Strange behavior...

motogman · 07-12-2011, 07:59 PM

I had a strange thing happen tonight when coming home late. I was on a country highway traveling about 70 and approaching a cross road where the light was red. I popped it out of gear to coast toward the light and for a few seconds all was fine. The motor then started to rev wildly and when I looked down was hitting in the red zone near 7 grand. I quickly pressed the clutch and it dropped back down to idle.

It does not seem to have done any immediate damage but was a bit alarming.

I checked carpet to make sure it was not fowling the throttle (ala Toyota and others) - no problem. My feet were well away from the throttle.

The car - an 81 SC mostly stock as far as I can tell (I have had the car about 2 months) does have speed control and perhaps something a bit hay wire with speed control???

Has anyone experienced similar stuff or remember threads along this ling?

dhagood · dhagood's Garage

it does sound like the cruise control was engaged. i'd try pretty hard to make sure it didn't happen again.

DRACO A5OG · DRACO A5OG's Garage

Check at the throttle body to see if you have any thing that is causing the binding.

Maybe the cruise control cable plastic adjustment holder may have broken off. That is a common symptom.

Next time shut her down quickly, turn to on position so you can turn the steering wheel and coast over to the side.

Jim

TibetanT · TibetanT's Garage

Yes! This has happened to me in my 1983 911SC.

In my case the throttle linkage got stuck on some piece of metal hanging off of CIS intake housing; driver's side of engine. I would take a picture to show what I mean, but my camera cannot upload photos to my Mac right now. Follow your cable from the servo to the linkage and see if there is anything that might have it bind.

Hope this helps.

motogman · 07-12-2011, 08:53 PM

Great leads. I was thinking this mioght be a phantom ghost but now I have some things to check out. Thanks

i will let you know what I find in a day or two.

dhagood · dhagood's Garage

the cruise control on these cars is in a strange position. it's directly behind (on my 84 carrera at least) the windshield wiper stalk:

pulling the stalk towards you will cancel cruise control. up is set/accelerate, and down is reset.

your story resonated with me for some reason, and i think this happened to me when i first got my car. it's really easy to casually bump the lever up, engaging cruise when you are not expecting it. there is no external indication that cruise control has been activated: no lights, no siren, no punch in the nose.

if you then put the car in neutral, the engine will bounce off the rev limiter until the cruise control is shut off.

given that the engine went back to idle when you pushed in the clutch (activating a microswitch that will turn off cruise control), it is very likely that this is what happened to you.

i would still check into the other suggestions if only for your peace of mind.

kodioneill · kodioneill's Garage

Pushing in the clutch pedal doesn't cancel the cruise it just puts it in a hold mode. When you release the clutch the cruise resumes whether or not you're in gear (bad design). I rewired my ecu so it cancels the cruise (same signal as applying the brakes) when the clutch pedal is pushed. I would suggest this mod for anyone that uses their cruise control.

motogman · 07-13-2011, 02:55 PM

I cehck for stuff fowling the throttle but found nothing.

I coasted a couple of seconds before it started reving... I suspect it is the speed control and as suggested I must have bumped the speed control lever.

The rewire makes sense.... hwo did you do that?

motogman · 07-16-2011, 08:49 AM

Turns out I am pretty sure it was the speed control and an errant bump in the night...

Can easily be reproduced and caught before it revs very high. Kind of a brain dead design.

kodiondell... how did you go about wiring the clutch switch in? Does it prevent the speed control from taking off if out of gear but clutch out? Maybe not...

dhagood · dhagood's Garage

the rationale behind the clutch cruise control microswitch suspending (as opposed to cancelling) active cruise is that porsche's design allows you to shift gears and keep your speed setting. please note that activating the brake microswitch will cancel (as opposed to suspending) active cruise.

this is not a safety first design, and it can and will catch you out. it can work safely if the drive understands the nature of the design and doesn't forget or misremember or accidentally bump a control.

i knew this after it happened to me many years ago, and i'd long since forgotten it.

kodioneill's ECU mod will fix this design issue by causing the clutch microswitch to cancel active cruise, just like the brake pedal. i believe this operational mode is now standard practice.

fbarrett · fbarrett's Garage

Just goes to show that cruise control is an invention of the devil!

Frank

motogman · 07-16-2011, 03:09 PM

Regardless of how the clutch switch is wired I don't think it stops the speed control from reving the engine if coasting out of gear with the clutch out and accidently engaging the speed control.

Now that I understand the system it is no big deal as it is not a common set of conditions and when I did it the other night it was a complete 'accident'. A tap of the clutch or brake disengages the system.