300E Cruise Control--jerky

[kmac]

This is my first post, the cruise control on my 300E 5 speed manual is very jerky at 100--120kph. Can any one suggest a possible cause or fix. It`s to bad to put up with when driving along
Thanks kmac

[Hendrik]

May be cold solders in your amplifier(?), although usually the symptoms are that the cruise won't maintain speed.
Although jerkiness seems to be a common thing for them, I test drove a 1990 300E with a jerky cruise.
May pay to to do a search on some of the bigger MB forums in the states, just about every 124 over there has cruise.
Did ya do a google?
http://www.mercedesshop.com/shopforum/s ... post883130
http://www.mbca.org/pages/tech/MBCA_Tro ... e_cont.htm
http://www.mercedesshop.com/shopforum/s ... se+control
http://207.210.95.34/~boostd/
http://www.mercedesshop.com/shopforum/s ... post600353
http://gdl-online.com/begin1.html

[John Green]

The main reason for cruise being jerky is incorrect adjustment of the linkage from the actuato to the other throttle linakges. There is a setting for it in the workshop manual along with the exact length that the rod should be. Yes it could be some issues with the control unit, but I would check the adjustment first. I am at home so can't look it up. Sorry. :(

[Hendrik]

The main reason for cruise being jerky is incorrect adjustment of the linkage from the actuato to the other throttle linakges. There is a setting for it in the workshop manual along with the exact length that the rod should be. Yes it could be some issues with the control unit, but I would check the adjustment first. I am at home so can't look it up. Sorry. :(

SO what you are saying if the linkage is out of adjustment it will be jerky because the cruise can't do it's job properly and has to 'hunt' up and down for the correct speed. Or in other words the computer tells the actuator to do something and the actuator does the wrong thing (pushes the throttle too far) and then has to correct itself over and over, thus giving a jerky effect.

[Tony From West Oz]

This is my first post, the cruise control on my 300E 5 speed manual is very jerky at 100--120kph. Can any one suggest a possible cause or fix. It`s to bad to put up with when driving along
Thanks kmac

Kmac,
Is the speedo needle stable? A swinging speedo needle will also cause the symptoms you describe. If so, move the speedo cable a bit until the needle does not swing.

Just looking from a different perspective,
Tony

[kmac]

Speedo needle dead stable, so far. John what length for the adjustment rod?
Thank you
kmac

[Tony From West Oz]

Kmac,
That's good, just another possibility eliminated.

Tony

[Hendrik]

Just got around to re-soldering my control unit/amplifier.
Touch wood the cruise is now working all the time and steady but haven't done a highway test yet.
A few tips for those who have to do this job.
Firstly if you don't know how to solder and don't own a quality iron, don't attempt this job. This is a tricky soldering job and it's not a good idea to use this to learn how to solder. A lot of the solder joints are very close together and if you make a mess the solders will join up and your control unit will not work and possibly damage other components in the cruise control system. The reason you need a good quality iron is so that the original solder and a tiny bit of new solder are quickly melted and can set nicely, using a real cheap iron will make this job twice as hard and possibly damage some of the electrical components on the board due to heat.
Now VDO in an attempt to make their units more reliable coated 90% of the board with a layer of some sort of varnish to stop the solder joints oxidizing and to help battle vibration. This coating of goo has to be removed, do not attempt to solder through this stuff. To remove it use a methylene-chloride free paint stripper, I used a citrus based one. Then use a wire brush in a drill to carefully remove the stripper and old coating, I had to do this twice to get all the stuff off. Make sure all this varnish coating is gone before attempting to solder.
Once this is done well, you should be left with a nice clean board to work with.
Basically now it is a matter of going over all the old solders and freshening them up, it is a good idea to put a little bit of new solder on some of the bigger ones.
I did all the solders on the board and it would seem that I am now with a fully functioning cruise :D :D
A couple of other handy things are a soldering iron tip cleaner and a stand, good idea to keep cleaning the tip to make sure it works at it's best.

[Hendrik]

Speedo needle dead stable, so far. John what length for the adjustment rod?

I am not John but have a MB tech book and know how to read, according to the book:

1. For the 103 engine remove the air cleaner
2. Disconnect connecting rod at the actuator or at the throttle control.
3. Move throttle control into idle position
4. Push lever of actuator to idle position
5. Adjust connecting rod in such a manner that it is approximately 1mm shorter than the actual distance
6. Engage connecting rod
7. Mount air cleaner

This adjustment is also valid for the 102 and 603 engines.