The Klima relay has several inputs, all of which must vote "YES" for the compressor's clutch to engage. A simple test is (with the engine NOT running) to disconnect the connector at the clutch and momentarily connect the clutch directly to +12V and ground (polarity doesn't matter). You should hear a "clack" noise as the clutch engages. No noise, bad clutch. Separably replaceable, I believe (ask Sixto for details).

The compressor has an RPM sensor that can tell if the compressor has frozen due to (for example) a bad bearing. If so, Klima turns off the clutch. This protects the serpentine belt.

Klima looks at engine RPM via the tachometer line. The purpose is to keep the compressor load off of the engine when the engine is first started. Engine RPM must be up to at least 600 or Klima won't allow the clutch to engage. No tach function, no a/c.

Klima looks at a full-throttle switch located in the linkage near the cruise control actuator. (This is NOT the kickdown switch under the pedal.) When you floor the accelerator, Klima assumes you need maximum power and turns off the compressor. If this switch or its wiring have failed, you get no a/c.

The a/c refrigerant line has a dual-pressure switch near the dryer (driver side, front of engine compartment). Too little pressure or too much shuts off the compressor.

Jeremy


The CCU must "tell" Klima to turn on the a/c compressor. CCU looks at various temperature sensors and the thermostat dial to make this decision. An electronic failure could result in this "request" not getting to Klima.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970