redline7921

I don't post often but would like to share how I fixed my cruise control.

1. Start by following the steps from the workshop manual to test the inputs and cruise control servo.

2. If everything checks out properly and you have the following symptoms:
a) not able to engage your cruise control
b) constant acceleration
c) increases by 5 - 10 km/hr or mi/hr when turned on
d) intermittent operation - works when car has sat all night (cold) but once warmed up does not or visa versa
e) will not engage but when you hold the control lever to on it does accelerate
Continue Reading.

I have personally experienced all of the above with two cruise control computers.

The Fix

Tools Required
Soldering Iron
Soldering Braid
Solder Sucker
Solder
Flux
Small wire clippers

Supplies Required
2.2 uf 63volt electrolytic capacitor (two required) - I used ones rated for 100 volts and with both leads on one end because it was all I could find.
10 uf 63 volt electrolytic capacitor (two required)
22 uf 63 volt electrolytic capacitor (one required)
Total parts bill approx. $5
Pics attached to show the ones needing replacement.

Take your time de-soldering the leads and cleaning the protective varnish on the board for the joints in question.

History
Initially I had only replaced the two 2.2 uf (light blue) capacitors but this did not fix the problem. My first computer which I thought was completely dead and with nothing to lose, I replaced all of the capacitors previously mentioned. In addition, I have also re-soldered every lead on the board and replaced all of the IC chips with the exception of the ones labelled with 2902 (not available at the time but plan on replacing still).
The second cruise computer, I have only replaced the capacitors mentioned and it now works. It seems that it is less smooth than the first computer (IC chips replaced). I will spend the time to replace the IC chips as well but only when the 2902 chips arrive so it can be done all at once. Once all of the chips are replaced I will report back on how it compares to current operation.

End Result
CHEAP REPAIR
Now I have TWO working cruise control computers.

BEFORE




AFTER

black944 turbo

How did you remove the varnish? I have many of the symptoms mentioned above. Mine works about 80% of the time, however it is very irritating when it acts up and I am scared to use the accelerate portion on it.

Alan 91 C2

My cruise control works, but I must engage about 2 MPH below the desired speed. As the unit will increase speed slightly after engagement.

What lead you to suspect the capacitors?

redline7921

I would like to add, that one of the first steps, as most have suggested here should also be to check and make sure you have no poor solder connections on the board.

After some research and from information on other threads regarding these cruise control computers, the best place to start was with the capacitors. They age and do not function properly over time.

http://en.m.wikiversity.org/wiki/Electrolytic_capacitor

There is a specific section on reliability and length of life.

When you heat the lead the varnish becomes compromised. Once the component is removed, I used a small slotted screw driver to carefully scrape the remaining varnish off the connection area. Do not apply a lot of pressure and be careful not to scratch the surrounding area.


1987 951

gruhsy

Great Post. I can do this fix at my work just was not aware of the process.

mikeyoman

You can often tell if capacitors have gone - the tops bulge out slightly. Not 100% test but if they are bulged they are gone.
We had issues with my mates cruise control a while ago, at the time we just re-soldered every joint on the board which worked fine. Didn't even think to check the caps.

redline7921

Often true, and initially I did not think the caps were the problem because on both computers they looked fine, with no buldges. Maybe the varnish reinforced them enough to hold their shape.

djantlive

Where is the board on my car?

gregeast

Excellent fix, thanks for sharing. Where did you source the components from?

redline7921

Location:
Drivers side under the dash. Near your left foot. An aluminum box, can't miss it. It's held on by two 10mm nuts.

I got the components from a local electronics store.

redline7921

Update:
I have replaced all the ICs on the one board and there was no change in operation after only replacing the electrolytic capacitors. It was to be expected, but I had worries during the resoldering that I had heated some of the joints too much.

Conclusion:

If I had to do it again, I would first replace only the capacitors and resolder only the joints that connect the pins to the board.

If that does not work, then resolder the entire board.

gregeast

Thanks again for this thread and for the follow up. My capacitors arrived just a few days ago, hope to give this a go in the next week or so.

Cheers!

redline7921

NP. Make sure you pay attention to the polarity of the caps so you place them I'm the correct orientation.

I would be interested to hear how it works for you when you are done.

gregeast

Will do. And if I F-it up, you may have a customer for your extra computer :-)

FRporscheman

Those pesky caps!

I've known for years that replacing the caps in older VDO clocks will usually fix them, but I just recently tried it on a tach (and it worked!), and my buddy even fixed his lopey idle by re-capping his ECU.