First thing to check is the low pressure switch on the dryer behind the left headlight. If there's no continuity across the contacts, there isn't enough refrigerant pressure. Or the switch is bad. With the engine running and ACC set to defrost, momentarily jump the wires to the switch. If the compressor engages, you're that much smarter. If not, check for 12V at one of the wires. 12V means you're getting a compressor signal from the cabin.

Sixto
83 300SE
98 E320 wagon

Sixto is telling you the right thing to do, check those pressure switches. There are two PRESSURE swiches on the drier. They are sometimes incorrectly referred to as temp switches, even in the FSM's. One turns the electric fan on when the system is over pressure. The other is a two mode switch that turns the compressor off when the system has EITHER too low or too high pressure. Sometimes you can mess things up by having too much refrigerant.

Some systems also have temp sensors, some have thermoswitches. Either has the same function: If the system thinks engine temp is high enough to shut off the compressor, it will also turn the electric fan on high. The thermo switches will be located either above the water pump or next to the thermostat, depending on motor.

The 210's had a completely different system. The way it works is that one random component automatically breaks on the hottest day of summer. When this happens, the OBD leads you to replace an unrelated component. By the time you figure it out, it's fall.

Junk yard control unit to the rescue.
44 degrees out the vents.
$125.
200 miles.
The End

lights on the unit do not work.
Are they serviceable?