Re: noisy AC servo
 

My w116 77 450 sel may or may not have an issue w/ the AC servo unit (I've also seen it called a climate control valve). I bought the car w/o a hood cover which would suppress engine noise, but the unit seems pretty loud from the passenger compartment.
When I first heard it I thought it was the fuel pump, but I hear it coming from the firewall. Then I put a stethoscope on the servo and that was where the sound was coming. The sound is intermittant, coming on for 30-90 seconds and then turning off. And it also comes on and off whether I've got the climate control system on or not. Is that normal?
My Climate Control unit seems to work fine, air blowing cold and heater blowing hot. But that servo sound is kind of annoying. Do I have a problem w/ my servo, or do I just need to get myself a hood cover?

RE: Re: noisy AC servo
 

I hate to tell you bad news, but my 79 450SLC project car, the servo started the same thing, and finally went. I had it rebuilt, but it is a pain in the neck to take it out and replace. Also, listen to the pump next to it, that is a failure point on these system. There is a cap on the top of the pump, pop it off and put a drop of two of household oil in there. That is a $95 part. Rebuilding the servo cost me $395.00. But new they run anywhere from $900 to $1500. Also check the top of the servo, there should be a black sealent covering the screw holes, if anti freeze leaks out of there, the servo need replacement. The other thing to watch out for, if the motor inside shorts out it will stay running after you turn off the car and drain you battery within a couple of hours. Once the car reaches operating temp and you turn on the heat or AC the servo will run. Since I had it back and got everthing running again, it seems to run all the time, as the temps around here have been below zero the past couple of days. But according to the instructions I got when it came back fixed, it should shut down if the sensors both in car and on in the blower housing should shut it on and off as needed, at least that how I understand it to work? Also, check the vacuum hoses on the back of the servo, there is one screw that holds them. see if any are cracked. Since I had it rebuilt I can hear it, and I only have 1/2 of the hood insulation left. Also, I re-read your post, when the climate control is off the servo is off. So your motor is going.

RE: Re: noisy AC servo
 

c280,
Well, dagnab it. That doesn't sound good. Fortunately, I live in the S. Calif desert, and don't risk freezing to death if I put off replacing parts. I will do the little checks and fixes you reference. Thanks for sharing your experience.

RE: Re: noisy AC servo
 

I did some further checking on the trouble shooting documents I have, and it could the the gears inside the servo that are going. If the servo still runs after the engine is shut off for more then 10 seconds +/- 5 seconds, then the inline fuse to the system which is located near the fuse box on the right footwell passenger side behind the glove box, you battery will run down very quickly. This fuse, when the engine is turned off put the servo in park and if it is blown the servo will keep on running. did you check the hot water heater next to the servo are the rubber loses hot that do into the servo? is not there could be a bunch of problems, believe me I had my servo rebuilt bought a new water heater. Had a new ampl, this could also let the servo run on, but usually cold air when hot is called for will usually indicate a servo problem. do you have a email address posted on the board? I can try and scan and send you a MS Word document with the trouble shooting steops. Oh it also could bed a vacuum leake. This system was invented by Chrysler in the fifites and Mercedes bought the rights to the system. Since I bought my 79 450SLC, I haven't heard a nice this about this system for HVAC, even thou when new they were the first of the real climate control systems based on ambient cabin and out temperature, and I am told when the work they work well. the trick after 30 years give or take is getting them to work. When I tookd the glove box out I was looking at a mas of vacuum lines and some electric lines. Don't know if these are worse then the pneumatci systems used back in the 40s and 50s. Also, Chrysler used this system on their Imperial, the top of the line back then. Also, I think GM used it also.
george_murphy@compuserve.com is the experts, expert on these systems and is a very nice person and helpful. Contact him and he will give you good solid advice.

RE: Re: noisy AC servo
 

c280,
Thanks for coming back to me with more. I've been out of town for a few, or I would have responded sooner. I think I have my email in my profile; if not, it's labigdog165@aol.com (I've got a 165 lb rottweiller - he's the resident german who translated 'mercedes benz' as 'take that, stupid american swine!'). :)

My servo does not run w/ the engine off, but rather comes on intermittantly even when the unit itself is not 'turned on.' I've been paying attention since your email as to whether the engine reaches operating temp before the unit first comes on, but I haven't formed an opinion yet (there have been a couple times when it seemed to come on early). Your email proves that things can almost always get worse though, doesn't it? Prior to your email I had considered the problem to be a possible grounding issue. My radio, antenna, and one window have quit working at various times - and then started working again all on their own a week or so later. As a fix against this, I will probably re-ground the units in question. I am also scouting for a location to install the battery ground
from its current position on the firewall to the motor and hook up a wire from the block to the firewall.

You mentioned a vacuum leak. That rings a bell because I am experiencing a vacuum drop in my brake booster right now. I'm going to go ahead and replace my master cylinder soon and I'll take a good hard look at the vacuum system at that time.

The engine and trans seem great on this car, and I dare say I enjoy this car more than I have just about any other car I've owned (altho I haven't owned real nice cars)[&o]. I have all the doctor's original receipts and know that he never missed a service interval, did not 'let the car go' before selling it, and took good care of it. So I think the car's worth keeping up. But I'm skeptical about dumping a lot of $ into the CC system. I'm actually not desperate and determined to have a climate control system at any cost, and would probably bypass the servo rather than replace it. Living in S. Cal gives me that advantage. But I also don't want to lose the unit if some preventitive work will keep it going. The original (and only other) owner of my car was a doctor who spent thousands on the climate control system over the last 28 years. He replaced the whole unit only 2 years ago at a cost of $2036. From what I've read, these units can just be counted on to fail every few years. That's a pit I intend to stay out of.

In any event, I'm very interested in any "fix it up" suggestions and tips you have. Thanks again.

RE: Re: noisy AC servo
 

Your last few lines are probably correct, we owners of MB's with these servo will spend a lot of money on them. I did a lot of investigation into these servo's and they don't last long, especially if you don't change the anti freeze regularly, the servo has to come off when you flush the antifreeze by the way and the servo cleaned with water. I downloaded tha PDF file on a digital replacement for this system, but the cost to me is prohibited and I am not so sure how reliable it is. In the troubleshooting and throughout the document, it says, the "original sytem and vacuum system must be in proper order, in order for the new system to work." The previous owner didn't have any heat of AC for 15 years. He only drove it in the summer. but that might have made problems worse. I do have heat now, only in defrost. it seems when I replaced the foam tubing to the in car ambient sensor, the bi level, auto hi or auto lo, buttoms won't turn the fan on. Before I replaced this hose, which was missing all the buttoms worked but no heat. I started looking for vacuum leaks and got nowhere. So as soon as it warms up I am taking out the in dash control unit and see if I disconencted something behind there. As you can change the foam hose by taking out the glove box, but your really blind after that, as the sensor is in the middle of the dash and with a hose in a big hand, I might of disconnected something. I troubleshooted the amp, check vacuum for the def to bilevel, and auto hi and lo and all are within specs. I am really at a dead end . The engine tranny all everything else is in great shape, on my 79 also and it run very, very well. But the HVAC system is a nightmare. I don't know if the 15 years of no use damage the system, but when the previous owner decided to sell the car, he tried to get it going, had the servo rebuilt, new amp, and he does have more mechanical skills then I and couldn't get hot air. At least I got that far.

RE: Re: noisy AC servo
 

all servos are noisy..if you hear them ,,there working ....check for cracks in it at the bottom ,,,coolant leaks was big prob with them ..

RE: Re: noisy AC servo
 

Yes, they are noisy when they start to go. I had mine rebuilt and I can't hear it from inside the the car. Usually, I am told, the noise is from the gears, which start to corrode over time. The blower is noiser then the servo now.

RE: Re: noisy AC servo
 

Thanks a lot for the valuable sharing of experiences, guys. It helped me decide what NOT to do, which is a big plus.
I understand that the CC/HVAC system wasn't even installed in Euro cars, and just wasn't well made. I won't bother to replace the unit if it fails.
It would be nice to be able to take the servo apart enough to clean and lubricate the gears, but frankly, I haven't been able to find any materials on the disassembly or maintenance of the unit. In fact, I have not been able to find good manuals of any kind (my Chilton is woefully inadequate, and I hear that Haynes is even worse). So that makes me reluctant to start taking stuff apart.
I'm going to focus on keeping the important stuff running, and not sweat the non-essentials.
If anybody has any more suggestions on CC/HVAC maintenance, or even knows about a good technical manual for 77s, please let me know.

RE: Re: noisy AC servo
 

Replace with new model servo from Unwired Tools they are a site sponser.