Tim, the RPMs have been hooked up for a while now. The controller has a input for RPM signal and can handle this right from the coil signal right from the DME.

The speedometer is just yet another signal I wish to also record, but the controller only has one more input left for this and it must be 0-5v DC square wave

First, I apologize for taking this post a bit "off topic."

It "has" prompted me to dig in to my own periodically functioning/malfunctioning cruise control. I've done more than a little soldering in my life, but even with a good pair of reading glasses, I'm not sure I'm steady enough anymore to touch up bad joints on a circuit board. But I was convinced it was the brain, having had one fail a couple years ago and putting in a used unit I bought off another club member.

But to my surprise, the I ran through the Bentley tests and it appears my clutch switch is now bad. Do these just fail with years of dirt and can be successfully "cleaned up" or are you better off just replacing it? It wouldn't be the dumbest thing I've dropped $80 on...

FWIW, the switch arm is not "mechanically" sticking so it is internal/electrical in nature...

Hi, has there been more progress on this subject? Tim Polzin?

I would like to know what (which electronic component) causes the 'surging' or 'speed oscillations' of the cruise control. For example, when I set the speed to 80 mph, the speed drops slowly to 75, then quite quickly rises to a little over 80, then drops slowly to 75 again, and so on.

Regards
Thomas

Replace ALL of the Electrolytic Capacitors. They dry out from age & fail to hold the charge.
This will cause speed surges &/or failure to operate the CC at all.
If you are working on any VDO made C.C. (Mercedes, BMW, Porsche, etc)this applies to all 12-pin, pre 1981 & 14 pin, 1981-1993.
The 14 pin has two 10 mfd axial, two 2.2 mfd axial. one 22 mfd radial & two non polarized yellow .01 mfd.
If parts are not at the local Radio Shack try http://www.tubesandmore.com
Search for capacitors on the Home Page left side. Good prices.

My cruise control on '80 911SC now only works at a speed set point of 35 mph, none other. Does that indicate what to look for on the board?

> Replace ALL of the Electrolytic Capacitors. They dry out from age & fail to hold the charge.
This will cause speed surges &/or failure to operate the CC at all.

As far as I could tell there are only 2 electrolytic capacitors (2 x 47 µF 63 V). I replaced those with Panasonic EEUFC1J470B.

However, this did not fix the cruise control (it is still surging).

What should I try next? I have read that replacing the two comparators LM2901N might fix the problem. What do you think?



http://forums.pelicanparts.com/uploa...1390066561.jpg

My final solution was to send it off to folks that do these circuits all the time. It works great, cost $100 with a guarantee. This was several years ago. Someone of this forum gave me to contact info to the folks. I posted on this forum with the good results and the contact info. Don't remember right now but my car cruise control still works fine.

I think I found that message, it was an old ebay link to Automotive Scientific Inc. Your #1 Source for ECU, TCU, ABS, EBCM, Climate Control and Speedometer Repairs, repair service for $89 (not advertising on ebay anymore)

But there is somebody else now advertising a repair for only $80 on ebay
911 928 Porsche Cruise Control Computer Repair Service to Your Unit 92861712700 | eBay

David Ruberg, 123 W. Sarnia St, Winona, MN 55987

has somebody used this Service?

That first one sounds right. It is worth the cost, I am a hands on EE and know how to do this stuff but also know when to not do stuff. Good luck. Also don't expect these old cruise controls to be as good as the new ones.

I will look and see if I can find the address where I sent mine but don't count on me finding it.

Try these guys, I think this is them
Truck and Auto electronics
212 East Washington St
Rogersville, Tenn 32857

Good luck

David Ruberg - yes but not for the cruise unit. Sent the overhead computer from my husband's F350 very fast and reasonable repair. Would not hesitate to use him again!


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I am unclear on the previous post about an F150 since I thought we were discussing Porsche 911SC not Fords. Does Peligan sell Ford parts now?
Anyway, the address I noted was the folks that got my cruise control electronics working for my 1980 911SC.

Simply that I had used David Ruberg for a circuit board repair, great job and very fast


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Oh OK, I understand. I notice you are in NM. Love that state, graduated from UNM, found my wife there, married in Santa Fe, my 911 put down many miles there. Once drove from Alb. to the lab at Los Alamos in 40 minutes, what a thrill, and no tickets.

I finally succeeded in repairing the Porsche cruise control, after replacing the two LM2901N quad voltage comparators; quite happy with it, even though for 12 years I have been convincing myself I don't really need a cruise control :-) .

Just want to add that I don't want to spoil anybody's business repairing cruise controls, if you are not used to soldering (unsoldering the 14 pin integrated circuits is the hardest) it might well be worth paying somebody some money to do it ... and anyway the person advertising on ebay (mentioned above) would only do repairs within the US.

In all you replaced the 2 caps and the 2 LM2901N chips, correct?

Do you suspect all 4 components where bad?

> In all you replaced the 2 caps and the 2 LM2901N chips, correct?
yes, and I resoldered all connections on the board.

I can safely say that I had at least 1 cold solder spot somewhere on the board (before I resoldered everything the cruise control did not work at all), and that at least 1 LM2901N was bad. I cannot say for sure that an elco was bad, because I replaced the elcos first and when I then tried the cruise control it behaved essentially like it did before, but from other posts it is quite clear that the elcos very likely go bad with time, so do replace them anyway, it is easy - much easier than the ICs - and they are cheap.

PS: Just comes to my mind I could test all the components I removed, but that really wouldn't add any more info, so I am going to throw them away untested :-)

Well alright, good job, congratulations! It is always satisfying when something finally works after lots of struggle.

Where is the electronic module located? Thx