ACC Heater servo replacement - 1980 300TD

YES, another servo problem. I've got the servo nailed as being the culprit. My VOM reads 3 ohms in "park", 3 in hot, and 1485 in cold. The motor will move (takes 5-10 secs for full travel) but heat/cold
control is erratic. Now, George Murphy at Performance Analysis Co. (super friendly and helpful!) has a new/rebuilt servo that uses an aluminum housing instead of that crappy plastic MB used. He's asking 595USD with a 1 year warranty. Germanstar (http://germanstar.net/123h.htm ) has a similair one for 365USD (don't know about a warranty). Does anyone have any knowledge on the differences, if any, and quality of them? Also, George recommends that the amp be replaced at the same time, just to be sure you've got a good signal going to the servo. Appreciate any help offered. AJ
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$261.55 at my source....http://www.mr-auto-parts.com/
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Thanks Tiger. Does MANTIS manufacture/rebuild these things using aluminum, ala the other guys? MB's original product, quite frankly, sucked. As I'm sure you know, the plastic housing was prone to cracks, then coolant leaks, and corrosion of the delicate parts inside the unit. Not to mention, they'd be happy to sell you a new OEM for around $800. AJ
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I suggest you call them.
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I have the Performance Analysis one in my car now for about a year. The first one they sent, sprung a leak in about a week. George quickly sent a replacement, which has worked perfectly. Not only is their a full warranty for a year, after that they will fix it for $100. Given the reliability of these, that sounds like a very good idea. Also, the PA one has an aluminum lower housing. The plastic one cracking and letting coolant leak is one of the primary failures.
Before you replace yours, try removing the upper portion of the housing. I'm not sure from your post what ohms you are measuring, but it sounds like they are from the feedback potentiometer, it may just be dirty. This pot is at the top of the unit and is driven by the position of the motor. It's part of the feedback system, the other components being the thumbwheel setting pot, the in car temp sensor, and the ambient temp sensor. That chain drives a voltage which moves the motor. If that pot in the servo gets a dirty/bad spot, it will cause erratic behavior. I watched mine with an ohm meter while I drove the motor through it's full cycle. At one small point the pot just went to an open condition. You can try spraying it with electronic cleaner. It worked once on one of mine.
It's also a good idea to cycle the system at least every few weeks from max heat to max cooling. That moves the pot through it's full range and helps keep it from developing a bad spot.
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snipped-for-privacy@optonline.net (Chet Hayes) wrote in message

Thanks for the post. I followed your instructions and "determined" that the feedback pot WAS bad, sprayed it with cleaner --- no change. Upon closer inspection, I discovered that this unit had been messed with before. The silicone "witness blobs" on the screws were missing, and the screws that held down the pot were partially mangled. The pot also had a horrible solder job done to it. What had happened was that the Joe that replaced it hadn't set it up properly, that is why I was getting incorrect resistance readings. After a bit of fiddling around with it, I had the exact readings to make this thing work. Still no go. IMO, the gears/post that controls the coolant flow is pooched. SOOOOOO, next step is going to be an after market servo. George at PERFORMANCE ANALYSIS seems to have the best unit/warranty, bar none ($595). Germanstar.net has one for $365. Mr Auto Parts has one for $261. The question is, are they all the same? I mean, that IS quite a price spread... AJ
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snipped-for-privacy@sympatico.ca (AJ) wrote in message

I guess you'd have to ask the other vendors if they have an aluminum lower housing, which the one from Perf Analysis does. The cracking of the OEM plastic ones is a major failure mode. Also, if Perf Analysis still offers lifetime repair for $100 after the one year warranty, that alone is a deal closer for me.
Since the one you have appears to be shot, you can take it out and fiddle with it some more. The motor is connected to terminals 1 and 5 on the forward most connector. The wires are brown and green on the harness. If you take the servo out, you can run the motor by hooking up a 9V household battery. Just reverse the polarity and it runs the other way. When it stops, in one direction it's at max cool, reversing till it stops in the other is max heat. Make sure you disconnect the battery immeadiately when the motor stops, or else you'll burn it out. In max cool, the water valve should be closed. In full heat, it should be wide open. This process elimates the pot and everything else, so you'll know for sure if the motor opens and closes the valve.
I wouldn't bother trying to disassemble it beyond what you have done. I took one all the way apart and it wasn't easy. It's hard to even figure out how it all comes apart without destroying it and there is little hope of ever getting it back together. How the hell PA rebuilds these is beyond me!
BTW, if the servo is running back and forth in the car, I would not bother replacing the AMP, as it should be fine.
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Greetings,
In the PerformanceProducts.com web site there is a "New" Digital replacement for the AC/Heater Servo. It is $699 for the kit. It appears to be a lot smaller that the original. Looks like about 5 pieces with a plug in wiring harness. They claim that it installs in less that 2 hours and comes with a 5 year warranty. There is supposed to be an online install procedure in PDF Format. Might want to take a look at this.
Bill
http://assets.performanceproducts.com/assets/product/109850/ServoManual.pdf

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Thanks all!! Just got an email from Jack at http://www.tristarparts.com and he's got an aluminum one for $300 plus shipping. I think I'll be going with that one.
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snipped-for-privacy@sympatico.ca (AJ) wrote in message

The aluminum servo from Tristar is in, works great! Very obvious that it is a rebuild/not new. I tested the motor and ohms before installing -- spot on numbers and sounds. AJ
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Good to hear it worked out. I think all the ones available now are rebuilt, as I remember seeing somewhere online that the original manufacturer stopped making them. I guess they finally figured out they were a piece of crap and weren't worth the trouble anymore.
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