermittently denying compressor engagement or the clutch might be slipping. An easy way to check for this is to observe the compressor’s clutch hub while A/C operation is requested. The outer pulley will rotate with the belt at all times, but the center section or face of the hub will only rotate if the clutch successfully engages. On some systems, it is normal to see the clutch cycle on and off – perhaps once each minute or two, but on-time should be prevalent. If you find, when requested, the clutch doesn’t promptly engage, an electrical or clutch fault is occurring.

If the clutch does faithfully engage, yet your air outlet temperature remains higher than it should, a refrigeration problem might exist. Common faults are contaminated or insufficient refrigerant, air in the system, blockage of a control valve or orifice, and a faulty compressor. In most cases such faults would reduce performance at all times or after a period of time.

Since your truck blows cold air, but sometimes after a delay, I’m betting against a refrigeration issue.

There’s also the chance your air delivery system is improperly mixing hot and cold air at times, but this is a long shot. Resolving this frustrating problem might require skills beyond those of the shop you are currently using.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

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AP-NY-09-10-04 0620EDT