CK0045

Greetings everyone! This is my first post to this forum. I have performed many different types of repairs to different brands but I am still new to MB vehicles. I have a 2007 SL550 that only has over 40,000km on it but has a few little issues. It’s usually stored indoors and is a wonderful vehicle. I like when everything runs flawlessly. I’ve been noticing a couple sounds coming from the front and when trying to diagnose it seems like one of the noises was coming from a fan belt pulley. One other thing I noticed was that the AC clutch was always engaged even though the AC system was off, not in defrost function, even with the HVAC fan being turned off.
I ordered a new set of idler pulleys, tensioner and drive belt. When I had the drive belt off I inspected the AC compressor and everything seems fine. Pulley turns fine, so weird sounds or anything binding. I installed the new pulleys, tensioner and drive belt, put this back together and tested everything again and the AC compressor stays engaged. When I have the fan blowing, it gets cold and works fine but when I turn the AC off or fan off it seems like somehow the AC clutch is still getting signal to activate even though nothing from the control panel is telling it to turn on. Same thing happens if I turn the HVAC off, turn car off and start the vehicle again. The AC clutch engages and stays engaged.

Does anyone have any suggestions of what to check or any idea what can cause this issue? Any recommendations of what to check? Any potential common trouble spots?

Thanks in advance,
Chris

Gene Fiorot

I Think I read here there is no clutch. It is always turning

CK0045

I thought I have a picture or video with the belt off but it’s not clear. I could easily turn the AC pulley without belt, but if I put 7mm socket onto center bolt it’s tough to move due to compression.

g0rsq

As per the previous response, Mercedes use a variable displacement clutch-less AC compressor, so is always engaged.

AC is activated by an electronic digital signal which corresponds to the percent of AC cooling needed. Max cooling is 99%, minimum cooling (off) is 0%.

You can read this with Mercedes diagnostics.

So the question is when you turn the AC off does it remain cooling the air, or does it stop cooling the air. Again the best tool to check this, as well as AC pressures, is a Mercedes diagnostic tool, though you could put gauges on the car if that is all you have.