Cruise Control Repair

ga_mueller · 05-10-2004, 09:20 PM

Hello all,
I realize that a cruise control on a Porsche is an anathema, but if yours is broken, here's how I fixed mine.
My daily driver is a '87 Carrera, and I do enjoy the cruise control for my commute. After it became nonfunctional (intermittant operation, worked ok in hot weather), I did some troubleshooting and determined that the control box (under the passenger side dashboard) was shot. See Bentleys for a good troubleshooting guide. I stripped the varnish coating from the solder side of the board with paint stripper, then resoldered the whole board with a 25w iron. This fixed the issue for a year or so, but it came back. Symptoms again were no cruise at cold temps, but OK at hot. I then attacked the circuit board. I replaced the 2 electrolytic caps on the board (Radio Shack, I don't remember the part #s but just match them, they go bad with age), and the single power transistor at the edge of the board ( TIP31, Radio Shack). After this, the CC has worked fine for a couple of months. Total cost about $5.00 . My ECU is another matter entirely, and I have left it to Loren's capable hands... .
Good luck, George

island911 · 05-10-2004, 10:33 PM

Which components?

SC-targa · 05-11-2004, 05:11 AM

You've got the opposite of my problems on my 82 SC. My cruise control works fine when it's cold outside, but loses speed rapidly when the temps get above 65 degrees ambient. Kind of a pain, because that cruise control can really save you from a lot of tickets.

Regards,

Jerry Kroeger

Rot 911 · 05-11-2004, 05:15 AM

I think this is a thread we need to expound on. My problem is my cruise only works below 55mph. Anything over that and it will not engage although I can lift up on the cruise lever and the car will accelerate so that eliminates any problems with the acutating lever in the tunnel or vacum problems. Guess I need to take the box apart and try the clean and solder method.

ga_mueller · 06:44 AM

This board is different from mine (date code 1981, mine was 1986) so it looks a bit different. The first thing to try is to resolder the board. You need to strip the varnish off with paint stripper from the soldered side otherwise you cannot do it. Then heat each individual solder joint until the solder melts. Add more if necessary. This fixed mine the first time. If this doesn't work, try swapping the parts. The power transistor is a TIP31 from Radio Shack. I could only see one electrolytic capacitor on this board; mine had two. They are identified by two numbers; the first is the capacitance and the second voltage. They had identical replacements for the two on my board at Radio Shack. I don't know if it was the capacitors or the power transistor; I just replaced them all at once and it's worked fine since then.
Having trouble reposting the previous pic due to BB inexperience....
George

Superman · 05-11-2004, 06:57 AM

George, Island's understanding of electronics components may not be much greater than mine. He may be asking for a simple description of what needs replacing.

Island previously had looked at what might cause the CC problems we see. There is the speed erosion problem, which my unit suffers from. but Island was largely looking at the problem of the CC unit lapsing, and alternately goosing the throttle. That is, sometimes our CC units cause the acceleration/deceleration to be not smooth at all. I think these are two separate problems.

Many of us do wish we understood these problems and could repair our CC systems to work properly. We appreciate this feedback.

brcorp · brcorp's Garage

My CC does not work at all, where do I start? Brake light work fine, new CC actuator and cable, what else?

island911 · 05-11-2004, 07:48 AM

The reason for my post was to supply a graphic to help anyone that was scratching their head, whilst asking 'electro-whata caps?

I will add; I wouldn't suggest resoldering the whole board. If you can spot a bad solder point, then fix it.

. . .example of a really bad solder point.

ga_mueller · 08:21 AM

Here are the component locations. I could only locate one electrolytic capacitor.

Resoldering the whole circuit board is easy (takes about 20 minutes) and is probably the most likely failure. I've read a lot of info on the web where most of these old circuit boards (20 years or so) which are now failing are just about the first electronics of this sort used in cars. Manufacturers know a lot more about "ruggedizing" this stuff.

For troubleshooting, buy the Bentleys manual which has an excellent cruise control troubleshooting section. You need to check the fuse/relay, clutch switch, actuator, etc. The manual is really worth it for everything else, as well.

btw, I'm not knowledgable about electronics although I am dangerous with a soldering iron. It's as simple as typing in the part number in google, getting the datasheet, and comparing to Radio Shack parts which are similar. That's what I did with the power transistor.

I won't detail the trouble this got me into trying to repair my Motronic ECU...

Also, I just noticed that the circle for the TIP31 is actually above the transistor by a little bit. It's the transistor with a hole in it which is stuck onto a tab.

targa80 · targa80's Garage

The next question does anyone have a detailed schematic and parts list? I know it is a dumb question but I thought I would ask anyway. I like having schematics on hand whether I need them or not.

ga_mueller · 05-11-2004, 10:09 AM

No schematic. I searched the crap out of the net and couldn't find one. It's called a "Tempostat" auf Deutsch. That's why I resorted to shotgun....

pirin911 · 05-11-2004, 11:22 AM

I read once that the speed bleeding (my case) is caused by an amplifier going bad in the CC unit. Any thoughts?

jgporsche · jgporsche's Garage

My 88 starts with out pushing in the clutch. Would that mean the clutch switch is bad? CC doesn't work, swapped out contol unit to no avail.

ga_mueller · 05-11-2004, 12:11 PM

The clutch switch just shuts off power to the cruise control so you don't rev the crap out of your engine. Nothing to do with starting.

As far as amplifiers, the only 2 IC's on the board are apparently comparators (LM2901). You could probably replace these and all of the transistors for $10 in parts.

island911 · 05-11-2004, 12:21 PM

Quote:

Originally posted by pirin911
I read once that the speed bleeding (my case) is caused by an amplifier going bad in the CC unit. Any thoughts?

I believe it is more likely a problematic electrolytic capacitor. They are supposed to hold a reference voltage (for set speed). Howver, they can 'leak' voltage, causing the reference voltage (speed) to drop.

akopac · akopac's Garage

This is a pic from the Porsche shop manual describing troubleshooting the cruise control

dshepp806 · dshepp806's Garage

Quote:

Originally Posted by island911

The reason for my post was to supply a graphic to help anyone that was scratching their head, whilst asking 'electro-whata caps?

I will add; I wouldn't suggest resoldering the whole board. If you can spot a bad solder point, then fix it.

. . .example of a really bad solder point.

BAD SOLDER JOINT? I'd call that a DAMAGED SOLDER JOINT!!!!!!!!

Best!

Doyle