Friends, I have an electronic/electrical problem with my A/C system on my ’97 C280:

The A/C compressor clutch will not engage. We find no voltage in the wire which comes directly from the main control head assembly (i.e., the dash mounted control to air conditioning, heater, fan, etc.).

We suspected the control head assembly was bad, so I had it tested and gone through by a company in Florida called Programma. They didn’t find a lot wrong with it. They cleared some codes and changed a button. I put it in the car -- same thing. Programma sent me a second refurbished unit just to be sure it wasn’t a problem they missed. Same result -– no current through the wire at the pressure switch.

Here are the notes from my tech: “Compressor clutch not activating. Noted no voltage source at the pressure switch. Suspect control head unit is bad. Retrieved codes 226, 228, 230, 232, 233, 422.”

226 = in-car temp sensor B10/4
228 = heater core temp sensor B10/1
230 = evaporator temp sensor B10/6
232 = refrigerant pressure sensor B12
233 = refrigerant temperature sensor B12/1
422 = serial interface K1

Apparently the current goes directly from the control head to the pressure switch, with no relays in-between. For the heck of it, I checked the three related fuses under the hood: #2, #12, and #18. All were good. I suspect a loose ground or broken wire. But where to look. Anyone else ever had such a problem? Note all the code references to sensors -- a clue?

(Although irrelevant to this problem, FYI, the compressor and pressure switch are almost new, and the system is full of refrigerant.)

Thanks in advance for any input!