Just spent most of yesterday switching out my cruise actuator (or throttle servo unit, to use the parlance of our time) on my '87 300TD. Perhaps this has been covered before, but I thought I'd write a description of the process in case anyone else was interested in doing this, because I didn't find anything info myself.
Overview: not really that difficult or complex, except that there was one screw that gave me a bunch of trouble. Could probably be performed in 1-2 hours if you knew really well what you were doing, but if not it might take 3-4 or more (which it took me, because of that screw; read on.) I'm explaining here what it took to do it on my car. It might be slightly different for your 124 or really different for your other model.
First thing, get yourself the replacement actuator. This will really help you know what your looking for under your hood. I found a new one for $196, which was the best price I could find online. Can run as much as $250. I hate to think what they'd run at the dealership.
Start by removing the windshield washer fluid reservoir. Disconnect the two electrical connections on the front (one is for windshield, one for headlights) and the electrical connection on the top for the low fluid sensor. Next, pull out the tubes that feed the fluid to the windshield/headlights. NOTE: the fluid will start coming out, so be prepared! You can either have a bucket to catch it or just move quickly to get it out of the engine compartment. Don't bother trying, like I did, to block the outlets with golf tees, they're too big! After everything has been disconnected, the reservouir will slide up and out. Very simple.
Now comes the fun part, mostly because the thing is fairly buried in the engine. Locate the actuator along the throttle linkage. If you look past where the washer fluid reservoir was, there is a cable (from the gas pedal) that enters a bracket with a bunch of other levers and ball joints all around it. In the center is a metal devise with its own ball joint and lever connected in the mix. You may be able to make out VDO on it. This is the actuator. Boy, that's buried in there, isn't it! How are you going to get tools/hands in there? At least that's what I thought.
What you have to do is disconnect all the throttle linkage and remove the bracket that supports the linkage and the actuator. Take a good, long look at how the total thing fits together, and understand what is connected to what. You'll need to reconnect it that way. There are four ball joints that need to come apart. This is easily done with a screw driver; they just pop off. There is also a small bracket held on by two screws on the front of the main actuator bracket. This small bracket holds another cable that heads off into the car. Take off the two screws and the small bracket. Also disconnect the electrical connection on the bottom of the bracket. You will also notice the long wire from the actuator itself that leads up to a connection near the ABS. You need to disconnet the 7 pin connector and remove the wire from all harnesses. After disconnecting these things, there shouldn't be anything left holding the actuator bracket to the engine block except for four allen screws, two on each side.
Using a 5 mm allen wrench, start unscrewing. This is pretty hard, especially since you don't want to loose the screws to the depths of the engine compartment. They're buried in there deep, and the allen wrench I had was only about 5 inches long. I didn't have too much trouble with three of them, but the upper right screw, which at first I barely knew existed, was the real challenge. Part of the engine manifold extends down, preventing the allen wrench from going into the screw head straight. At first I thought I had it straight enough, but as I turned it, I could tell I was stripping the screw. So I spent about an hour and a half trying to figure out how I was going to get that screw out. I finally decided I would have to bend the allen wrench with a torch to off-set the tip. After several trys I got it to go into the screw head straight, but the first time I tried to loosen the screw, I had stripped it too much! #&*%!! So I ended up drilling the screw out. Not ideal, but I was left with no other choice. So the most important lesson I have is: ***make sure you have the right tool if you run into this problem!*** Something where you can off-set the head of the allen wrench.
After getting the screws out, the rest is pretty easy. Maneuver the bracket/actuator out of its little space and you'll have easy acess to the three screws that hold the actuator in the bracket. Remove these and switch actuators and reassemble everything the opposite of taking it out. The hardest part of reassembling was snapping the ball joints back together, particularly because of the limited space for getting pliers in there, but tenacity will win out.
That's the most of it. Good luck. Remember, once you start this, you have to reassemble or the car isn't going anywhere. You've removed the throttle linkage. So despite the potential for frustration (which I had my share of with that one screw), it's best to see it through.
Thanks Keystone,
How did you determine the actuator was bad? How many miles do you have on the car? I presume everything works now.
On sedans the washer reservoir is opposite the intake manifold. Is it in a different location on the wagon?
The RPMs were surging when I went up hill with the cruise engaged, and I was told that this is a classic sign of an actuator problem. Look at
http://www.gdl-online.com/begin.html for loads of info on MB cruise control systems. The washer reservoir on my car is on the driver side of the engine block, but regardless of the reservoir, this is where the actuator should be because of its place in the throttle linkage off the gas pedal.
Linear Mode
