It's just a pass through or a relay, I wouldn't overthink and use any binary-electronics logic here.

If the pressure is above the min and below the max, voltage is carried to the compressor clutch. Two pins should suffice. Any "thinking" by the switch on whether to let the clutch engage goes on internally. If there was some requirement to provide an output as to WHY the compressor wasn't engaging, then you might need another pin. The compressor is dumb and doesn't care: on or off, +12 on the output side or not.

I pressure tested the uniary switch. It is normally open at atmospheric pressure. Switch closed at around 45 psi (may be off by 10 psi because gauge not very accurate). I'll have to look at the Mercedes AC wiring diagram to see how the pressure switch controls the compressor. Hard to imagine but the binary switch has to somehow work like this:

Switch is normally open When the compressor turns on and reaches normal pressure, the switch stays open. If the pressure drops below or above a certain pressure, then the switch closes and cuts power to the compressor?

I also tested the temp switch. It is normally open, it closes at approx 135 F. Does that sound about right? Do you have specs?

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The pressure sounds mostly right. The FSM says ON at 2.6 bar (1 bar = 14.7 psi) and OFF at 2.0 bar. This makes sense because you don't want the system cycling if the pressure moves slightly above/below a specific value. I would re-phrase the statement to say "in a normally charged system, the switch is closed". With a uniary (?) switch, it should only be open if you've had a leak and the refrigerant has leaked out and depressurized your system.

Go through the system logically: the CCU says "time to turn on the compressor" and outputs +12. That signal goes to the pressure switch and it gets a veto authority: not enough refrigerant = no +12 on the output = no clutch engage signal. 3 bar (or 45 psi) = +12 on the output = clutch gets to engage. The switch isn't "interactive" with the compressor, it's just a pass-through - if there's enough pressure then the compressor (or clutch) can do what the CCU asks.

This is one reason there's so much advice that you can't troubleshoot a compressor without gauges. If the resting pressure (no compressor running) in an A/C system isn't greater than the low pressure cut off (LPCO) switch then the compressor can't engage because the clutch shouldn't see +12v because the LPCO says so.

Think of the LPCO as an "It ain't gonna work so don't even try" switch and the HPCO as "It's gonna blow so we're shutting you down" switch.


The FSM says the aux fan temp on the R/D is 62 C or 142 F.

My thinking was wrong. The pressure switch, when installed, never sees atmospheric pressure unless there is a leak in the system. It should always be at around 90 psi depending on the refrigerant. 2 terminals is sufficient.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked