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Ticking Noise

My 1999 Mercedes ML 320 with 180K started ticking when my wife turned the temperature control from cold to hot last week. Before this, it ran fine with a fairly new a/c installed - ~1 year ago. It was a cool morning. I started the car and she turned the control from blue to red. The ticking sound started immediately and only cooler air came out. The a/c also does not blow cool air anymore either. The ticking is constant and low when the controller is in the blue and louder and faster when the control is turned to red. Can anyone suggest what happened, what the problem may be, and repair options? I haven’t taken it in to my mechanic yet and would like to get some preliminary thoughts. Thanks for any help!

Stuck blend door would be my guess. It is not as simple as it sounds though, in some cases, the whole dash has to be disassembled and the AC system depresurized and recharged after. It can get pricy.


The blend door is actuated by an electrical actuator. I can say from personal experience that you’ll have to remove the dash to get to that actuator. The good news is that you don’t have to recover the refrigerant

Had the same thing happen on a Dodge with the defrost door. Made a ticking noise. Luckily I was able to reach up with a socket wrench and a long extension to get it out.