Servo Vacuum Leaks - PeachParts Mercedes-Benz Forum

Graplr · #1 04-10-2012, 08:58 AM

In chasing down vacuum leaks in the climate control system, it seems the majority of the vacuum leaks associated with the climate control are coming from the servo unit itself (black box under the hood). I have a handful of lines that won't hold vacuum. Some do. I can post which lines hold and which one don't if it makes any difference as I have it written down.

All tests were done with the car off and vacuum pulled at the line connections on the inner fender near the battery. Some immediately dropped to zero, some dropped and held at 2-3 and some held vacuum.

I'm sure this is not how it is supposed to operate but I do not know for certain. Does this indicate the servo unit is not good?

I really haven't driven the car too much since acquiring it last fall as I have been going through systems getting them 100% operational before putting it on the road. The climate control does respond to changes in the switches and heat does come if I press defrost. That is about all the further I've gotten with testing operation of the unit.

kerry · #2 04-10-2012, 09:50 AM

Not sure of the answer to your question, but I did provide a link to the online troubleshooting manual for a servo a few years ago. If you search on my username and servo manual, I think it should show up.

charmalu · #3 04-10-2012, 10:51 AM

Here is the manual, I had it saved.

1971 Imperial & Chrysler Auto-Temp II Service Book (Session 281) from Master Technician Service Conference

Charlie

1980sd · #4 04-10-2012, 11:51 AM

Here is some more stuff:

https://s3.amazonaws.com/sspublicweb/mercedes/Mercedes+116+Climate+Control+Service+Manual.pdf

https://s3.amazonaws.com/sspublicweb/mercedes/83-604+climate+vacuum+diagrams.pdf

https://s3.amazonaws.com/sspublicweb/mercedes/w116-vacuum-climate.png

Thanks to sshanky for the info!

Graplr · #5 04-10-2012, 09:16 PM

So if I am understanding the servos function correctly, it is to take a constant vacuum supply from the engine and then direct it to the necessary pods and blower to cause the correct function to happen?

If I am thinking about this correctly then when the blower is "parked" as described in the literature, there will be "open" vacuum circuits in the servo that won't hold vacuum when tested with the MityVac. Thus my vacuum tests on the servo are irrelevant in testing for leaks?

Do I sound correct?

1980sd · #6 04-11-2012, 07:45 PM

It has a little electric "servo" motor inside it that drives some clockwork gears that do 3 things. It drives a vacuum switch that changes which vents have air coming out via vacuum pods that actuate flaps in the dash. It also controls the blower speeds and opens/closes a water valve at the base.

There are some pics of mine disassembled here:

It's heater season! ACC fun...

Graplr · #7 04-12-2012, 03:40 PM

I just wanted to update this thread for future searchers. I was right in my last post. Vacuum leak checks originate at the servo and work back into the car as seen in service article 83-620 in the service manual.

I've started to dive into it but it will take some time to find all the leaks.

1980sd · #8 04-12-2012, 03:54 PM

Wow! great reference that I've never seen.

Can you hear the servo motor running? It should make some gear noise when the temp wheel is turned or when it parks after the ignition is turned off. You can hear it with the hood raised.

Graplr · #9 04-12-2012, 04:07 PM

Quote:

Originally Posted by 1980sd

Wow! great reference that I've never seen.

Can you hear the servo motor running? It should make some gear noise when the temp wheel is turned or when it parks after the ignition is turned off. You can hear it with the hood raised.

I believe the servo, amp and push button unit are all in working condition. The PBU responds to wheel adjustments and heat comes out when called for and does not come out when not called for. So I am just chasing down the leaks in the system and then will put it all back together and check how everything functions.

The pulley for the compressor was broken off with no belt when I got it and I have since fixed it. It is the old York style. The lines are charged (was when I got it) but I haven't taken the car out since fixing. It is a low charge and the compressor cycles while idling so I haven't been able to check for cooling at the vents. Honestly I've only driven this car around my driveway since purchasing it last fall. Its been a garage queen while I'm bringing all the little stuff up to par. Overall it is in great shape. A PO brought it to MN in 2006 and had it repainted and the interior redone. I bought it from fellow forum member TheSwede last fall who bought it from that person.

So far I've replaced the brake booster and bled all the lines, upgraded the alternator to an AL129X and the wiring, power steering filter and fluid, took the rear hatch lock out as a key was stuck in it and hatch wouldn't open and I'm in the middle of checking into the climate control system leaks. I want to replace the dash as well then I should be able to drive it.

Graplr · #10 04-15-2012, 07:14 PM

Done with going through the entire Climate Control vacuum system per the manual. Found a couple rubber lines that needed replacing and one leaking vacuum pod. It is a two-stage vacuum element for the defrost located by the driver's pedal. One way holds fine but the other side leaks. If I understand how 2-stage pods work correctly, it is the side that deactivates the pod that is leaking. I currently have the line plugged and will either order a new one or rebuild it. Luckily it is the easiest pod to get to even though I plan on putting in a new dash soon.

It was tedious to go through the system but well worth it as know I have an understanding of all the vacuum routing.