Agreed. (n/m) posted by JMartz on July 24, 2011 at 07:53 AM
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I was doing some homework on the system for the wiki. The ONLY Zs that use a throttle suction valve are R12 models with the auto climate control. All other models (R12 with manual climate control and R134A models) use a thermostatic "amplifier" at the evaporator which cycles the compressor on and off. While it's possible this amplifier (or it's associated circuit) has gone by, I still hold that the pressure switch on the receiver/drier is most likely the culprit. Or rather, that the pressure is too high or too low, tripping the switch and disabling the compressor.