webrest

Hi All,
My car is a 1985 911 with the older style vacuum operated CC.
My cruise control works as it should apart from the annoying fact that it gradually gathers speed once set at a given.
Not dramatic increases, gradual slow climbing.
Is this a module unit problem or a servo related issue?
The module is still under warranty from a previous repair so I would like to have some idea of what is happening before that runs out.
Looking forward to suggested remedies.
Regards Dave

webrest

Bump.
No knowledge out there with this problem?
Dave

pingman

Sorry webrest,
I don't have an explanation for you. But I am watching this thread because I have an '87 that gradually slows down over a period of time. No big problem, I just reset it when it does slow down. I think all of them on the older cars have a vacuum assit with a servo. Someone will chime in soon, most folks can't resist giving out advice! Mark

MUSSBERGER

Vacuum leak? Just a guess.

rgrimm

runaway acceleration ?

About all I can suggest...if you know someone else with a car of your vintage, see if you can briefly swap out your "brain." They can do some pretty strange things when they begin to fail. It's either on the back of the trunk or under the dash (can't remember when the change took place).

webrest

Thanks for your help guys.
I know the operational part of the system pretty well as I have installed the system from scratch as a retrofit, ba.t.rd of a job. The brain that I bought off evil bay didn't work even though it was supposed too so it had to be repaired here in Australia. The system does as it should, all functions perform correctly, but this creeping is annoying. I don't think it would be a vacuum leak as that would allow it to lose speed not gain. I think that I will have to send it back to the repairer and get them to check it again.
I wonder if the servo can be bad electronically?
Dave

theiceman

The slowing down one is a vaccum loss for sure , probably at one of the switches .. but the increasing one is weird ...

Edgy01

The cruise control or 'tempostat' from that era was a total engineering nightmare. Several of the answers are on the right track, from my experience. The loss of speed could be either the black box or the vacuum devise that that cable comes out of that goes to the throttle. Years later Porsche completely redesigned those things because of all the trouble they created in the 1970s and 1980s. You're lucky that it works at all because the clutch switch usually is problematic preventing it from working at all (it always thinks the clutch is in). Personally? I would just attribute it to an older car and live with it.

Lorenfb

"The slowing down one is a vaccum loss for sure"

Not necessarily! The 10 pin cruise control ECU is an analog design with
the reference speed stored as a voltage versus a digital number as
used on the 14 pin later ECU, e.g. '88 and later 911s. This loss of speed
is one of the most common 10 pin cruise control issues. Most likely a
marginal rebuild if that was what actually was done.

"The cruise control or 'tempostat' from that era was a total engineering nightmare."

Not really! It's fairly basic design and actually works really well.

pingman

Thank you Loren,
I had checked for vacuum leaks around the cruise control and not found any. So this is why I had been following this thread. You in your infinite wisdom, have once again provided the answer to my cruise slowing down as opposed to webrest's speeding up. Thanks, Mark

dshepp806

You've some failing components within the box's internals....my guess.

Doyle

bhtaco21

86 coupe. Cruise control goes to WOT when the stalk is pressed up or down. So in the "set" position, it goes to WOT. And in the "resume" position, it also goes to WOT. Seems like CC ECU needs to be repaired or replaced?? Where would one send it for repair?
Thanks
Butch

1986 Targa

Try these folks:

http://www.autoecu.com/R&RFormOL.pdf

theiceman

id just leave it and put a post it note on it that says " boost "

bhtaco21

I like your thinking iceman! I spend a fair amount of time on the interstates, so it is nice for those situations. The back roads are another story - right foot gets a workout!
Thanks,
Butch