Cruise Control mysteries

[jeffbottman]

I've resurrected my cruise control to some extent ('86 Targa) by reflowing all the solder joints and replacing the two large Al electrolytic caps, which were bad.

Trouble is, the speed ends up ~5 MPH above the 'set' speed (surges initially), and then alternates speeding up and slowing down over a few MPH range, as though the hysteresis of something in the circuit is too wide.

Any ideas or insights out there on how to proceed? I am thinking of replacing all the tantalum electrolytic caps also. The ICs, resistors, and film capacitors should not be troubled by age.

[Chuck.H]

Check that the plastic ferrule near the end of the CC throttle cable is firm in the square peg holder. I have to hold mine in place with a small zip tie because some part of the black plastic broke like 300k miles ago. Sorry no picture, but I simply zip tie mine in there firmly. It holds good and I use my cruise ALL the time (mostly to avoid tickets).

I suppose this could also be the CC actuator not having enough grunt to pull the throttle open quickly? Does the throttle move smoothly in and out if you pull on that lever the CC cable attaches to?

BTW, on my '89 I can lightly put my foot on the throttle and feel the CC pulling the throttle as I encounter a hill - maybe you can also feel some pulsing at the wrong times? For example a big drop down and then a gradual adjustment up which might indicate something is sticking? I think the CC makes small adjustments all the time, and if a small adjustment 'jumps' for some reason it will end up large than the CC intended and then it will re-adjust to compensate, hence speed up and down. Just a thought.

Chuck.H
'89 TurboLookTarga, 362k miles

[jeffbottman]

Thanks for all the ideas, Chuck. I will look into the mechanical aspects for sure and let you know what I find.
-Jeff

[p911dad]

If your efforts to fix the cruise control don't work out, I suggest you send the unit to Systems Consulting. (systemsc.com). They fixed the CC on our 84 Carrera.

[island911]

Originally Posted by jeffbottman

Thanks for all the ideas, Chuck. I will look into the mechanical aspects for sure and let you know what I find.
-Jeff

there are cones of rubber that make the valving in the actuator. I suspect that wear and 'hard set' leads to poor sealing and the surging issue, but have not verified. Let us know what you find.

[jeffbottman]

Has anyone removed the servo actuator, and if so how hard is it to do? In my limited experience everything on a 911 seems hard to access, with fiendishly place fasteners.

[uwanna]

Many folks, including me have had the CC "slow down-speed up" disease. Many, many threads on this forum with same symptoms. Almost all have fixed the problem by replacing control unit box with a known good one (I did) or sending their unit off to one of the repair facilities. I appreciate your electronic "chops" (I was an electronic tech in a former life) but it sounds like replacing the obvious bad caps etc aren't going to fix it.
As far as the servo, I've never seen anyone find that your particular problem was caused by the servo! Some folks have had good luck with this guy: (no affiliation)

Porsche 911 928 Cruise Control Computer 92861712700 | eBay

For $79 I think you will put an end to your frustration

Good luck,
Grant

[jeffbottman]

A couple more data points:

1. I tightened the cable via the plastic hex nut to reduce any slop in the linkage. No difference with the 'hunting' behavior.

2. The 'hunting' largely goes away at freeway speeds, above 60, which is where a person needs cruise anyway. So on the freeway, it works beautifully!

I will try replacing the tantalum caps anyway just for fun, and let you know what I find.