Carpokes Porsche Forum

Prelude
Have you ever taken anything apart and wished you had taken better notes or photos? I have.

This post is part of a continuing series of posts that detail the internal construction of various things. This is not intended to be step-by-step instructions for repair/removal/disassembly/etc. Rather, I hope this post (and future posts) can serve as a reference for people who would like visual clues in making their own repairs.


Previously Unopened Cruise Control Brain Module
- How to Read a Capacitor
- Another Capacitor Guide including default assumptions and conversion between micro/pico/nano Farad

Front Side - High Resolution
On computer: Right Click on image > Open Image in New Tab > Click anywhere to zoom in (may vary depending on browser)
Back Side - High Resolution
I may have taken this photo after replacing the electrolytic capacitors. Shouldn't matter for following traces on the board, though.
On computer: Right Click on image > Open Image in New Tab > Click anywhere to zoom in (may vary depending on browser)
Cruise Control Caveats
In the future I will post a how-to regarding replacement of the small number of electrolytic capacitors (the cylindrical cans). The numerous rectangular boxes are poly film capacitors and have a very very low failure rate, even when quite old, so they probably won't be a problem. However, I have personally seen a professionally repaired cruise control brain module with a few poly film caps also replaced. Over the years, I have read a few reports that some of the IC's (integrated circuits, ie "chips") also fail.

My point, "re-capping" your cruise control brain module may not fix your problem. In addition to bad caps and potentially failed IC's, problems can also exist in the under-hood actuator, improper slack in the cable from the actuator to the throttle, and in the wiring to/from the brain module for speed input, column switch input, brake pedal activation, actuator control/feedback, etc. I believe Clarks Garage has a guide for testing the electrical inputs/outputs at the brain module harness connectors.

Furthermore, I believe you need at least one working brake light for cruise control to engage (quirk of the design?). If you have switched to LED brake lights (without any load resistors), the brake light circuit will appear to be an open (broken) circuit (or at least high resistance compared to the ~5Ω of an incandescent bulb) and the cruise control won't engage.

Hi Usury - thanks for posting. I'm in the middle of this process so far and there are a few things not in the Clarks reference.

How to bench test for servo operation?
Servo Pin 2 - GND

Test Motor - Motor spins
Servo Pin 7 +12v
Servo Pin 1 GND

Test Clutch Clutch - Audible switching of actuator
Servo Pin 5 +12V

This is probably not a complete test, but if these aren't working, you know its dead.

Mike

usury wrote: Thu Nov 17, 2022 12:51 pm
Furthermore, I believe you need at least one working brake light for cruise control to engage (quirk of the design?).

Not a quirk. The ground for the clutch in the mechanical servo is provided through the brake lights. This provides a redundant cruise control release mechanism in case of some kind of cruise module failure.

whalenlg wrote: Thu Nov 17, 2022 2:36 pm Hi Usury - thanks for posting. I'm in the middle of this process so far and there are a few things not in the Clarks reference.

How to bench test for servo operation?
Servo Pin 2 - GND

Test Motor - Motor spins
Servo Pin 7 +12v
Servo Pin 1 GND

Test Clutch Clutch - Audible switching of actuator
Servo Pin 5 +12V

This is probably not a complete test, but if these aren't working, you know its dead.

Mike

Hey Mike, can you elaborate on these tests? Is this while the box is in the car or out of the car or what is the process? Thanks - Paul

The test is performed with the connector to the servo in the engine compartment disconnected, and voltages applied to the servo side. Unfortunately, this does not test for the most likely failure, which is the carbon potentiometer strip in the servo wearing out. There are a number of videos on youtube that discuss the cruise system.

Clark's gets one thing wrong, if the speed jumps 5-10 MPH and stays there when you set the speed, it is not the servo unit, it is the control unit storing the wrong speed. If the speed is erratic, it is likely the servo potentiometer.

Oh, gotcha - just to test the servo unit. I was thinking this was for testing the control box functionality. Thanks!

The cruise control was my nemesis for some time. Never could get it to work. I finally did and I detailed the saga in this post on other forum. I hope this is ok to reference.
https://forums.pelicanparts.com/porsche ... ntrol.html

Any idea why the control unit stores the wrong speed? That is my finally CC problem.

dme wrote: Tue Jan 14, 2025 12:06 pm The cruise control was my nemesis for some time. Never could get it to work. I finally did and I detailed the saga in this post on other forum. I hope this is ok to reference.
https://forums.pelicanparts.com/porsche ... ntrol.html

It is!

Thanks for sharing - it will help someone or several someones in time I'm sure.

dme wrote: Tue Jan 14, 2025 12:08 pm Any idea why the control unit stores the wrong speed? That is my finally CC problem.

I've never found a reversed engineered schematic for one, so I am working from my belief on how it operates. I think that you have a cracked solder joint leading to the input of the latch that stores the speed, or the latch itself is bad. This causes the speed to be stored wrong, but the rest of the CC is working, and it controls the vehicle speed to the incorrect value.

This is where Clark's is wrong, as it states that the servo is suspect when the speed jumps up. Yet it is obvious that the servo can control the speed quite nicely, it is just being directed to the wrong one. If the throttle action was jumping around, I would suspect the servo position feedback resistor in the servo.

I had my CC apart four times:
1) Did not see the cracks.
2) Found and resoldered a few cracks.
3) With stronger magnification, found and resoldered a few more.
4) Resoldered every connection on the board.
Now it works.