early W123 servo bypass described/pics - PeachParts Mercedes-Benz Forum

unkl300d · #1 11-17-2007, 10:52 PM

I finally took the dive and bypassed the AC Srvo of my 1979 300D with the help of a friend.

I left the servo intact with vacuum lines and electrical harness plugged in.
I bypassed the coolant aspect.
I left the operational aux water pump inside the loop.(update, I disconnected the wire harness to aux water pump to render it inoperational- a measure taken to reduce water pressure against the closed valve and reduce minor trace warm air when valve was off.)
Update: I also disconnected the cold engine lock out switch under the servo and bypassed it by connecting the yellow and black vacuum lines together.-Prior to this my vent flaps worked fine. This measure was taken after reading more archives. No noticeable difference afterward.)

Kits usually include only a single small piece of hose and a reducer with the assumption that you will only bypass the engine coolant side of the servo along with the manual valve.

Sometimes the servo can have an internal leak so water can come out of the abandoned servo stems unless you plug them with tube and plug snubs.
I preferred to just by pass both sides and not hassle with it.
(at this point in time this is my choice, I may alter it later.)

I left the set up completely reversible.

I decided to buy the materials independently because I could buy whatever lengthd of 5/8 inch and 1/2 inch hose that I needed and a 6 foot 'choke cable' rather than supplied 5 foot cable.

Materials:
6 foot stainless steel choke cable kit from muscle cars USA item code 2332
$10.00 plus $5 shipping (909-390-7900)

The heater valve I bought from a place recommended by a forum member.
Bouchillon Perfomance engineering (843-744-6539)
Item 3115 heater valve $26.37 plus 8.98 shipping. (push to close, pull to open).

The 5/8 inch to 1/2 inch reducer bought from Napa auto parts.
Item 9910 $3.69.
You need the reducer in order to splice the aux water pump line to the main 5/8 inch line and valve etc.

I made a 90 degree elbow for the fender side coolant return line that comes around that side of the servo and under the aux water pump. I did not want to kink the hose with a false bend so I just made an elbow with 1/2 inch copper pipe.

Bought new hose clamps, at least 4 since some can be reused. You might want to buy more.

I ran the control cable through the speedometer cable opening in the firewall.
No need to drill new hole.
I found a clamp that was nice to clamp onto the carpet lining above the gas pedal and at the same time it clamps the cable body. Wrap some electrical tape around the cable body at that point to give it some thickness.

The cable is easily strung along the engine bay firewall under any parts which may get in the way and under the metal heater pipes on the firewall and under the air filter housing. Used loose wire ties to hold in place until finally installing everything.
Take your time and string it along avoiding kinks and sharp bends. Give yourself a bit of slack and commit the first sheath cut and see how easy it is to pull and puch the control knob. If it is hard, you may have too much slack. Reduce the slack and accompanying cable bend-bow and make another cut. Take your time until it feels right.
Better to start with slack than cut too short and buy a new cable !

Remove old heater hoses at each side of servo. Splice in the valve. The valve's cable clamp is positioned toward the firewall. I did not see directional arrows on the valve. Install the new coolant return line (if you choose to go that route).

Measure up the cable end relative to the valve cable clamp.
The cable has an external metal jacket and a solid wire cable inside.

Mark the place where you want to cut the cable so that it sets in with the valve cable clamp. Pull out the inner solid wire by pulling out the control knob inside the car about 3 feet. This clears the external metal sheath so you can cut that sheath to length. I used a grinder rather than hack saw for ease and cleaner cut. File down the cut edges and make sure there is no cutting edge left that may wear down the inner cable by friction.

The inner cable is wound around the valve arm. drop an end behind the wind to secure it. About 2-3 winds is enough. Press the winds togther with a plier and find a brass wire clamp of approximate size to the valve arm 'pin'.
I had to drill it a bit wider. Fatsen this clamp on theend of the pin in order to keep the solid wire loop from slipping off the 'pin' as the valve is opend and closed.

The pictures will show the coolant flow diagram, the materials and shots of the finished set up from variuos angles.

The final result was good. Although I now have heat, the down side is that my AC expels warm air when on with compressor, the basic cool air and blower setting with manual valve off also blows warm air from center vents.

This may be the result of having bypassed the return line outside the servo.

My AC pump activates with the OEM switch and my vent flaps work fine with the climate control buttons and vacuum lines.

I never use my (operational) AC anyway and normally just open windows for cool air. So this current bypass set up is OK for now.

The valve is not perfect. I suspect that even when closed it allows a bit of engine heated water to sneak by. Mechanical limitations of the valve??? update- so I disconnected the aux water pump to reduce idle water pressure. I can still feel very very slight mild warm stagnant air only from center vents with heater valve off and blower off.

Also be sure to back flush your heater core and radiator once all the hoses are dismantled. Watch all the residue come out of the core !
After the project is done, keep an eye on the radiator coolant level as you may expel an air bubble and need to top off. Try to burp the system. Also keep checking the hose clamps as they may loosen as the fitting settle.
Here are the pictures.
Good luck !