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.

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:

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
"Wan-ning Tan" <suntan (AT) erols (DOT) com> wrote

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.


"THOMAS PRUITT" <vertlp (AT) verizon (DOT) net> wrote

On 2005-08-17 06:15:40 -0700, "Karl" <aufever (AT) prodigy (DOT) net> said:

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