I am not familiar with this car but is there a coolant temp sensor for the a/c system ? I am basing the following based on my 87, 260E.

The same component (coolant temp sensor), but a different switch within it, is also likely to control the aux fan. Because your fans are coming on right away when you start the car, it tells me that the a/c relay is getting a temp signal that is indicating a hot engine, this cuts off the compressor and turns on the aux fan. If coolant sensor is the problem, then disconnecting the sensor should keep the compressor running. Try this. You may have a 4 pin sensor, 2 pins for the a/c relay and 2 pins for the aux fan relay. If you disconnect all, and the compressor comes on and the fans go off (do this test when the engine is cold), then you found your problem.

Another signal that cuts off the compressor after 2 to 3 seconds of its start is a compressor speed sensor. The coil resistance for this sensor is between 550 and 650 ohms. And when on idle, the sensor should produce around 0.3V AC.

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Saumil S. Patel