MY 87 cruise control rebuild - where does this hookup to?
 

Hi guys,

I’m trying to troubleshoot the cruise control (it’s never worked as far as I remember).

I thought it might be due to the control unit capacitors been blown up, however I just figured out the Bowden cable seems to be broken.

Could anyone confirm if the two circled spots were supposed to be hooked up?

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From my '84.

Kris at cruisecontrolrepair.com got my module going again. Read about him here.

Yes, normally they are one. Even the metallic part on your throttle is part of the (almost unobtainable) bowden cable coming from the vacuum unit in the top left of the picture.
You need to find some way of connecting those again (parts from a bike brake cable ending can help) so that the cable has no slack but is also not 'pulling' on the throttle when idle (or off).

Another thing I needed to do is to replace a few capacitors on the board especially the 2 'frako's' in the middle of the board, 47 uF at 16V were fried and the reference capacitor that holds the reference speed in the lower right of the board (0.1uF) with 'one leg up' to the FET transistor in the lower right corner. (The C5 in my temp schematic here - I have a better one now with some mistakes eliminated).

Checked out all the wiring and the Ohms on the vacuum (pin 3 to pin 7 : 11-17 Ohms) unit and it works again in my case and pretty stable.

If you need further help, let me know.
It is a pretty simple system and perfectly user-fixable.

Regards,

You can see that is it fixed in place with a small lead end-stop.

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The more I know about Porsche, the more I love the support from Porsche guys like you all.

Thank you VERY much for the valuable information!!!


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There is a troubleshooting guide in the Bentley manual. Even with your broken cable, you could run through the checks to rule out other issues such as a faulty cruise lever on the stalk or the control unit. Chances are, those are fine and its just the cable, but its fairly easy to check.

I also used Kris to rebuild my cruise module, all good now
Very pleased with his service

Blair

Pay attention to the plastic adjustment connector (grey part on the cable, just below your yellow circle above). Hard to tell exactly, but it looks like one of the latching fingers may be broken. This is a very common failure, but that connection will now be loose, and can cause binding (including possible stuck throttle). There are several threads on this. Easy fix is to surround/secure the connection in the bracket with a small cable-tie (or two).

Thanks for your comment!


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Just know that the test table has an error : when measuring 'terminal 6 vs ground' (connection to the brake light circuit) your Ohmmeter won't read infinity but it will read the Ohms of the (parallel) brake lights : 1,5 Ohms in my case.
Key in ACC or Engine running : If you depress the brake pedal it should read 12V on terminal 6. If brake pedal is not pressed, it should be almost ground. That is the correct working of pin 6.

The Bently manual is in error about the test procedure. The manual is simply WRONG. Find a OEM manual or search the Pelican tech forums to find the proper procedure.

Thanks everybody for your feedback.

Today, ?? Years later, the cruise control is back to live.

As suggested below, I changed 3 capacitors on the cruise control board, and fixed the Bowden cable by connecting an extension coming from a brake/clutch repair kit ordered on Amazon.

Total cost of the repair, $5 the capacitors and $10 the repair kit. It took me a couple of days doing research trying to figure out what was doing. I might now use it very often from now on, but the satisfaction for being able to bring it to life is priceless.

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thanks for pict, do you have the detailed list of components ?
value of capacitors you changed on the board ?
nothing else changed ?

thanks !!!

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I like to wrap a couple of black zip ties in a cris-cross pattern around that plastic connection. The factory cable has some plastic clips to hold that cable in place. If the clips let go, it can directly pull the throttle, and your engine can go to wide open throttle at a bad time.

It is a pure belts and suspenders backup safety plan. But I like redundant safety.

If the original four clips are tight and not broken you might be OK. On my cable the some of the clips are broken from age.

It's extremely rare that the cruise control module has a component failure.
A simple re-soldering effort is all that's necessary.

In most cases I agree with your statement. Several years ago I sent two inoperable
CC units to an Ebay guy offering a repair service for $80 (now charges $160).

I looked over both units upon return and found the only thing done was the removal
of the varnish on the rear of the PC board and a reflow of EVERY solder connection.
Not a single component replaced! Both units worked perfectly.
Interesting thing, the guy put a seal on the unit and voided his guarantee if
you opened it up to have a look. Perhaps didn't want you to know he only did a solder reflow!

I still had several defective CC units in my parts stash and decided to try the
same thing the Ebay guy did. I successfully repaired three units with the solder
reflow operation.

A bit of caution, it is a VERY tedious job. I used a magnifying hood and a small
plastic vise to hold the board. Also used some VERY fine solder and very fine tip soldering iron. There is sure a helluva lot
of solder joints on this PC board! Kinda nerve racking but the outcome was worth it.

I was absolutely horrified by the state of my 944 climate control unit. Dull/grey/dry/cracked joints and puddles of flux everywhere. I've seen better soldering on a $5 clock radio....

My buddies early Motronic that I converted to 28 pin was little better. I failed to restrain myself from remaking most of the joints while we had it open...

For me the 2 47 uF caps in the middle needed replacing (due to a previous overvoltage), they can only withstand 16V. So I replaced those with 2x 47 uF 63V electrolytic capacitors to be more resilient to voltage peaks.
Then the reference cap on the lower right side of the board that feeds the FET transistor and has 'one leg up' (not going through the PCB) : 1x 0.1 uF radial 250V (as that was the only radial type available, lower voltage is OK here).

And : good varnish/paint remover ! As yes, I did remove all varnish from the backside and resoldered all contacts (takes about half an hour).

Didn't tough anything else and brought my 2 control units back to life this way.

thanks !!!

what did you do to remove all the old varnish ??

As said, went to the DIY store and got some varnish remover. It’s a type of gel you smear onto it and it dissoves the varnish. Clean off well, even with a little demineralised water, clean with alcohol, dry the board.

Use the varnish remover on the back side only!


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