The air conditioning system on my 90 560SEC does not put out cool air.
The compressor is engaged and the sight glass has only small bubbles,
no foam. Two weeks ago it was cooling to 60 degrees F. Now it doesn't
cool at all. Do you think the expansion valve may be faulty? Any help
will be appreciated.
Thanks

Does the low pressure hose get cool OR cold?
Does the drier or high pressure hose get HOT (be careful, can be 140 deg)?

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Or the monovalve could be putting heat in to the system. You might notice some cool air with the AC on until the engine warms up (3-5 minutes after a cold start), does this sound familiar? You would still get water from the evaporator though, do you notice the typical dripping of water on the ground under the car?

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Sight glass is not a pressure check. Most cars need "topped" of as they just leak slightly when getting old. R12 can still be bought its not illegal just harder to find. Get a gage with compressor engaged should be around 40-50 at idle 35 at around 1500 rpms. If not your low.

If pressure is right the i agree with the mono valve.

Good luck