Shane

Last time I got to Drive my car (Last July ) I engaged my Cruise Control and it began ok but soon began to oscillate over a 5 to 10 mph spread.
It would go from 60 to 70 and slow back to 60 and repeat. Needless to say I turned it off, can't have the surrounding traffic think there is an idiot driving such a nice car.
What needs to be checked first? Vacuum, or CC brain.

Brian Bowman

Shane,
There's a simple diagnostic which I read about some time on here.. don't know how accurate it is, but here goes:

If you can accelerate with the cruise... (i.e. NOT the gas pedal, just pushing the cruise control arm forward), but it won't hold speed, you have a brain problem. If you can't accelerate in this manner, you've got a vacuum problem.

Again, don't know about the accuracy of this testing method, but it's something to try!

-Brian
<img src="graemlins/xyxwave.gif" border="0" alt="[bigbye]" />

Chris

From John Pirtle's web site <a href="http://members.rennlist.com/pirtle/svc.html" target="_blank">http://members.rennlist.com/pirtle/svc.html</a>

Cruise Control
The Automatic Cruise Control (Tempostat) often fails for several reasons. The electrical connection at the vacuum servo corrodes, the vacuum system leaks, or the control brain fails. The workshop manual includes a good troubleshooting procedure in section 27.

Check fuse #4 (S4). The same fuse runs the stoplights.

Test the vacuum to the reservoir by connecting a vacuum pump to the line running from the check valve (connected to the brake booster) into the fender. It will take quite a number of pumps to bring up the vacuum in the reservoir.

Remove the left front wheel and inner panel.

In the rear of the fender is the vacuum servo. Check that the electrical connection is clean and tight. The servo is removed by removing the bolt at the bottom. Also check the vacuum line for twists or kinks.

If everything checks out, replace the fender trim and wheel.

Remove the two side carpet panels of center console.

The control is a silver metal box mounted on top of the tunnel next to the firewall. Remove the bolt on the right side.

Remove the multiple pin plug.

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."

Check the switch controls with a volt/ohmmeter. Turn on ignition (do not start engine).

Connect the voltmeter between terminals 5 and 12 (ground). Terminal 5 should have battery voltage.

Connect the voltmeter between terminals 6 and 12. Terminal 6 should have voltage when the brake is pressed.

Connect terminals 8 and 12. There should be voltage. Pulling back on the cruise control lever (cancel) should go to 0 volts.

Terminals 9 and 12 should have 0 volts, but pushing the lever forward (set) should show voltage.

Terminals 10 and 12 should have 0 volts, and voltage when pushing the lever down (repeat).

An ohmmeter should read 0 ohm between terminal 12 and car ground.

Terminals 3 and 7 should read @ 14 ohms.

Terminals 11 and 12 should alternate from 0 to infinite ohms while turning one rear wheel and holding the other.

If everything checks out thus far, bend open the metal tabs of the control and pull the circuit board out.

Carefully use a soldering iron and heat each joint.

Replace the control making sure to correctly orient the multiple pin plug.

Hopefully your cruise control now works!

Shane

Thanks Brian and Chris,

I tried the acceleration with CC lever and that works fine. But in 33 days I'll check on the things that Brian posted. Very detailed and I thank you for that. I'll be ordering the Manuals today.

Shane

Sorry the things that Chris Posted. I'll check on those. And hopefully I might see you around Spokane this Summer. I'll be in the area visiting some friends and family there and over in Idaho (Orofino area)
Thanks again

Randy V

Shane, just an FYI that you can edit your previous posts. Just click on the little 'paper and pencil' icon within the post you want to modify.

Shane

Thanks Randy

_Thomas_

Hi does anybody have more details about this problem? Which electronic component(s) should be checked (and how)?
Regards Thomas

Angel84-928

re-read post #3.

use the WSM and this site: http://members.rennlist.com/pirtle/svc.html

_Thomas_

What I meant was 'does anybody know the board electronics'. I have done everything suggested in post #3; my cruise control works, the problem is the the same as the original poster's (Shane's), he never reported here whether he could fix the problem.


Which electronic component (IC?) is the most likely to cause speed oscillations? When I set the speed to, for example, 80 mph, the speed drops slowly to 75, then quite quickly rises to a little over 80, then drops slowly to 75 again, and so on.

_Thomas_

*bump*

has still nobody without a commercial interest found a solution to this problem?

The closest I found so far was by Tim Polzin
http://forums.pelicanparts.com/porsc...ml#post3789513

but the 'surging' is not discussed unfortunately.

Landseer

Some folks re-flow the solder connections within the silver brain box (located under the HVAC ductwork, on the center console, all the way forward against the firewall).

Search the forum for reflow, solder, cruise, and similar words. Might bring-up some of the threads I've read here on the subject of cruise oscillations. I'm having similar situations and intend to attempt to fix in this manner. Figured it couldn't hurt to try. I have not done this, however, just to be clear.

Diagnostics may or may not pinpoint the issue as they may not anticipate an age-related breakdown of multiple connections.

Cosmo Kramer

On mine the reflow got the cruise to work again, but I got the oscillation. I ended up replacing the brain. I think there is an electronic component in the brain that fails that causes the oscillation problem.

_Thomas_

Thanks Ted, that's what I thought.
I would be very glad if somebody could tell me which electronic component has to be replaced.

docmirror

I gotta do this. Mine runs crappy, won't hold speed.