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*Premium Member
2008 SLK350
3,236 Posts
Discussion Starter · #1 · (Edited by Moderator)
What @efair says. It is not a simple on/off compressor. The amount of cooling is governed by an internal valve in the compressor that controls the amount of cooling liquid running through the system.
This valve is controlled by a PWM signal from the SAM. SAM's in early R171 often fail producing a PWM signal. Result: no cooling.
Best way to determine if it is the SAM or compressor in your case is to apply an external PWM signal to the compressor and see if it cools.

If the PWM signal fails the compressor may kick in but the valve stays closed and there is no cooling. It may look from the outside the compressor is shot.
In your case it may be shot but I don't think you can say for sure based on the tests they did as you described them.
If you are interested,here is explained how a variable compressor works

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