Type 44 A/C Troubleshooting Assistance

[Gary Meier]

Fellow Listers -- 
Patient is a 1988 5KSQ - converted to R-134 about 5 years ago
Symptoms:
A/C compressor will not engage - jumpered relay compressor runs - suction pressure is good - A/C cools well -- I suspected the relay - tried 2 different relays from the salvage yard, compressor will not start - seems unlikely, although not impossible that 3 relays would have failed in the same manner. 
Prior to the start of this problem there were times when the A/C - Climate Control would seem to get lost and stay in heating when cabin temps were above setpoint and vice versa. Is it possible that my problems are caused by the cabin temp sensor, the circulating fan for this sensor is functioning properly? What other possible sources for this/these problems should I be investigating? Any help will be greatly appreciated.
Thanks in Advance
Gary Meier