1990 300E A/C suddenly stopped blowing cold
 

This car has always had the best A/C out of all of my cars. It can get so cold, even when 90 out that I actually have to turn the temp knob so that the 22 deg C number is at the very top of the knob.

When I got back into the car after lunch, it stopped blowing cold.
When I got it home, I went through my normal checks:

1. When a/c is on the clutch doesn't move.
2. Clutch does turn by hand with no noticeable resistance or condition issues.
3. By passed the low pressure switch, still no clutch engagement.
4. Inspected the bullet fuses (replaced with copper versions 3 months ago), no broken fuses.
5. Replaced Climate control unit, still no clutch movement.
6. Removed and re-installed MAS (90's don't have dedicated Klima relays), no change.
7. When I turn on the a/c, the engine rev's lower briefly

Seems like I've lost power to the a/c clutch somehow.
What else can I look for?

Thanks,

AL
What else can I do to get the clutch to engage again.

Clutch not engaging means low refrigerant. You need to check for leaks and do a pressure test before messing with the parts.

By passed low pressure switch
 

I was directed by my MBZ indy of 12 years that by passing the low pressure switch overcomes low R-12 level if only briefly so that the clutch would engage. So it would seem that another there's another cause. Thanks.

Anyone else?

Check the wiring on the clutch. If you have the m103 engine, it's fairly easy to gain access to the wires, maybe a faulty ground. Locate the wiring on the ac and check for power using a multimeter, start at the compressor unit to see if power is reaching it.

Oooops, I guess I should've posted how to access the wires: 2 Philip head screws on the top of the compressor release the connector, then one 8mm bolt anchoring the wire a little further up under the intake manifold. Once those are out you should be able to access wiring to compressor

You never said if you have 12 volts going to the low pressure switch. If you do have 12 volts there, then the problem is likely the compressor clutch. If you don't have 12 volts there, the problem is upstream - maybe the relay or the control unit.

Pictures, Pictures
 

OK, got the multi-meter out. First, yes I do get 12v at the two connectors for the low pressure switch. Kind of a pain to get the plug to release from the compressor (see pic 1). With the engine running and the A/C on at the control panel, I do not get 12v with any combination of the 3 slots while testing the plug end. How can I energize the clutch via the 3 posts (see pic 2)? The plug continues upstream toward the fire wall (see pic 3). Anything upstream I should check? Yes, I do like to keep a tidy engine bay in all my cars- especially since I'm the one getting dirty if I didn't :)

Thanks,

AL

It was the MAS unit!!!
 

Replace the MAS and all is good and cold again!

This was the first MAS failure I experienced with all the 1990-93 MB's I had (5 at various times).

Recap of symptoms and diagnosis:

1. Car always had strong a/c, suddenly stopped being cold when I got back in the car after an appointment on Wednesday. It was 97 deg. outside.
2. Clutch could be spun by hand with only a light amount of resistance and did not feel loose.
3. To check if Freon (still using R-12) was too low, bypassed low pressure sensor and jumped the two leads, with the a/c power on and the engine running.
4. Confirmed with multi-meter that 12v was present when the two leads were connected.
5. Inspected plug and wiring attachment at side of compressor, no apparent issues.
6. Replaced MAS is my 1990 car with one from 1991 (could not find the exact part number match from the salvage yard), a/c is functional again.
7. Car is not throwing any DTC's (diagnostic trouble codes).
8. Added 1/2 can of R-12 with dye for leak detection as abundance of caution.

Thanks for all those that chimed in their help.

Done!