A/C Compressor only turns on via jump

[AwfulMechanic]

Hate to start another A/C thread. After Googling for 2 hours and having 20 tabs open I figured I'd throw in the towel and ask directly. I'm stumped.

Car is a 1984 300D. Where do I start to diagnosing this climate control? Lots of threads were talking about replacing the monovalve, not sure if this is necessary. The heat seems to work (and turn off) nicely.

This is where I'm at.

The previous owner has already replaced compressor, temp switch, expansion valve and drier in august of 06. (He left a nice thick binder of all maintenance since 84', I love him <3 haha ) The system now uses R-134a and I'm ok with this.

I checked the lowside and it had no refrigerant, added to 25psi and nothing happened. I jumped the compressor on at the pressure switch on the drier and filled the entire system again to 25psi. While the system was filling the A/C inside started to blow nice n' cold, and also the AUX fan started after about 15 minutes.

The A/C compressor still fails to engage with the system pressurized. What do I check next?

[MBeige]

Seems like a faulty pressure switch.

You can also check the AC compressor relay which is on the black box atop the driver side inner fender. One closer to the firewall.