What is the actual function of the speed-sensor on W124 A/C compressor (specifically 1988 300TE)? Since we went to a 1995 C36 M104 AMG engine, the A/C compressor on the C36 engine does NOT have the same speed-sensor signal so we had to disconnect the speed-sensor. Is there a way to tap in the the tach/RPM signal and at 5500 RPM have the A/C clutch disengage the compressor at high RPMs? Thanks, :-) neil 1988 360TE AMG 1993 500E

From my 93 300E, I always thought the speed sensor was to shut off the compressor if it was jammed. In other words, the circuitry looks at the speed sensor and if it is not turning at the proper speed, it will not engage the clutch. This is so that all the other things being driven by the serpentine belt, like the water pump and alternator, to continue to operate. I seem to remember seeing the specs one time but I cannot find it now.

I found something but I am not sure it will help. I had remembered wrong--it was from an official Mercedes publication for the 190D using the Nippondenso compressor. The book states:

Nominal value at approximately 750 rpm (using an AC voltmeter to measure): min. 0.3 V ~

Nominal resistance of sensor: 530 to 650 ohms

It also states (and again for the 190D); "The compressor is activated as soon as an engine speed of greater than 500 rpm is reached. The control unit compares the speeds of the flywheel and compressor 2 seconds after the compressor comes on (to compensate for clutch slippage during cut-in). If there is a difference of more than 30% as the compressor comes on, the speed difference is checked for a period of 200 milliseconds. If the rpm difference equalizes within 200 milliseconds, the compressor remains engaged. If the rpm difference remains (i.e. due to a jammed compressor) the control unit will immediately cut-out the compressor."

Thanks!

Now the C36 compressor has a speed-sensor but with different values (something like 1.1-1.5V AC). I can't interchange speed-sensors, anyone have a idea on how to have the A/C compressor cut-out should it seize?

Thanks,
:-) neil

I have a similar problem. I had an engine swap from om603 to om606 in my w124. The 606 comes with a compressor without speed sensor. The compressor is not engaged when I switch it on. My technician tells me it is because of the speed sensor. It there anyway to have it work without the speed sensor. Or is there any klima relay for that compressor?

It's because you aren't feeding a signal to the Klima relay. It's possible to replace the Klima with two relays, but you will lose some safety functionality. I posted the info here. If you try to implement this mod, contact me, as the diode isn't in the right position:

Compressor speed sensor.