W123 Mercedes VDO Tempomat Cruise Control Actuator Replacement

The Mercedes-Benz VDO W123 Tempomat cruise control is now going on 42 years of service for the first years of production. Or perhaps it is no longer in service due to mechanical failure. 

These units, based on the research I have done, are pretty robust and are serving a good long time. That said, my 1981 240D with the bellows style VDO Tempomat actuator has been baking under the hood in the desert for many years and is finally not holding proper vacuum. 

Mercedes W123 VDO Tempomat cruise

NOTE: Before you get a new actuator, they are $640, make sure it is not the amplifier (the brains) under the driver side dash or a vacuum leak elsewhere. The amplifier is subject to intense heat and vibration and they do get fickle with age. Several symptoms of a amplifier going bad is being able to increase your speed using the cruise control lever but not being able to set that speed once you let go. Other scenarios include the cruise control cutting out while engaged. 

I have already replaced my control unit with a remanufactured unit several months back as new units are not available from Mercedes.

Once the cruise control began working, I realized it had probably not been used in many years and the actuator began to malfunction after several trips on the highway. The symptoms: Once the speed was set, it would steadily lose it a few miles per hour every several seconds. Either the Tempomat was losing vacuum due to a dry rotting bellows or the internal vacuum/vent switch was not engaging properly.  Either way, in my case, it was time for a new unit as I knew I did not have any vacuum leaks.

NOTE: It is also important to note that the cruise control actuator is to be hooked up to the 2/3 way yellow and gray vacuum check valve (some check valve used for the door locks) which draws vacuum from the vacuum reservoir tank located in the trunk above the gas tank. Make sure this is not leaking or it will cause problems with your cruise control among other problems.

REPLACING THE VDO TEMPOMAT CRUISE CONTROL ACTUATOR

The replacement of the actuator is not complicated at all, but one that will require a great deal of finesse to remove the bowden cable, if you wish to save the cable. While there are a number of NOS actuators available from Mercedes, there are evidently only two bowden cables remaining in the global Mercedes stock. 

While the cable is not fragile, all the plastic clips that are necessary to keep the cable in place are! If you break any of these clips the cable will not function.

STEP 1
Disconnect the vacuum, and vent lines along with the electrical plug. Now is the time to make sure no vacuum hoses or rubber connectors are failing.

Disconnect the three plugs show in the photo. The gray nut for the vent line is plastic!

STEP 2
Remove the nut holding the actuator in the bracket.

STEP 3
Carefully release the bowden cable from the throttle linkage (valve cover area). Remember, the plastic clips are very brittle and are ready to crumble. Gently using needle nose pliers will work well.

STEP 4
Unscrew the plastic threaded adjusting "nut" and once again using the needle nose pliers release the next section of the bowden cable.

STEP 5
Push the cable toward the actuator to give a bit of slack to remove the next clip. This clip is awkward to remove and is too ready to fall apart. Go slow and gentle.

STEP 6
Unscrew the three screws at the actuator retaining plate for the bowden cable. Give yourself enough slack for room to work. 

STEP 7
Release bowden cable from cover

STEP 8
With the cable removed, the installation process is the reverse. 

STEP 9
The following steps are for the W123 second generation diesel models only! To adjust earlier models, W115 and W123, please refer to the Mercedes Service Manual.

Adjust the cable. If the cable is not properly adjusted according to the Mercedes service manual the cruise control will not function properly. Make the following check and adjust cable length as necessary with the plastic threaded adjusting "nut": Press and hold the emergency stop lever (diesel) in the off position and check the slack of the cable.

Mercedes R107 450SL Bumpers – U.S. Market Cars

In the early 1970's the U.S. Department of Transportation required upgraded bumpers to accept a 5 mph "tap" without any significant damage. As many will remember, these bumpers were pretty ugly and extended the length of the car significantly. 

Mounted on twin shocks, with rubber corners, rubber block overriders and rubber impact strips, they have now taken on a certain retro charm. 

Disassembling the bumper for restoration or repair is not complicated and is well worth it if you are wanting to do your car right. 

STEP 1
Both the front and rear bumper are held in place by two shocks mounted to the body of the car and by two mounting brackets on each side of the driver and passenger fenders (front and back). Start by detaching the side brackets. Next, simply remove the shock bolts and remove the bumper. NOTE: The bumpers are fairly heavy and awkward. I would recommend making this a two person job to avoid damaging the car or the bumper. 

STEP 2
Once the bumper(s) have been removed it is time for disassembly. My stainless steel caps were fairly dinged up, but purchasing new ones will cost you a considerable amount. With that knowledge at hand, I decided to have the old caps restored. You can either take the caps to a metal polishing shop who will beat out the dents and polish or, should you have the skill and tools to remove the dents it will save you a good bit of money. I chose the latter as I have metal working experience and once dent free, took the caps to be polished. I would recommend not attempting to polish at home unless you have professional equipment. Getting the mirror finish can not be achieved with consumer grade tools.

STEP 3
Next, remove the turn signal units to avoid any damage to them. Should you need one or two they are still available, as are just the lenses. The OEM unit is ULO and can be obtained from AutohausAZ

STEP 4
Now remove the shocks from the bumper. Remove the 4 bolts and extract shock along with two brackets and 2 rubber buffers.

STEP 5
With the shocks removed, the retaining nut for the overriders will be exposed—remove.

STEP 6
Remove the rubber bumper end caps by removing the 3 nuts and bolts. 

STEP 7
Now remove the rubber impact strip inserts. The front bumper will slide out, while the rear bumper will require the removal of one overrider/shock bracket to remove the impact strip.

STEP 8
At this stage the stainless steel bumper caps are now ready to be removed. Work carefully when removing to avoid bending the metal. Slightly lift up each tab to loosen the cap. Gently pry off cap from the back side (not the front lip). The tabs do not need to be opened up very much to get the cap off. 

Be mindful that you will be working around a rubber gasket and soft rubber sponge strip used to eliminate water and vibration.

With the bumper now disassembled and while the caps are off getting worked on, now is the time to clean and prep the bumpers. 

As you will see, the lower portion of the bumper is factory painted with a "rocker panel" stone guard. SEM makes a very good product for this application - Part No. 39793. Make sure to scrape, sand and clean very well before reapplying a new coating. Spray in even long strokes and do not apply in heavy coats. 

At this stage and once the bumper is dry for a good 24 hours at 75°F, the bumper can be reassembled and installed. Again, I would recommend this as a two person job.