Various cars have various switches connected to the A/C lines. If the refrigerant pressure is too low, it will shut off the compressor. If the pressure is too high, it will shut off the compressor.

You need to have just the right amount of refrigerant - not too much, not too little, THEN the switch will allow the compressor to run.

So look for those sort of switches - usually around the accumulator/receiver/dryer.

Problem is power not going to clutch, not too little or too much refrigerant. 0v at a/c connector for clutch and 0v at the a/c cutout switch (attached at receiver/drier). As stated before I ran a straight +12v to the compressor from the (+) battery terminal (car was off) and I heard and saw the magnetic clutch engage. I then wired up the relay bypassing everything and can no get the clutch to engage and disengage with the inside cabin switch that also controls my high speed fans. With this bypass set-up my a/c is now fully functional and pressure on hi/low side is good, but I still would like to know why there is no power. I even tried changing the MAS relay and still 0v.

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1990 190E 3.0L

The binary switch is sensitive to low or high pressure. If the pressure is not correct, the binary switch will not allow 12volts to engage the compressor. By running a 12 volt wire to the compressor you have bypassed the binary switch which could cause other problems. I would recommend that you jump the terminals on the binary switch for a few seconds and see if the compressor engages.

In your description above, you clearly point out a problem with the MAS relay. You direct wired 12V and got the clutch to move, but when you used a relay and "bypassed everything" (I don't have a clue how you were able to do that) you couldn't get 12V again. It seems that your scheme to bypass everything didn't work. You even said you changed the MAS and it didn't work. I would suggest that unless you used a brand new MAS as a replacement (unlikely since new ones run $400+) you may have a bad "used" unit as a tester. They are cheap enough, get another MAS, clear the codes and try again. These a/c systems are sturdy and are pretty straight forward. The only other issue could be a bad climate control unit, also not that expensive to replace.

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03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio

I am a little confused. You say the car DOES NOT have a Klima relay. Is that because it has never had one, or it has been removed? According to the wiring diagrams your car should have a "compressor cutout control unit" that could be called a Klima relay. It appears to be separate from any other relays.
I believe you when you say this is not a pressure problem because I saw the picture of the gauges. However if there is no compressor cutout control unit (Kilma) then it doesn't have any bearing on your problem.
Could you give any information on any alterations done to the A/C system. Not parts changed, only changes made to the way it was made.

Thanks,

PaulM

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84 500 SEL (307,xxx miles)

No modifications to the a/c system and the compressor cut out switch is on the receiver/drier. All components of the ac system were replaced when I put on the compressor: all new o-rings, expansion valve, ac pressure switch, ac cut out switch, receiver/drier, and serpentine belt. Condenser was cleaned, evaporator was cleaned. I cleaned out the entire area where the blower motor and evaporator sit. Everything was buttoned back up and vacuumed for 45 mins....Then I tested for leaks for 2 hours. Nothing out of the ordinary.

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1990 190E 3.0L