I need some advice as to what could be the problem with my A/C system.
The A/C in my 1988 300E quit working, and I have not been able to get the compressor to engage. However it has engaged on a few occasions (3-4 times) when the car was being driven at highway speeds, 75 plus MPH. At that time the A/C starts working and works the way it always had. However as soon as I slow down or stop it quits and does not come on again, even if the speed is increased. Any suggestions would be greatly appreciated. Thanks!

Have you checked your aux fan operation? Also be sure the system is full of freon and the belt is very tight. A loose belt will get you every time.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

I agree with the belt tension comment. Not sure of the arrangement of the belt on a 300E, but my 88' 190E has a single serpentine belt that drives all the auxiliaries. There is a spring loaded belt tensioner pulley that maintains proper tension on the belt. For me, the tensioner spring had weakend over the years and was not applying enough tension to keep the A/C compressor pulley fully engaged except at highway speeds. I replaced the tensioner and the tensioner damper (a small shock absorber attached to the tensioner) and the A/C works fine now.

I went thru similar problem with my hybrid 190 E36 --- what A/C tech found was an air lock in the system. You should try all the tensioning suggestions posted earlier, then have someone evacuate your a/c system .. pull vacumn and refill ... then try the system, and check for leaks using uv dye. If that fails to improve, then ask tech to check the dryer- reciever, and high pressure switches. If not those items, then check the Klima relay... hope this helps .. lemme know.

Just this past week repaired a 1986 300E that
the compressor would only stay engaged if the motor was running at 2500 or higher....
convert that to road speed if you like. The
RPM sensor reading was 215 ohms and not within MB specs of 590ohms or +/- 10% of that figure, so replaced it and now it is back to normal.
Sounds close to your problem....

Good luck,
TobiasMB
92 300SE
88 300CE
84 190/5.0

Thanks for the help it was the RPM sensor! Ordered a RPM sensor but after removing the compressor I noticed that it was not Mercedes but a Japanese unit with a totaly different RPM sensor.
Any ideas on who would be able to cross reference an off-brand compressor and maybe get a RPM sensor for it?

Thanks again
David Busenitz
C36 ('97)
300E ('88)
190-16V (2x)('86)
230SL ('65)

All of the later MB cars use the Nippondenso compressor with the rpm sensor in the rear. There are a few other cars, Saab or Volvo that also use the same type sensor system but I don't think are interchangeable. Because of location constraints etc it is about impossible to install any other than the OE compressor. Earlier (1984-86) used a GM R4 compressor and 1984 used a Nippondenso without the sensor at all but would not work in your system as it is looking for an impulse to engage the clutch. If you still can not find a sensor replacement try calling
Hancock Industries, listed in the Star mag if you are a MBCA member or contact the Parts Shop.

TobiasMB
300SE,300CE,190/5.0