The evap temp sensor is an NTC thermistor. If it is open-circuit, it will be infinite resistance and the PBU will think you're at -100000 degrees (maybe a bit exaggerated, but you get the jist).

The good news is that you've now proven the wiring all the way back to the PBU. "cycling" of the compressor is done based on the temperature of the evaporator. If the evap temperature sensor is screwed up or open-circuit, it can cause it, otherwise the PBU is probably toast.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]