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Old 03-16-2017, 11:15 PM
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Join Date: Mar 2017
Posts: 1
Does the Air Conditioning Compressor Disc always turn on a 2007 ML350? No cold air.

I'm not getting cold air with the AC. The disc associated with the AC pully on the poly belt, the lower right one looking from the front of the engine, is always turning with the engine running, whether the AC is turned on or not. I am used to vehicles where that only turns when the AC is on. Does anyone know if it always turns on an 07 ML350?

I tried to add coolant to the lower pressure side filling attachment. But the Mercedes receptacle is so deep I am not sure the filling attachment I have is reaching down far enough to open up the valve to allow coolant into the car's AC system. Does anyone know if a special attachment is necessary for filling a Mercedes AC system?

The coolant level must be really low because when I use a small screwdriver to depress the low pressure valve inlet only a tiny bit comes out, like a tire with 3 psi in it. With the engine running, when I turn the AC on with my friend's head under the hood right over the engine compartment there is no audible change in noise, no clicks, no nothing.

Anyone know what I can do to get my AC going?

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Old 03-17-2017, 10:41 AM
85 300TD
Join Date: Jul 2006
Location: Cameron Park, CA
Posts: 124
Your ML's A/C compressor is a clutchless unit and you will see the pulley turning at all times. It varies the pressure by use of a "swash plate" instead of a cycling clutch. The swash plate pitch is control by PWM signal from the A/C control unit. If the center bolt on the compressor is turning with the pulley, then the pump is at least turning as well. On the newer vehicles, attempting to just add a little won't work. There are a lot of redundant checks with pressure and temperature, which requires the proper amount of refrigerant to be charged. The fittings are standard R134a which do not require any special adapters. The biggest problem is if a fault has been set in the AAC control unit, even if the fault has been rectified, it must be cleared and rest before the system will begin to function (not all the time, its dependent of what code was set).

The best thing that you can do it take it to a technician that is familiar with Mercedes-Benz vehicles to have it diagnosed and repaired.

1985 300TD - "Panzer Wagen" - undergoing full restoration (resto - mod)
2014 GLK250 Bluetec - Wife's car

Mercedes-Benz Master Certified Technician.
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