No Power to ac compressor
 

I just installed a compressor and I have no power going to it. Need help asap. Car is 1990 190E 2.6 and the searches have not given me anything useful. There is no Klima relay on the car and I can't even find the fuse.

Instead of a Klima relay, it and other relays were combined into the MAS (silver box with a knob in front of the battery). Turn the knob to the "0" position to be able to remove it. "I" position to install it. On my son's 1990 300E, it also wasn't getting power, even though I had 12v when I jumped the two leads from the a/c low pressure switch (at the top of the dryer). You local pick n' pull has one for $20 or eBay for $35. Let us know what you find.

I am not familiar with the 201 but I looked at the wiring diagrams for your car and I see a unit called "Control unit for compressor protective cut-out". I couldn't find out where it is but I see that it has control of the compressor. It looks at pressure switches, temperature sensors, WOT switch, ECM, CCU, etc.
If you go the the stickies at the top of the Tech section you can find the manuals for this car. Go down to the climate control section. You can find locations, wiring, vacuum, trouble shooting and R&R.

PaulM

Ok, so I am definitely not getting power to the compressor. I swapped out the MAS behind the battery and got no change. I ran a straight hotwire from the positive battery terminal to the clutch and finally got it to engage. What I did was wire up a relay to the compressor so now I have a working a/c again. I obviously still need to find the problem, but I needed something because the temperature here is now in the 110° range and will be this way for the next 3 months ago a/c was imperative. Now that I narrowed it down some I can start looking at the electrical wiring. Any more suggestions are greatly welcomed and appreciated.

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.

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.

What is the binary switch? My a/c pressure is normal and unless my manifold gauges are faulty, my system is properly charged.

The switch can be faulty. Have you checked the switch? Have you tried shorting it as suggested? This switch would be at the accumulator as said above.

As explained, the binary switch monitors the pressure in the system, shutting down the compressor when the pressure is abnormal. The binary switch is installed in one of the freon lines normally located near the receiver drier.

Hi side ~240 psi, low side ~ 50psi, fluctuates close to those ranges. A/c is cold. I'm positive this is not related to pressure or lack of charge or over charge

If you know the answer, they why are you asking for advice? Our advice was given...

I never said I knew the answer, but I am confident that lack of refrigerant isn't the problem. I'm hoping that someone has encountered something similar with their a/c that can help. Also, as stated in post #6, I tried the ac switches and got nothing 0v. Could there be something else that would cause the compressor to not have power? If someone wouldn't mind offering their own hi side/low side readings with a similar outside ambient temp of 107°F that would also be great, just to confirm that my readings fall within proper range.

bad MAS- test with another or bad climate control unit
 

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.

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