W126 Cruise Control
 

I have a 1991 420-SEL. The cruise control recently started acting up. On one trip, it did not work at all. The next trip, when I set it, it started accelerating. Each time I set it, it would continue to accelerate. Does anybody know which module I should investigate.

I have not changed the tires, etc.

I had a problem with my w202 where the Cruise Control would engage at 30mph and drop out at 40mph. I had the Brake Light Switch replaced and this cured the problem. I have been told by a reliable Mercedes source that Tyre Pressures can affect the operation of the CC and ABS. You can try and reset the Brake Light Switch but as you have to take it out to do this, you may as well spend the £14 for the new one. It barely takes 1 minutes of time (if your a Mercedes mechanic).

Thanks, Sidsson. I'll check the brake light switch, but from everything else I've read, I am beginning to suspect the problem is in the CC amp.