300 SDL A/C problem

Without stating the obvious, anyone know what the job of the speed sensor is on the rear of the A/C compressor?
At idle, my A/C runs fine for hours. After about 30 minutes of highway
driving, it shuts down and won't start up again until the ignition switch is turned off and then on again and the process repeats. Been battling this for 2+ years. Independent--but knowledgeable--mechanic is stumped. Along the way, I've changed the following: Compressor, receiver/drier, expansion valve, in-car temp sensor, evaporator temp sensor, aux fan and pre-resistor fan relays, aspirator blower motor, compressor cut-out control unit, pushbutton switch assembly, 105/120 C temp switch, high and low pressure switches, mono-valve. The only anomally I can find is low output voltage from the speed sensor (but the system runs fine with this low voltage when at idle).
Anyone seen this type of problem before? Really need help.
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Once the AC is called for, the control relay waits for about half second for the compressor to catch up then it constantly compares the engine and compressor speed. If the deviation is more than 30% (IIRC), it assumes the compressor has seized. To keep other accessories running on the same serpentine belt, the relay cuts off the compressor clutch. This cut-off logic can only be reset by turning off the ignition.
The speed sensor on the compressor provides the input to the control relay. On W201 (which uses the same logic though different relay part), there is another speed sensor on the fly wheel. I don't recall if 300SDL has the 2nd speed sensor since I have had no problem there.
Another possibility to cause the speed deviation is a slippery belt/clutch or a worn belt tensioner. If the replacement compressor is new, I would suspect the belt or tensioner or even oil deposit on clutch plate.
THOMAS PRUITT wrote:

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Thanks for the info. It makes sense.
Do you know if the compressor control unit is comparing voltages or pulses from the sensors involved? My M-B shop manuals say the compressor speed sensor is to put out 900mV. I measure just 75mV at idle and the system works as it should. Reving the engine doesn't increase the voltage. The compressor was a remanufactured one. I suppose I could have inherited someone else's bad speed sensor.
The A/C wiring diagram does show an engine speed sensor on the ring gear that feeds the Electronic Diesel System. Do you know if this ring gear sensor is also used to drive the tachometer--the tach works fine.
I appreciate the help.
Tom

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The compressor speed sensor is measured in mV AC. Not DC volts. Same as the engine rpm sensor that the tach uses. Yes, the Klima relay compares the tach rpm signal and the compressor rpm signal.
Your problem is the compressor rpm sensor. It has to rise with speed. One side of it is bad. Use your meter and check the AC voltage output to ground on each leg. You will find 1/2 is dead.

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Is it true that this RPM sensor cannot be pulled and replaced without evacuation of the system and removal of the compressor from the car?
I think my 1986 190e needs this also, as the same symptoms present. Shutting the car off and restarting gets the cold air blowing again also...
Thanks, Marty
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