What's an 'ice cube' relay?

The pressure switch on the receiver/dryer is a dual switch. One switch will open if the pressure is too low, and the other switch opens if the pressure goes too high. The connections on this pressure switch come from the push-button control, and the ACC (klima?) controller. When the proper button is pushed, this line is grounded. It is the ground signal that tells the ACC to turn the compressor on. The ACC will turn the compressor off if the engine temperature gets too high, or if the compressor speed does not agree with the engine rpm. From you symptom, (compressor stops and will not restart until the key is cycled) I'd first want to look into the speed signals going to the ACC controller. Jumper power to the compressor clutch and measure the signal coming from the speed sensor. Mine was under specification at about .17 volts at idle, but works fine. If it changes after 4 minutes, find out why. A slipping belt or incorrect compressor clutch gap are possibilities, as is a failing compressor. If the signal is steady and the compressor continues to run with direct power to the clutch, then you'll need to look at the engine speed signal, the coolant temperature sensor, and the compressor request signal. If all those signals are correct, then it's the ACC controller at fault.

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1989 300 SEL that mostly works, but needs TLC