Cruise Control...Uncontrolled Acceleration!

Venetian · 02-15-2013, 02:57 PM

The cruise control worked fine when I first got the car. Several years ago it failed. If I try to set speed, it accelerates quickly till I shut it(cruise) off. I assume its the computer? Or could it be something else. I don't use it often but would like to get in working order. Thanks.

wwest · wwest's Garage

The early models used a vaccuum powered servomotor. The system uses two electrically actuated solenoid air valves, one to open the servomotor volume to atmoshere, decline to a lower speed, and a second one to open the servomotor to engine vacuum, accelerate to a higher speed or "make-up" for vacuum loss. The most probable failure would be the open to atmosphere solenoid valve.

Venetian · 02-17-2013, 06:15 AM

Thanks for the response. That appears to be a $500 item. I will see what I can find used.

PCA7GGR · 02-17-2013, 06:23 AM

Check to see if the control unit has a Audi symbol - ;>)

spuggy · 02-17-2013, 10:24 AM

It's certainly worth checking the valves, but it could also be the computer; I had a series 2 (1986) 944 with precisely those symptoms - checked everything per the manual with the Fluke, swapped the brain and it worked perfectly.

If you know anything about soldering, you'll immediately see on opening these 80's units that most would benefit from judicious use of a solder sucker and some fresh solder; "cold" joints everywhere, big puddles of flux etc. However, if this doesn't fix it, this may annoy the folks who rebuild them and they may not accept it as an exchange core anymore, so that's kind of a gamble ($250 for a rebuilt unit with core exchange, versus $650 for new - or one that you really don't know either way from Evil Bay)...

I know SC's use the same cruise control as series 1 944's and 928s (928 part #). Don't know what the '87 911 uses (looks like 78-86 is one type, with 87-89 another).

I think I'd try to find/borrow a known working computer that you can swap in just to diagnose and figure it out from there.

kodioneill · kodioneill's Garage

Have you taken your speedo out lately? If yes, it could be the wiring. You can have the ecu rebuilt for $90.00 if you find it's the problem check ebay for a repair center.

wwest · wwest's Garage

Quote:

Originally Posted by Venetian

The cruise control worked fine when I first got the car. Several years ago it failed. If I try to set speed, it accelerates quickly till I shut it(cruise) off. I assume its the computer? Or could it be something else. I don't use it often but would like to get in working order. Thanks.

The "computer", is working, in the sense that it can be switched "on" from an off state, actively venting engine vacuum to the servomotor thereby commanding a higher speed. Which begs the question, once that servomotor is "filled" with vacuum through what method do you switch it off?

atc_marty · 02-18-2013, 11:48 AM

The valves and servo sound like the culprit, but if that fails you can have the solder points redone and the capacitors checked on the CC computer. The CC wasn't working at all on my 87 when I bought it. I took it to a local electronics shop and they fixed it right up. At $80 it was cheaper than a new computer, that's for sure!