Cruise Control Brain

Tim Polzin · 10-05-2007, 07:49 AM

Over the last few years, I've been periodically trying to get the cruise control working properly. I've managed to collect at least 3 brains and swapping each out gives me three different set of problems.

Brain 1 - sometimes works pretty good (+/- 3kph), but intermittent. When not working, sometimes a good swat makes it work. I have tried reflowing solder on most pads. After a lot of looking with a fairly high powered magnifier I could not find any cold solder joins.

Brain 2 - just keeps accelerating when engaged.

Brain 3 - works really well, very little variation in speed (+/- 1 or 2 kph), but when engaged for more than 5 minutes starts loosing speed.

The board is not complex. Has anyone found a circuit diagram for it or figured out how to fix the common failures?

Tim

Rot 911 · 10-05-2007, 08:22 AM

Tim, I've had the same experience that you have. Two brains and both would work intermittently. I finally just gave up.

KFC911 · 10-05-2007, 08:32 AM

My car doesn't have CC, so I use a brick instead

Superman · 10-05-2007, 09:37 AM

Some of this has been looked at before. I had hopes that Island911 would solve the riddles. There are more than one. Part of the CC problems are in the servo unit, in the engine compartment. Again, there are two or three distinct malfunctions. Wouldn't it be nice if someone identified them and outlined the fixes. I would actually like my CC work work properly. The stalk control is SO elegant. Up-on/accelerate. Pull-off. Down-resume. German simplicity.

Tim Polzin · 10-05-2007, 09:56 AM

Quote:

Originally Posted by Superman

The stalk control is SO elegant. Up-on/accelerate. Pull-off. Down-resume. German simplicity.

And then there is the heat and fresh air controls....

The photo I used as actually from Island911. We hashed this around before, but never resolved it.

Tim

Lorenfb · 10-05-2007, 10:03 AM

"Part of the CC problems are in the servo unit, in the engine compartment."

Hardly EVER!

"Again, there are two or three distinct malfunctions"

Not really:

1. Doesn't work at all.
2. Works intermittently.
3. Doesn't hold the speed.
4. Continues to accelerate.
5. Causes the vehicle to surge, i.e. slow "hunting" between two speeds.

Remember, Pelicans Parts shows them in their list for those that don't
want to make repairing them a "life's effort" as with most DIY projects.

Rot 911 · 10-05-2007, 10:17 AM

Loren, I appreciate you weighing in on what the problems are, but I am willing to experiment more on what I have before I empty the piggy bank out for a new one! Especially after I looked up the price.

Cruise Control Computer, 911 (1978-87)
Brand: Genuine Porsche [Photo] [view in parts diagrams]
$651.00

jbrinkley · 10-05-2007, 11:01 AM

I feel bad, I just threw one away while cleaning the spare bedroom the other day.

dshepp806 · dshepp806's Garage

I doubt it to take a lifetime to address, only if your electronically astute. I've found most all automotive circuits to be quite rudimentary relative to other circuitries in other industries/specialties...usually easily navigated with proper docs/schematics...not much "magic complexities" in this backyard.............keep at it COMPLETELY...maybe will only take 1/3 of your lifetime...have you instrumentation such as a meter, scope, etc.?

Finally, were you able to obtain a schematic of this box?

Best,

Superman · 03:56 PM

With all DUE respect Loren, I wonder what you're even TRYING to say. That there are not two or three, but rather at least FIVE failure modes? Okay, I can agree with that. The notion that we can just dismiss the potential malfunction of a vacuum controlled unit that's upwards of thirty years old......nope. Not even if I were friends with somebody who rebuilds electronics.

Tim Polzin · 10-05-2007, 12:24 PM

Quote:

Originally Posted by dshepp806

..maybe will only take 1/3 of your lifetime...have you instrumentation such as a meter, scope, etc.?

Finally, were you able to obtain a schematic of this box?

Best,

You may not have caught one of my threads about 5 years ago in resurrecting a DME, that one just about consummed me.

Yes, all test equipment, but no schematic.

Tim

dshepp806 · dshepp806's Garage

No schematic presents a special challenge, eh?

Best,

DonMo · DonMo's Garage

My guess it would not be difficult to pin out the connector from the cable to one of the modern aftermarkets like an audiovox and use the existing inside the car equipment and the new brain from the audiovox. Anybody tried it. Oh by the way, I have a brain but no engine compartment cruise control equipment courtesy of the PO if you car to make me an offer (PM me).

tcar · 10-06-2007, 07:46 AM

Did you follow the troubleshooting guide in Bentley? - need a tester...

reflowing the solder joints has worked well on a couple that I had.

Tim Polzin · 10-15-2007, 08:03 AM

Quote:

Originally Posted by DonMo

My guess it would not be difficult to pin out the connector from the cable to one of the modern aftermarkets like an audiovox and use the existing inside the car equipment and the new brain from the audiovox. Anybody tried it. Oh by the way, I have a brain but no engine compartment cruise control equipment courtesy of the PO if you car to make me an offer (PM me).

I actually considered that but on the ones I looked at, a 4 contact stalk switch is needed and the stock one only has three as I really did not want to change the stalk to non-standard. Thanks for the offer for another brain, but I already have three.

What I'm really interested in is a schematic.

Tim

Phantomflyer · 02-16-2008, 03:26 PM

Hi Tim,

Sorry to bring up this old Thread, but it fits to the one i opened.
do you still have the 3 Units?
I think it would be interesting to find out what the differences of the brains are electrically. Maybe you can find a difference in resistance from all the terminals to terminal 12 or brown ground wire.

Frank

Chris Lovell · 02-16-2008, 05:12 PM

I had my cruise control brain rebuilt at GDL in Morro Bay, CA 805-772-5727. Pelican is first choice but a few years back, these guys did a good job for me.

porschenut · 02-16-2008, 09:34 PM

Quote:

Originally Posted by Kurt V

Loren, I appreciate you weighing in on what the problems are, but I am willing to experiment more on what I have before I empty the piggy bank out for a new one! Especially after I looked up the price.

Cruise Control Computer, 911 (1978-87)
Brand: Genuine Porsche [Photo] [view in parts diagrams]
$651.00

General Development Laboratories will fix your brain good as new for $245.

khamul02 · khamul02's Garage

I think the problem here is how good is "good as new"?

pranderson · pranderson's Garage

I have an 83 SC with a problem Identical to #3. , works fine then starts decellerating after 5 miles or so. I've pondered messing with it----well, having a mechanic mess with it, but never have just because I figured it would be an expensive can of frustrating worms. Luckily I can have a fairly steady foot when needed.