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Old 07-21-2002, 09:53 AM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
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.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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