Some time ago in our 2010 ML350 4Matic A/C stopped working. I have replaced dryer unit and A/C compressor with a brand new Nissens. System was checked for leaks, vacuumed and charged.

Next day my wife started to hear loud humming noise in the cabin from the engine bay. There is no noise difference if A/C is on or off, if cabin fan is on or off.

Noise can be heard when idling, but it is fainter. Noise became louder and more rhythmical if you rev up the engine while driving.

Doers anyone have any ideas what can cause such noise?

 

Did you put the belt back on correctly?

 

Yes, the belt is installed correctly.

 

OEM compressor is Denso. Other brands are not recommended as they may be noisey. Remove the belt and run for just a few seconds to see if the noise goes away. Alternately you can use a shorter belt to bypass the compressor.

 

I have tried running without belt and my wife confirmed that noise is still there. However noise is best heard when driving.
Do you know part number for shorter belt for extended testing?

 

Engines M272/273 – 20 x 2330 to 2340 mm (A011 997 04 92)
OR:
https://mbworld.org/forums/c-class-w...ypass-tip.html

What is the age/condition of your engine mounts?

 

Thank you for the belt info. That would have being my next step.

I think the mounts are original.

After some search online I have found a review of different model of Nissens compressor on amazon. Reviewer stated very similar rythmic noise on Audi. Looks like I have made a big mistake by ordering aftermarket. I have ordered Denso compressor and will replace the aftermarket one. I thought FCP Euro will sell only highest quality tested parts...

 

Wenter, please update your profile to include the year and model, is it gas, CDI or Blue and the country you live in.

 

Remove the belt and spin the pulley 3 & 6 as fast as you can with your fingers. If they spin fast or make noise, they must be replaced.

 

Issue was solved by replacing Nissen a/c compressor with Denso a/c compressor. Lesson learend - no cheap aftermarket can beat OEM.

 

Exactly.