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The symptom was rapid cycling of the compressor clutch. Unfortunately there does not seem to be a high-side pressure test port on the R-134A 1993 Volvo 240s. Weird. Without the test port of course it wasn't possible to check for excess pressure.
Several online resources suggested that the orifice tube might be plugged and/or the accumulator gunked up. Since both appeared to be the original units I replaced them and did an evacuation and recharge. Still the compressor fast cycled.
After much electrical troubleshooting I finally discovered that as some point before I got the car someone disconnected the ground wire which is supposed to ground the compressor to the block. The clutch circuit was thus only getting a highly resistive ground though a leakage path. Cleaning and reconnecting the ground wire gave me a system which works like new. I'm surprised that the clutch worked at all without the ground attached, but there ya go.
One thing which made all of this easier going is that Harbor Freight had their A/C gauge set on sale for $30! It is an amazingly good unit for that price:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92649
John
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