There is a speed sensor at the back of the compressor. But this is a renewable part according to MB manuals. Check with MB dealer. How does the mechanic know this is bad ? The resistance across the sensor should be about 550 to 650 ohms. And when the car is running, there should atleast be 0.3V ac voltage across it.
I have posted a systematic way to check the compressor cut-off, check this:
http://www.peachparts.com/shopforum/showthread.php?t=158591&page=5
I am not sure with a bad sensor, how does the compressor remain on after about 6 seconds. And as Mike said, this test is only performed once after the car is started, so if you stop the car once the compressor has cut off and start it again quickly, the compressor should come on again for atleast 6 seconds.