2000 Chevrolet Silverado • 135,000 miles

2000 Silverado 2500, 6.0L, 135,000, 4x4 in the NW has intermittant A/C. Works fine from startup but once it has ran for awhile, if you shut is off or the cycling switch takes it down (just guessing on that one as it does quit going down the road), the compressor clutch does not re-engage. You can turn if off for awhile and it will usually engage and cold air blows once more. Had it evacuated and filled with 1.8 lbs, ran a scantool on it and all the permissives are present, can feel the A/C relay come in when requested, but no engagement. Jumpering across 30 and 87 on the relay and checking with a DC tong I get a reading of around 3.2 amps out to the clutch coil. The current is constant whether the coil brings in the clutch or not. Checking the air gap it looks to be in the.020-.030 range. Allow it to rest (cool?) And once again the clutch comes in.
As a side note, the only thing that has happened to this truck recently (4 months ago) is it lost the water pump and hot coolant went everywhere. Don't know how that would affect the clutch but thougth I'd throw it in.
Thanks for any ideas.
August 25, 2011.

Scan indicated AC request when on so PCM not at fault so possible maybe clutch coil. What is voltage at coil AC on? If good OHM coil

Aug 26, 2011.
With A/C on, motor running, 14.1 volts. Reading through the coil I get 3.1 ohms.

Aug 29, 2011.