Brian,

I found a diagram on the W126 service info CD. If this is correct, the pressure switch only opens with low pressure. There is no high pressure switch. Test the switch for continuity. Should be closed with refrigerant in the system. The ground signal comes from the pushbutton controler, through the pressure switch, and arrives at pin 10 of the ACC socket. This pin should be the ground for the control circuit of your 'ice cube' relay. This ground is controlled by the pushbutton control. You will also need power at two pins of your relay. One feeds the control side, and one feeds the load. Since you say you were able to jump pins and get continuous compressor opperation, it indicates that the load power supply, the wiring from the relay to the compressor clutch, and the compressor clutch ground are all good.

You are either losing the power supply to the control side of the relay, or you are losing the ground. Test by grounding the control circuit at the pressure switch. If the compressor runs, look at the pushbutton control. The pushbutton control monitors the temperature of the evaporator and heater cores. This is how the temperature of the evaperator core is maintained at a point above freezing. A faulty evap temperature sensor would shut the compressor down.

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1989 300 SEL that mostly works, but needs TLC