Shane

Nope not fixed on any of my 928s. Sorry!

JHowell37

I just spoke to a guy in TN who works for a place that rebuilds the CC brain. They've got an auction on e-bay where they're selling the service for about $90. I think I'm going to give it a try. The guy said that when they do an auction like that, it's because they do a lot of those repairs. He also said the rebuilld comes with a lifetime warranty. I'll pull it out, send it to them, and see what happens.

TM___

Ok, this is a 20y old thread, but there are so many on the cruise control malfunction that I didn't want to start a new one.

I have just retrofitted cruise control in my '84 Euro S (310hp) 5sp manual, and have ordered a refurbished brain from a ECU repair service in the Netherlands.
It seems that my speed is also oscilating, but not always, so I wanted to hear what the normal behavior is for the brain?
I have all new parts installed: steering column stalk and vacuum servo brand new, overhauled brain from a supposedly reputable source.

Now if I engage the CC at lower speeds (50kmh/31mph), the brain is fluctuating the throttle, especially if it doesn't find the right setting right away (eg. uphill driving). It speeds up the car very quickly, going over the set speed, it slows down and repeats. This cycle is slowly slowing down until it is at a steady speed (and steady surface).
If I get it just right when engaging (when I keep my foot on the pedal a little longer), it immediately keeps the speed steady.

It happens less at higher speeds (the car doesn't speed up that fast), so it stabilizes more quickly.

The resume function is always showing this behavior, as it always has to bring the car up to speed first.
It is as if the servo is actuating the throttle way too hard, going over the set speed, and then again when it slows down under the set speed.

Is this normal behavior, or should I request a replacement unit from the seller (I have a 1yr warranty on the unit)?
Or would this indicate a vacuum leak?
Thing is, sometimes it does it just perfectly, not fluctuating the throttle as much. A vacuum leak would not really allow that I guess?

honerboys

don't know if he's still doing these repairs but here is where i sent mine in 2017 - still working perfectly - i seem to remember it's the capacitors that go bad..but ?

https://rennlist.com/forums/928-foru...agnostics.html

mkhargrove

make sure the cable it tight

TM___

Which cable do you mean? The cable from the servo to the throttle? It is a snug fit, a tiny bit of play (as per the WSM).
As I'm getting used to it, I can mostly use it without much shaking, but it does happen, especially when resuming speed.
Maybe it is just the way it was built originally, there probably isn't a circuit to slow down the acceleration when it almost reached the set speed.

The brain is a refurbished one, just done by a specialist. So I'm inclined to believe it works as it should. That's what I want to find out for sure.

TM___

It may be more present in a manual car as mine, as the gear I'm in allows for swift acceleration...