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Old 11-20-2012, 01:08 PM   #1 (permalink)
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Date registered: Jun 2009
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Dreaded F****** Servo!

Hello to you all. It's been awhile since I last posted here on Benzworld. I own a '79 300D with approximately 304,000 miles. My car has the upgraded aluminum body heater servo unit. The defrost/heat hasn't been working for the last three days and there are no visible leaks coming from or around the servo unit. I've already checked the fuse and that looks good. Car is running at normal temp and radiator fluid isn't low either. The next thing I am going to do is grab a second servo control amplifier at my local junk yard. Any other suggestions? Thanks!
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Old 11-20-2012, 01:39 PM   #2 (permalink)
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Date registered: Mar 2007
Vehicle: 1980 300cd
Location: Saint Louis
Posts: 112
I needed a new servo (old one cracked,) amplifier and potentiometer wheel adjustment to get my 1980 300cd's climate system working again.

I used a multimeter with spade terminals on its ends to test the servo, amp and sensor chain. The test plug for the sensor chain is located near the passenger's feet on the firewall. You'll need a power source to test the servo's motor if you pull it.

You can get amps, climate pods and servos here:
Performance Analysis | Climate Control | Diaphragms | Blower Motors | Pushbutton Panels | Vacuum Door Locks | Autotemp | Cruise Control | Engine Oil Analysis | Mercedes-Benz Parts | Mercedes-Benz Club of America | Oak Ridge | Car Care

If you follow the tests here, you'll be fine:
The Servo Gods Have Heard Your Prayers: - PeachParts Mercedes ShopForum

Good luck!
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Old 11-20-2012, 01:57 PM   #3 (permalink)
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IF the defrost isn't working then it's not the servo. Defrost operates on a safety bypass of the server as a safety so that you can always have heat\see. I'm not sure by your description if your saying your getting cold air, or no air movement at all.
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Old 11-20-2012, 02:06 PM   #4 (permalink)
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Thanks for the replies! I am only getting cold
air from defrost button (even when car is up to
normal operating temp.).
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Old 11-20-2012, 02:15 PM   #5 (permalink)
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Then it's not your servo (and there was much rejoicing) so it would seem that your not circulating water through your heater core. The first thing I would check is your AUX water pump, which is found right next to your servo. Pull it out, feed it 12volts and see if it's circulating water. They have a tendency to seize up and impede water flow. While you have that out of the loop you should also check to see if your heater core is plugged. Isolate the two heater hoses going into your firewall and see if you can pass water through them (a funnel and gravity should suffice here, it should move quickly and freely into one hose, and out the other. If it is clogged here's a good how to on unclogging it, better yet hook a garden hose up to one and see if it flows freely out the other)

Last edited by dude99; 11-20-2012 at 02:26 PM.
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