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  #1  
Old 01-28-2008, 12:34 PM
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Location: Redmond, WA
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Cruise Control Actuator Rebuild Thread (pics soon)

I went to the Pull n Pay today and grabbed 3 VDO cruise control actuators from W123s and W126s and am planning to work on rebuilding. Plus my own. Total of 4

Three of them have motors that spin, one doesn't have any but I can probably use it for parts in some other fashion.

What I've discovered is that the three that do spin, the motor spins like its turning but the actuator arm that controls the throttle linkage does not move. This is because there's a spring tensioner inside of the motor which has worn out and is no longer keeping two of the gears together like it should. I'm going to try adding a shim or re-crimping the spring and seeing what comes out of it.

I'll post pictures of the whole process when I actually get into it soon, or at least a walkthrough of the inside of your VDO cruise unit, even if I can't fix it. I figure, the ones with good motors but no turning are good candidates for rebuilds.

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Old 01-28-2008, 09:31 PM
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Some Updates:

I've determined what I think is one of the most common causes of failure. Keeping in mind my sample size is of 4 electronic VDO cruise servos from W123 and W126 Diesel engines only.

The servos came from the following vehicles:

W123 -- 131k Miles
W123 -- 230k Miles
W126 -- 140k Miles
W126 -- 339k Miles

Both W123 and one W126 servo engage when given +12V power between pins 4 and 5 of the cable. However, the actuator arm which attaches to the throttle linkage never moves.

Opening them up inside, there are two points of failure:

First is a tensioning spring which holds a retaining clip in place. While "open", this clip allows for the removal of the internal gearing. While "closed", the clip holds the gears together so their teeth mesh. This spring stretched out in all 4 of the servos opened, resulting in no gear meshing. I fixed this problem with a shim, made out of a thin piece of plastic.

Second is a compression-return spring which holds a metal plate to a quarter-circle gear. This spring ensures that the gear, which directly moves the actuator arm itself, continues to turn with the metal plate. This spring wears out and the plate and the plastic gear move independently of each other. I haven't been able to fix this one quite yet, but I'm going to try epoxying the two together in place so they don't need to be retained by the spring.

None of the units showed any wear on any of the 4 large gears of varying sizes which are inside at all. My guess is the retaining springs wore out before they would've worn down from use. One unit showed significant internal corrosion, the gasket between both halves of the unit probably leaked and ruined it internally, but even its gears were okay.

Pics shortly.
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Old 01-28-2008, 09:37 PM
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I did a stint as a reliability engineer in the past. I would be hesitant to change anything at all in these servos. What looks like a quick fix by substituting what seems like a similar part could be changing a fundamental engineering requirement and will most definitely change the characteristics of the design. My guess is MB has tens of 1000's of hours of reliabilty testing on these since they are a safety component, your substitutes will not. Seems pretty iffy on something as critical as a hard link to your throttle.

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