Help with Cruise Control Diagnosis/Wierdness

[Nickshu]

Got motivated today to try to troubleshoot my cruise control....also motivated by a recent speeding ticket! (Get comfortable...this is going to be a long one)

It used to work, but stopped years ago. Read lots of old threads and tested things per the Bentley manual and also some old posts here with some other tests listed.

Here's a summary:

1.) All tests of cruise control steering column switch, brake light switch, resistance at servo (terminals 3/7) and ground all show OK/good/working normally.

2.) Took apart the cruise control unit, I see no evidence of broken solder joints on the board and no evidence of burnt areas/shorts.

3.) I have no vacuum leaks at the servo....hoses replaced and in good shape.

Here's where the weirdness starts:

4.) Clutch switch does not change ohm reading when clutch is depressed, Bentley says it should show 10ohms when clutch is depressed. So I removed the clutch switch and it bench tests good, reinstalled and aligned. CC relay clicks on/off when pressing the clutch showing that the switch works. (What a waste of time that was!)....after reinstalling it still shows no change in ohms at terminal 3.

5.) With engine running I applied a 9V battery to terminals 3 and 7 (saw this in an old post). There was no change in idle speed (servo not activated).

6.) Speedo input is strange....terminal 11 shows battery voltage when engine running without the car moving and does not change when the car is moving even above 30mph (yes I drove around the neighborhood with a multimeter taped to my windshield). Also with the car off, when measuring ohms at terminal 11 while the car is rolling I see no change in ohms.


SOOOOO I need some help from the experts here. It seems like I may have two problems, both a bad speedo input and a bad Servo in the engine bay.
The speedo input being bad seems unlikely because in the Zillion old posts on cruise control troubleshooting no one ever seems to find that's the problem.
So I seem to be narrowed down to the two most common culprits...the computer and the Servo.

Is there any way to test the servo before I spend $200 for a used one? Also has anyone had the computer board look OK but turn out to be bad? Should I be looking into the speedo issue further? Speedo works fine otherwise.

[Nick Triesch]

When I bought my 85 911 I noticed that I could set my speed control but it would drop down in speed after a few seconds. I read that the cause most of the time is the cruise control module . After years of bumps and shaking it just goes bad. So, I did a search and found a replacement on eBay for $65.00 installed it and it works great. I was lucky . But I bet that's it for your car. Folks hardly use speed control overall so the other parts should be OK.

[Nickshu]

Ok. I was hoping to avoid diagnosis by exclusion but I'll send in the cruise unit for rebuild and see what it gets me. At least it's only $80-100.

[ClickClickBoom]

Cruise Control Fix

[Nickshu]

I sent it here today for repair:

911 928 Porsche Cruise Control Computer Repair Service to Your Unit 92861712700 | eBay

Will let you know how it goes.....at least will rule out one piece of the puzzle.

[salo65]

Just ordered! how'd yours work out?