Servo valve - Mercedes-Benz Forum
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#1 (permalink) Old 01-28-2011, 11:40 AM
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Servo valve

If the servo valve is the source of battery drain due to being stuck; is there a way to get it working again without taking it apart?

The tutorials for rebuilding a servo valve are daunting to me. Is there a way to flush or free up the internals in place?
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#2 (permalink) Old 01-28-2011, 01:57 PM
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Originally Posted by GrandpaLarry View Post
If the servo valve is the source of battery drain due to being stuck; is there a way to get it working again without taking it apart?

The tutorials for rebuilding a servo valve are daunting to me. Is there a way to flush or free up the internals in place?
Not to my knowledge. I have seen valves torn down, and haev replaced on on a friend's car, but I have not torn one down personally.

BEst ditch it fo rth eUnwired Tools digital replacement... ScotinSoCal can give you advice and feedback on its performance.

RM Smith www.rmsmithco.com

Where is it again that we are going... And why are we in a handbasket?
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#3 (permalink) Old 01-28-2011, 03:08 PM
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If your amp is bad the servo could be fine. The amp is what supplies the power to move the servo to it's requested position. Remove the servo cap and change the temp settings and listen and watch to see & hear it run. The temp guage in the dash, the temp control work together to bias the amp to move the servo to the correct position. The servo turns the hot water on and off and routes the vacuum to the correct pods.

Timing has a lot to do with the outcome of a Rain dance.

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#4 (permalink) Old 01-28-2011, 03:17 PM
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Originally Posted by Nascar Fan View Post
If your amp is bad the servo could be fine. The amp is what supplies the power to move the servo to it's requested position. Remove the servo cap and change the temp settings and listen and watch to see & hear it run. The temp guage in the dash, the temp control work together to bias the amp to move the servo to the correct position. The servo turns the hot water on and off and routes the vacuum to the correct pods.
He is chasing a battery drain, and I suggested the servo may be locked up and not parking... Will the servo not park, regardless of the amp, when the ignition is shut off?

Incidentally, a locked servo has been reported to kill the amp, like shorting a monovalve will likely kill a climate control panel on the later models ...

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#5 (permalink) Old 01-29-2011, 09:58 AM
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Servo Valve

Okay,

I am attaching a picture of the servo valve with the cover off. What should move or rotate when you ask for heat with the cabin controller. I can't see anything that looks like it would move without further dis-assembly. Motor noise makes it hard to hear if the servo is moving and the aux pump is not running either.

I do have current with the key off and the negative disconnected. I think it reads 0.4 on the 10 amp scale so there is a good draw but I haven't checked fuses yet.

Does this draw indicate that the amp is okay?

I plan to sell this car so maybe I will be best off to just tell the next guy that the heater doesn't work Maybe someone in Florida will buy it.

I hate to give up on anything though. But money can become a problem pretty quickly with these cars.
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#6 (permalink) Old 01-29-2011, 11:04 AM
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I disconnected the electrical from the servo valve and the current drain stopped. One problem solved but now what to do about the servo.....
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#7 (permalink) Old 01-29-2011, 03:17 PM
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The little pot has an arrow in the middle and the pot turns with the servo. If that is the servo you got from me, it was a working unit. I don't know about the parking issue with a bad amp. I hate this system of heat/AC.

Timing has a lot to do with the outcome of a Rain dance.

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#8 (permalink) Old 01-29-2011, 03:25 PM
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I disconnected the electrical from the servo valve and the current drain stopped. One problem solved but now what to do about the servo.....
If you had a current drain with the ignition off, the servo is trying to park and can't. This usually means down deeper than what you have exposed, the water valve has corroded and locked up or at least partially locked the shaft, and the valve shaft can't can't turn to put the electrical switches in the park or "no current drain" position. If the valve can't turn, the proper vacuum elements can't get vacuum, and the appropriate doors will not open to direct air as desired. Also, the water cannot be regulated for the heating, nor can the fan speed be controlled as designed. Also, since the servo can't park, it is constantly drawing current, trying to turn the valve shaft(and all the other connected electrical switches and vacuum switches).

I am going to try and find the link to the chrysler imperial manual (from which our systems originated) and post here... Stand by...

Edit for link: http://www.imperialclub.com/Repair/L.../281/index.htm

Obviously, not everything in this attached document is applicable, but you will recognize what is and isn't. I have also attached the link for several articles regarding the Chrysler system. You may find other references that apply.

http://www.imperialclub.com/Repair/Air/index.htm

Enjoy reading, and good luck.

RM Smith www.rmsmithco.com

Where is it again that we are going... And why are we in a handbasket?

Last edited by Ears; 01-29-2011 at 03:30 PM. Reason: added link
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#9 (permalink) Old 01-29-2011, 03:45 PM
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There used to be a link from the chrysler page, to a guys "imperial" page, with cool pics of a servo in various stages of disassembly. The quality of the pics left a little to be desired, but were clear enough to get the point across. Page 13 of the manual I posted (in the first link of my last post) shows the wiring diagram. That is the best diagram I have seen of the "system"...

An electrical engineer friend of mine and I were working on a replacement (similar to unwired tools product). Then I saw the UT ad for the servo replacement, and we shelved the project.

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#10 (permalink) Old 01-29-2011, 04:07 PM
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Don't worry about it. I will look at it a little closer if the weather holds and I can get one car out and this car into the garage. My problem may be the amp. I'm not sure how to test it though.

Those little hoses on the top of the servo are kinda loose also if they have to transmit vacuum and hold it.
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