Author Topic: New servo burning up amp  (Read 230 times)

p. rex

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New servo burning up amp
« on: 23 September 2019, 10:24 AM »
Hello all,

I have been overhauling the HVAC system on my ‘79 6.9.  We recharged the system and have the AC working well, but the car has a broken servo.  I ordered a new servo from George Murphy and had my guy install it.  He called this morning to tell me that the new servo isn’t cycling to heat.  The new servo is also burning up the climate control amplifier — the thing is smoking.  What could be wrong here?  The old servo was cracked and wasn’t reliably cycling between cooling and heat, hanging up in certain places and killing the battery when the engine was shut off.

Does it sound like my new servo is defective?  I don’t think you can really wire the thing wrong, since it connects with one big electrical plug.  Have any of you guys found yourself in this situation?  I searched the forum (and a couple of the other vintage MB forums, too) and could not find anything on point.

*edit*  I was discussing this further with my mechanic.  He is convinced that the sensor chain is okay, since the old servo was behaving pretty normally, maintaining temperature and moving the wiper arm as appropriate.  That suggests to me that the sensor chain is working properly and that there's an internal wiring fault in the new rebuilt servo.

FYI, we're replacing the old servo because it's hanging up intermittently and the housing is cracked.  Could we get away with leaving it a little longer?  Maybe so, but better to replace it now.

Thanks!

Jordan
« Last Edit: 23 September 2019, 01:51 PM by p. rex »
1979 450SEL 6.9 #7184 “Die Marschallin”

Jed

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Re: New servo burning up amp
« Reply #1 on: 23 September 2019, 06:44 PM »
Hey Jordan

Sorry to hear about your problem.  I can't get over what a crappy system that old chrysler servo is.  For what it costs to replace or even have rebuilt it really sucks.  I had one rebuilt two years ago. Like so many others it lasted two years and about 1000 miles then started leaking...sent it back to the ebay rebuilder out in california...nice enough guy...he said it was corroding internally due to poor coolant which had been flushed when the servos was installed. He replaced some gaskets, charged me again and I installed it.  Guess what...it burned up my amplifier, not once but twice before i figured out it was the servo.  The rebuilder assured me it wasn't the servo, that it was the control panel/temp adjuster.  After screwing around with that for a couple weeks without success I put in a known good servo from another car (with a newly rebuilt programa amplifier and problem solved.  Has worked as it should all summer.

I don't know whats wrong with my old servo but it seems something internally is shorting out the amplifier.  Interestingly I can cycle the servo from opened to closed with a 9v battery without a problem but I don't dare reinstall it because I am tired of buying amplifiers!

If I were you I would send it back to George and tell him whats going on.  From what I can tell he is as familiar with these things as anyone.

Good luck and report back when you figure it out please


1979 Mercedes 6.9 #5206 - restored
1979 Mercedes 6.9 #6424 - ongoing restoration
1976 Mercedes 6.9 #484 - restoration?

p. rex

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Re: New servo burning up amp
« Reply #2 on: 24 September 2019, 12:58 PM »
Thank you, Jed.  George has sent out a fresh servo and amplifier.  Hopefully that'll solve the problem.  If it does, George can take a look at the first servo and maybe we'll have a better idea of what causes certain servos to do this.  Gotta be some kind of internal short, like you said.

I am really hoping that whatever servo I end up with lasts a good few years.  The thing is apparently a basket case for fundamental engineering reasons -- there's a reason that folks on certain other MB forums call it the "EVIL SERVO" -- but some folks get lucky.  Wouldn't it be nice to make it ten years before I have to monkey with the wretched thing again?
1979 450SEL 6.9 #7184 “Die Marschallin”

UTn_boy

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Re: New servo burning up amp
« Reply #3 on: 27 September 2019, 10:27 AM »
Any time the servo hangs up, even if just for a short moment or while, it'll fry the amplifier.  Judging from your post it suggests that the amplifier wasn't replaced initially.  If that is to be true, then take heed to the rule that the servo and amplifier are to always be replaced in unison.  Also, it's not uncommon for rebuilt units to hang up....which will also fry the amplifiers.  Whenever possible I buy new ones from Mercedes.  Sure...they're a little more expensive, but it's plug and play, and good to go for another 10-15 years.   
1966 250se coupe`, black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3, papyrus white/dark red leather
1973 300sel 4.5, silver blue metallic/blue leather
1976 280s astral silver/blue MB Tex
1978 450sel 6.9 Euro, Anthr/velour

p. rex

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Re: New servo burning up amp
« Reply #4 on: 01 October 2019, 11:59 AM »
My mechanic has installed a newly-rebuilt aluminum servo and amplifier.  Word is that everything is working perfectly.  Will have to see when I pick up the car this afternoon.

Of course, the mechanic also says that he's seeing signs of leakage at the compressor front seal.  I'm not overly excited to do the compressor, so I figure I'll enjoy the car as is for now and have the compressor done in the spring.

George Murphy has been helpful and responsive on this, and got a good servo and amplifier out to me in a jiffy.
1979 450SEL 6.9 #7184 “Die Marschallin”

UTn_boy

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Re: New servo burning up amp
« Reply #5 on: 01 October 2019, 05:11 PM »
The original seals on the A6 compressors were a hard ceramic seal.  They worked about as good as a British made seal in that the British design their seals to keep the dirt out, not to keep the oil in. haha  However, you can use a viton lip type seal for an R4 compressor and it'll not sling oil near as bad, if any at all.  I reseal all of my own compressors and on my customer's cars.  It's never failed me yet. 
1966 250se coupe`, black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3, papyrus white/dark red leather
1973 300sel 4.5, silver blue metallic/blue leather
1976 280s astral silver/blue MB Tex
1978 450sel 6.9 Euro, Anthr/velour

p. rex

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Re: New servo burning up amp
« Reply #6 on: 17 October 2019, 10:54 AM »
UTn Boy --

Figured you might be interested in a status update.  The whole climate system seems to be working correctly, apart from occasional rattling from one of the flaps.  Oh well, it will have to be done eventually, but not yet.  I took the car on a 1,000-mile road trip this past weekend, used the A/C quite a bit, and it seems to work pretty well, so I think it's probably a slow leak rather than a huge one.  Unsurprisingly, the heat will drive you out of the car if you want it to.  The Germans don't mess around with that.

As for the compressor, is it possible to replace the main shaft seal with the compressor in place?  If so, that would be good.  If the compressor has to come out, I am probably just ordering a new one from Alma.  The labor cost is going to be bad enough that I'd really want a brand-new compressor when I'm done.  Supposedly Alma made a design change that solves the main seal problem. 

On a slightly related note, MAN does it feel good that the car is nearly "done," at least from a driver's perspective.  I felt like a god blasting down those rural two-lanes, whooshing ahead effortlessly on that big wave of torque.  It really drove home why we love these cars so much.  Sure, I got 12.5 mpg, but I was having too good a time to care.
1979 450SEL 6.9 #7184 “Die Marschallin”

raueda1

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Re: New servo burning up amp
« Reply #7 on: 17 October 2019, 11:33 AM »
UTn Boy --

Figured you might be interested in a status update.  The whole climate system seems to be working correctly, apart from occasional rattling from one of the flaps.  Oh well, it will have to be done eventually, but not yet.  I took the car on a 1,000-mile road trip this past weekend, used the A/C quite a bit, and it seems to work pretty well, so I think it's probably a slow leak rather than a huge one.  Unsurprisingly, the heat will drive you out of the car if you want it to.  The Germans don't mess around with that.

As for the compressor, is it possible to replace the main shaft seal with the compressor in place?  If so, that would be good.  If the compressor has to come out, I am probably just ordering a new one from Alma.  The labor cost is going to be bad enough that I'd really want a brand-new compressor when I'm done.  Supposedly Alma made a design change that solves the main seal problem. 

On a slightly related note, MAN does it feel good that the car is nearly "done," at least from a driver's perspective.  I felt like a god blasting down those rural two-lanes, whooshing ahead effortlessly on that big wave of torque.  It really drove home why we love these cars so much.  Sure, I got 12.5 mpg, but I was having too good a time to care.
Bingo!!  And if you had an egg between your foot and the gas you'd probably just get 13mpg anyway.  Live a little!
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

p. rex

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Re: New servo burning up amp
« Reply #8 on: 18 October 2019, 08:18 AM »
You don't need to tell me twice!  I'm certainly not shy with the loud pedal.  I have always considered myself a bit of a leadfoot, although I stay away from license-losing territory now that I've matured a little.  I'm actually really pleased that I was doing better than 12 mpg, I interpret that as an indication that my motor is fairly strong and my injection is more or less in tune.

Of course, when I'm romping it from stop light to stop light in the city, it's more like 8-9 mpg, but I am told that that's totally normal.  Whatever, I figure that if I'm having a good time I'm getting my money's worth.
1979 450SEL 6.9 #7184 “Die Marschallin”