The VDO Cruise control unit used on the w123, w126, w124 and likely many other mercedes models was also used on a pile of other cars, including Porsche, BMW, even some Rolls Royce I’m told.
They worked good… in 1982. Not so much nowadays, many have died slow deaths, starting out surging and not always engaging, until one day they stopped working entirely.
Let’s change that. I’ve created videos showing the troubleshooting steps for each component. I’ve been working on a document that has detailed instructions, It is not finished. but I’ve attached the current state of it and will continue to update the document as I figure out everything I need to know so I can convey accurate information.
Here is the first video about testing the Actuator under the hood:
Part 2, About testing the control stalk, the power and ground wiring and all the electronics aside from the amplifier:
Part 3, Repairing the cruise control amplifier, Not a difficult task, but rather time consuming.
Here is the link to the current document:
August 2022 Added wire color codes for amp connector in case it pops apart on you.
13 Comments on “W123 VDO Cruise control repair Saga”
Great vid, but i was unable to download the pdf document, i think you want to double check your hyperlink. cheers, bill
You’re absolutely right, Turns out there was an update for the software I use for the website, and apparently it broke the button links.
I’ve fixed them now, or at least they work on my end now, hopefully you can get the pdf now!
Thank you so much for this awesome resource! This document + the video really makes troubleshooting so easy. I am going to start testing today. Thank you!
Glad to be of assistance, Having the cruise working makes the cars a dream for long road trips!
Thank you very much for generously sharing your time and knowledge. I had all but given up on the cruise control in my 240D. I have since made multiple trips to the salvage yard. and I am excited to expose where the angry little monster has been hiding all this time. This repair escapade has become a war of the wits that I have unfortunately been losing. You have put me back in the game!
Glad to help!
It is certainly a finicky, sometimes maddening system to figure out, but when it works it adds so much to the comfort these cars are so famous for. Long trips are a dream in one of these machines!
Jarod, I am just ordered my fourth cruise control actuator….I gave up on ebay (a relatively cheap waste of time, but a still waste of time if defective parts are all that is out there) and have commited to a much more expensive OEM part. My original, and each of the ebay three all had functional motors, but non-functioning “engage solonoids” (pin 7 neg, touch pin 6 pos). My data is telling me that the failed “engage solonoid” is a fatal flaw in the vintage MB design. Once I get the OEM actuator (hopefully it will pass the tests), I’ll be able to determine if I need to advance myself to your repair of the cruise control amplifier. I would have been at a total standstill if not for your instruction. If there was a technique for repairing solonoids, I bet there would be a sustainable market.
I’ve got a couple spare actuators, I started taking one apart to see the viability of replacing the solenoid, or repairing the positional feedback stuff, I haven’t done nearly as much as I wanted to with it, Its certainly on the list though!
I’m just hoping I can cut my work back a bit soon to give me more time to explore some of these things that I greatly enjoy doing.
I am confused. Can you help me understand.
The video on testing the actuator does not match the August documentation specifically on tests 2 and 3. On the video, test 2 is pin #6 is 12v+ and #7 is ground. On the August documentation it is the reverse with pin #6 is ground and #7 is 12v+.
On test 3 the video shows #5 and #7 ground with #4 & #6 12v+. In the August documentation it shows #5 and #6 ground with #4 and #7 12v+.
I have the same year 300TD as you if that helps.
I updated the documentation, but cannot update the video as it is already published, A viewer pointed out that I had the polarity backwards for the solenoid energize connections, now it doesn’t really make a difference on most actuators because a solenoid should energize regardless of how you hook it up, but evidently some of the later model actuators have a diode or something internally that will cause them to not work if hooked up backwards, and during my initial testing I was unaware of this and my actuator does not care which direction power is applied.
A technical oversight on my part due to lack of existing documentation, I was writing it as I went lol
hi Jarod, thank you for your work, I have been working on repairing my cruise control for too long now. I have just carried out all your tests from the PDF, and everything is fine apart from 1 thing…
you say, 9 Gnd (12) Between 2.8K and 4K ohms Actuator potentiometer, but I’m getting 2.42k ohms
The inside of my actuator and circuit board is immaculate. looks like it was never used so,
what problems could this possibly cause?
also, I have the official VDO wiring diagrams on paper and there is some variation with the colour of wires in the amplifier plug, some do not match your picture, and number 14 pin is empty, can I get your email so I can send you some photos and info please?
Hey, just checked the comments here, sorry I didn’t see it earlier. My email is: tom at trythistv dot com
The info in the pdf and such about colors is just my observations of the actuators I have, and what I have been able to glean online, I’d love to take a look at any info you have.
There may be variances from year to year, or even model to model. 2.4k may be perfectly OK for the actuator, you could try moving the arm by hand to see that it has a nice clean sweep up and down as you move it from end to end, the most common issue with the actuators is the trace gets a spot worn through at around the throttle needed for 55-65mph where they got a lot of use.
I’ll have to do better about checking if comments are getting stuck.