John Orr

My 1987 S4 has developed a little problem, and I just don't know where to start looking. The crusie control worked fine till yesterday, now its stopped working all together!! Checked the fuse, and its fine. Where do I start looking to fix this prblem? Any help is always appriciated

John
87 S4 AT

Adrian

Dear John,
The first place to look is under the brake pedal. There is a microswitch under the brake pedal which inhibits the cruise control or turns it off when you hit the brakes. The most common problem is that this switch is damaged and is telling the cruise control that the brake is applied.
If you had a manual you would find a similar microswitch under the clutch pedal as well,
Ciao,
Adrian
911C4

John Krawczyk

The cruise control amplifiers are also notorious for failures. Check that too.

go to the main 928 list and do a archive search. you will find many a post on the subject.

Brian Bowman

Adrian,

The bit about the switches under the pedals intrigues me. Do they exist on the earlier cars as well? What do they look like and are they easily replaceable? Thanks!

-Brian Bowman
1980 928S 5sp.

Randy V

Another area to investigate (after the switch, before the CC brain) is the vacuum hose(s) off the brake booster that power the CC mechanicals.

John Orr

To all:
Thanks for the list of things to check. I will start looking this evening when I get home from work.
Adrian, is there a way to bypass the switch to test the whole system before I replace the switch?

John

Adrian

Dear John,
Bypassing the switch is not hard as long as you find out if the switch is normally open with the brake pedal at rest or normally closed. The alternative is to check the switch right now and see if it is open or closed. Reverse the situation and see if the cruise control can be activated.
It is a legal requirement that all cruise controls be de-activated as soon as the brake pedal is depressed. The only way known is via switch system which detects this movement. There may a brake sensor detection system but I doubt it. The input is normally a discrete. An earth.
Ciao,
Adrian
911C4

PS: The early 964 cruise control came from the 928. In the 964 there is the switch under the pedal and another under the clutch which can make for some interesting effects.

John Orr

Thank Adrian:

Bypassed the witch this morning, and still no CC, so now will test the vac. lines,s and if they are ok, I guess I move on to the brain.
John K. you mentioned the amplifier. I am not sure where or what this is. Can you give me a short description and location?
JOHN

BlueEyedGirl

Hee, hee.

I realize you are worried...

but,

cruise control on an 87 928-S4...my baby would probably run away if I actually used it on her.

No offense...just humor

BE and Trixie

Tabor

On 944s it is very common for the cruise control "brain" to go bad.

Jay Wellwood

A possible fix to your CC brain if it is dead is to re-solder all 130+ points on the board.

John Pirtle has done this (as some others) with positive results.

I have a spare 'resoldered' brain awaiting installation as I type this (driving weather is still good here).

I'll let ya know in the Spring once I get my Shark out of hibernation (and Intake removal/repaint).

John Orr

How bout this one everyone!!! Last evening, I tested the vac. line, and it was fine!! Went for a drive and put the CC on, and it worked just fine!!! I have no idea what happened before, and why its working now. No matter what I did, it would not work at all, now its working fine. Any ideas what might be happening?

JOHN

Randy V

Intermittent vacuum loss - replace vacuum line to CC (it attaches to a hard plastic line that runs down behind the brake fluid resrvoir).

Also, inspect the vacuum check valve and replace as required.

John Orr

Thanks for the suggestion Randy, I will do just as you said to do this weekend. I will let you know how things turn out.

John

DougM

When the CC goes bad, it usually happens over time, sometimes working, sometimes not. You can get the brain rebuilt for @ $175 with a 2 year warranty. Or you car try the solder it yourself method. Don't buy a used brain unless it is rebuilt because it will also be 15 years old like yours and may go bad at any moment.