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Old 06-18-2008, 09:37 PM
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Join Date: May 2008
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W116 Auto Climat Control only Defrost Works

So, I have a W116 300SD with the Auto Climate Control and only the Defrost works. I have posted elsewhere and have gotten great starting points and would like to add to them. Auto-hi, Auto-lo, Bi-Level don't work...neither does the A/C compressor. I've checked thoroughly the vacuum lines and cable connections and everything seems fine. The servo is working as I can hear it spinning when the Defrost is on. I hope that means it's it??? I can't seem to locate the Fresh Air Door Actuator as I'm told that might have a leak. I've taken apart the glove box and the piece under it...where can it be???

I am actually having fun trying to solve this and getting my hands dirty but I could use some advice.

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Old 06-18-2008, 10:58 PM
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Mercedes has these systems default to Defrost, the rationale being that safety demands a clear windscreen so if anything works, defrost should. That said, it means that your system is not working, since "off" essentially means defrost. Can you confirm that the fan is working, that you are feeling air at the windshield vents? Is it hot air or room temp air? Can you change the speed of the fan? Sounds like not.

You can test the a/c compressor clutch by removing its plug and connecting 12 volts to its terminals. It should "click." That will tell you if you have a clutch problem or some other problem.

There is a pressure switch in the a/c line somewhere. If the system has no freon in it, the compressor won't be told to engage the clutch.

I'm sorry I don't know more about these servo-type systems.


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Old 06-19-2008, 02:28 AM
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Join Date: May 2008
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Hey Jeremy,

Thanks for the input. I always get a little more info. like the pressure switch. The compressor is definitely not coming on but I haven't directly applied 12v yet. You say there is a pressure switch connected to the actual A/C hose that measures the freon. This is very helpful and I'll just follow the hose route and hope for the best. Just one more thing for me to try.

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Old 06-19-2008, 09:22 AM
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On that type system OPERATION is all about vacuum. Any vacuum loss & the system won't come on in any mode BUT defrost.

You need a vacuum gauge & diagram to check for vacuum leaks.
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Old 06-19-2008, 10:20 AM
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Thanks for the reply. I am just about at the vacuum test stage(which probably should have been stage 1).

My question is can I do a vacuum test with one of those hand vacuum pumps or do I need something a little more sophisticated. I've got online access to the vacuum diagrams at so I'm set there.

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