I'd test driven a 560SEL, '86, recently. Upon checking if the AC blew cold, I found it not to. I asked, and was told by the resident mechanic that the AC won't turn on unless the outside tempurature is a certain amount. It was between 41 to 49 degrees outside. It sounds pretty washy to me, but I'd like to ask before I discredit him.

Is this true?

Yes, most if not all (modern) cars have a compressor cutout connected to a temperature sensor. This cuts out the compressor below a certain temperature to protect the overall system.

So in cold weather it can be difficult to evaluate how well a car's AC works.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

Generally the cutout point is set around 35 degrees - below that point the compressor won't activate. The cutout function is supplied by the freon pressure switch, the same one that deactivates the compressor when the system has a refrigerant leak. On cold days the static pressure within the system drops low enough that the compressor doesn't run.

How would you tell if the air was cold enough anyways? Low 40s is a typical temperature for the duct air.

Long story short. There's no way to really evaluate the a/c system in that kind of weather. I think the compressor should have switched on, but the only way to verify that would be to open the hood and see if it was spinning. I would not expect to be able to tell if the air was cooled or not.

- JimY