Speedtoys

So..lets test the CC I say..87 S4.

I get to say 70, I set it..doesnt set, I set and hold...it accelerates, but doesnt hold.

Hmm..I pull down to resume..it starts a moderate acceleration that never stops..


That's the last time it did anything last week. Whats the CC repair checklist??

Petza914

I have a similar issue. Wouldn't engage at all so I replaced the silver boxed brain. Now it engages, but doesn't really hold steady. It can lose up to about 10 MPH in speed while staying engaged and on level ground will slowly increase the speed at which it's set, like a 5 MPH increase over a mile distance. I'm thinking it's the vacuum control in the driver's side fender that might be bad but am looking for confirmation on that.

soontobered84

Jeff, IIRC, the symptom you describe is a CC brain mal-function. And IIRC, it was determined that there is not a corrective action for this symptom other than replacing the CC brain. (or CC brain can be rebuilt professionally. Roger, I believe offers this service)

Speedtoys

Ok..thanks. My old 88 was the same way..

Where is this located so I can ez-bake it.

syoo8

Not to be snarky to the OP but this topic has been answered again and again on the board. Use the search function.

This is my post from June 12, 2013:


Do you have the workshop manual? It is section 27, pages 10-13.

The Cruise Control (aka tempostat) amplifier is inside the center console, You remove the right carpeted cover (to the left of the passenger's left knee) and I think there are two or three bolts (and I think they were an odd size, like 7mm) which will release the amplifier.


The amplifier is a silver metal box, with some taupe colored plastic at the end, mounted on top of the tunnel next to the firewall. Remove the bolt on the right side.

The workshop manuals include a check of the servo by bridging terminals 3 and 5 on the plug. "Connect a push button switch on terminals 7 and 12. Turn on ignition and press the push button switch with engine stopped. The vacuum servo should move the throttle uniformly to full load position. This requires vacuum."

Remove the multiple pin plug. Then, with a voltmeter, you can check the pins. With ignition on:

Between terminal 5 and 12: battery voltage.
Between terminal 6 and 12: battery voltage when brake is pressed
Between terminal 8 and 12. battery voltage, unless you pull cancel on the CC lever, at which point it should go to zero.
Between terminal 9 and 12: 0 volts, unless you push the lever forward on the CC level, at which point it should show voltage.
Between terminal 10 and 12: 0 volts, unless you push the lever down (reset), at which it should show voltage.

With an ohmmeter:
Between terminal 12 and ground: 0 ohms
Between terminals 3 and 7: 14 ohms
Between terminals 11 and 12: should alternate between 0 and infinite ohms when you turn rear wheel.

If you check all these and they are "spec" then your CC amplifier is no good. Both of my 928s had this problem. You can either reflow the solder in your CC amplifier (which I have tried several times with no success) or buy a used one. IIRC it is actually a 911 part, and there are quite a few of them on the used market.

The Patman

did the above, mine was bad, got a rebuilt from Roger. Works fine still today.

Speedtoys

Ya..I deserved it..my search was too limited, too rushed.

syoo8

You didn't. I had a huge headache, was crabby. I apologize.

Roger from 928sRUs sells refurbished cruise control amplifiers- with warranty. I bought two, and they both work flawlessly.