The climate control servo in these systems often causes one of the most common battery drains. About 500ma.

I have never actually traced the current flow through the defective servos but have thought the problem to be corrosion caused by coolant leaking into the electrical areas. I suspect that someone might have disconnected it either cause of this problem or to prevent it on a system that wasn't being used (you said the A/C was out).

I learn new things on these systems still, but I can tell you that I have learned very little from the manual. I hate fault tree testing and only resort to it in a last ditch way on reverse engineering design concepts. I can not diagnose something I do not understand. Reading fault trees to understand a system needs a rosetta stone.

The reason I didn't mention the adjustment earlier was that I wanted to be sure that the system was responding. As long as the system changes blower speed in response to temp change the servo is moving. It can still be moving based upon the wrong calculation but if it is consistant and just skewed then the adjustment is an answer.

Years ago my brother built us a tool that analyzes this system. MB dealers around the world would have saved thousands maybe millions of hours if MB had designed and handed out such a simple device. It did these things:

First it mounted in line with the one plug on the servo that is control (the other is all blower connections). It had two lights one red and one green. The lights designated the polarity of the motor and thus which direction the servo was moving (green -colder, red - hotter). There was an analog gauge that read the feedback potentiometer (this tells the amplifier the same info for calculating control amplitude) and gave a reading of the relative position of the guts of the servo. And perhaps the nicest feature was the ability to over ride the amp and control the servo independently. This ability allowed us to discover one of the most common intermittant failures. This failure was where a gear had a broken tooth and locked up the mechanism. After seeing this and finding that we could back the mechanism up and it would then pass through this position (position predictable because of the meters view of feedback pot), we eventually took apart a few and found the problem.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

When fixing the whole A/C system, I also had to get a new temp sensor (mounted within the speaker grill on the passenger's side). Without this sensor though, the system "thought" that it must be freezingly cold (since it didn't get any current from that particular sensor) and the heater blower WAS definitely responding my switching blower speed (higher and higher). As soon as I connected the sensor, it never would do that again, making me think that it's either just not cold enough, or that I will just have to make the adjustment on the little white gearwheel you were talking about.
The other thing is, that I really would like to figure out, when and by what impulse the aux. water pump should come on, since opinions seem to differ here.
Steve, you said it's a matter of temperature - could I trick the sensor (where is it? it's not the interior sensor, right?) by cooling it down with a plastic bag full of ice cubes or whatsoever?
The testing machine for the servo your brother built should definitely have turned you guys into millionaires! I wish I could build stuff like that!
T.

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1980 300D, 156K

I'll definitely check out the 2 amp fuse right away!
keep you updated, T.

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1980 300D, 156K

Kyle,
checked out the fuse you were metioning - it's the weirdest
fuse I've seen in my life.
It's a small glass pipe w/ a blue and a green stripe on it, and there is WHITE SAND in it! ????
Do you have an idea what's that supposed to mean?
And - sorry to bother you with all that fuse stuff - what is it for again? The aux water pump?
Have a great day, T.

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1980 300D, 156K

This is a great thread, was enough to get me started,

Here is what I did,

Used the search for Control Unit Solder to find this thread
Went to Home Depot and got the 25W soldering iron = $12.97 + Tax
Took the ashtray and radio out, the unit itself
Opened the plastic plate, examined the board and saw right away 3 bad points that needed soldering. I did about 6 of them in all.
Put it back in the car, WOW everything works !!!!!!!!
I have heat now !!!!!!!!!

Funny how much it matters when you are driving in a cold car freezing your butt off ....

Cool thanx for the help guys

I fixed mine as well by carefully resoldering the amp board , it solved 99% of my ACC problems , I love the fact that all these little parts can be taken apart and fixed . At the moment I am fixing a weeping aux water pump on the 300CE by replacing a 5 cent O-ring , any other car would have a sealed unit that couldn't be taken apart without self destructing , eg Ford or GM .

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Democracy dies in darkness, you have to kick at the darkness till it bleeds daylight