Thanks for the reply Alex.

Here is the symptoms for the 190E (1991) compressor. The Ac system was working well except when you stop at a traffic light and had to wait. At this time, the system circulated warm air. I have looked at the ac clutch and cannot see the clutch engage. I have check the fuse and it is OK. It appears that the cooling has been slowly getting worse.
1.How do I test the MAS relay and where is the location?
2.When I turned the heat on yesterday, Hot air came out from the side vents but cold air came out from the middle vents. Why?

Thanks,
Rascal