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
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.
After 3 years I finally decided it's time to try to fix the last item on my original list of needed repairs. The cruise control had never maintained the set speed. You could tell that it was activated and helping but just couldn't hold the speed. Set the cruise at 75 and right away it's just creeping down, regardless of uphill or down.
I had read that a common fix was to resolder the circuit board and this was a pretty reliable fix. So I decided to find out. I do have an extensive, if not faded electronics background. (I'm building a fuel injector tester in my spare time.)
So by removing only the passenger side, center console cover the cruise control unit comes out just fine. Oh, I did check all of the other function per the manuals 2 years ago. I removed the cover, removed the circuit board and resoldered all of the connections. This does take a while, by the way.
Today, I plugged the control unit back into the car and went for a drive. I'll be, It worked! The cruise control holds speed just fine now. So why did I wait 3 years?? Time, as I tell Jason, It's all about available time and desire. I still don't know when I'll ever use the cruise control. I tend to use the throttle to much!! But it's fixed now.
'86.5 928S Auto
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