Electrical

Cruise Control Amplifier Rebuild

My 81 Euro cruise control exhibited the normal symptoms which everyone seems to experience
eventually. I carried out all the factory tests then replaced the cable, all to no avail. Reluctantly I
turned to the "Brain". VDO would not supply a circuit diagram so I resorted to changing
components which might normally be suspected of failure. This sometimes had some interesting
results, like screaming away from a "set" speed of 80kph up to 140kph then dropping to 100kph
to oscillate there for a while, then fall to 80kph then drop out completely. In response to this I
simply replaced the components I had removed. It was obvious that even slight changes in
component tolerance has a dramatic effect on the circuit operation. After changing and or
testing 80% of components and testing in the car after every change I decided to change the
main comparators LM2901 and to my delight my cruise in "working like a bought one". Total
cost of parts 5$ Aust. PS VDO's excuse for not supplying a circuit diagram was that "Germany"
did not want people playing around with such a safety critical item, I agree BE CAREFUL

Peter Mathew


Hi All, I guess around 18 months ago I sent a post reporting that the roller costa ride resulting
from me setting my Cruise Control had been fixed by the replacement of the two comparator
chips on the control board. Which indeed it did up until about a month ago. When the CC
ceased to operate at all. Following the standard tests, the fault appeared to be in the control unit
again. Under magnified examination I found about 6 suspect solder joints. Once again my CC
was functioning perfectly. I started to think about the comparator replacement, having time on
my hands at the moment, I dug out the original chips and swapped them with the "new" chips
from 18 months ago. Low and behold the old chips work fine! My conclusion is that my PC
board was ( and maybe still is) riddled with dry joints,time will tell if I have got them all. So for
now I believe that if you are having trouble with your CC and have isolated the problem to the
control board, and you have the expertise, try critically examining ALL solder joints on the
board. I'm sorry if others have tried the comparator fix and found it did not work for them, but
beleive me initially (18months) it appeared to be my fix.

Peter Mathew