993/964 Cruise Control Incident

[Funracer]

Title says 993/964 because I believe the 964 Cruise Control is similar. Other years may be as well.

Owned my 1995 993 for nearly 3 years, 5k miles. In that time the cruise control has been turned on for maybe 5 minutes a couple of years ago.

Did a lot of work to the car prior to this years Porsche Parade, to include removing the pedal cluster and replacing most bushings, new master cylinder and updated kinematic lever etc.

The plan was 11 hours to Dallas, spend the night with family, then on to Oklahoma City.

Seven hours in the car was running great. I got bored and decided to engage the cruise control (cc). Traffic was light to medium cruising at 75+ on highway 20 westbound.
Played with it a bit, accelerating and decelerating, turning it off and on just to get familiar with it. Noticed there is no Cruise Control Active light anywhere in my instruments. With that exception it seemed like pretty much any other cc.

Traffic ahead of me began to slow, and the SUV about 100 yards in front of me put on his brake lights, slowing gradually. This time, instead of pulling the cc stalk towards me to disengage, I stepped on the brake pedal.

I was expecting two things to happen: the cc to disengage and the car to slow down as the SUV in front crept closer. Instead the exact opposite occurred. The cc did not disengage, and the car sped up.

My first thought was that I had left a micro switch unplugged or out of adjustment in the pedal cluster somewhere when I put it all back together. Thinking I might catch the switch with further pedal travel, I hit the brakes harder this time. The cc fought me, revving the engine to maintain 75 mph.

Now the SUV was even closer, probably 30 yards. Every car I’ve had will kick off (or pause) the cc if the clutch is depressed. I pushed the clutch in and put the gearshift in neutral, at the same time stepping on the brake to slow down.

There was a 1 second lag, then the engine took off and started bouncing off the rev limiter. Clearly the cc was still engaged and trying to keep the car at speed.

Brakes were now effective so was able to slow. Just then an exit ramp appeared and I took it at probably 60 mph. Got the car slowed quickly and, with the engine still hitting the rev limiter at 7K rpm (?? not sure exactly) the only thing I could think of to disengage the cc was to turn the key off. Finally everything was quiet.

Coasted onto the margin of the off ramp, trying to sort out what had just happened.
After a minute I started the car again, ready to turn the key off again if necessary.
It started at normal idle. Drove it on some local roads at low speed everything seemed ok. I checked the cc stalk disconnect (pulling the stalk toward you) and it worked just as it had when I was playing with it earlier.

Went to Parade, which I will save for a later post, and the round trip including a couple of other stops was about 35 hours of driving over 2000 miles. The car ran great. I never tried the cruise control again. I was tempted to pull the floor board and take a look, but decided not to use the cc and deal with it when I got home.

This was a mistake.

[Funracer]

Got home Saturday night after a long day on the road, the last 3 hours at night.

Finally got to looking at the Cruise Control switches. They were plugged in. Really unhappy to see this:


Did a bit of reading. The CC pedal shutoff works through the brake light micro switch. Brake lights have to be on (or at least the plunger extended) to complete the disengagement circuit. Not only could I not disengage the cc with the brake pedal, I also did not have brake lights available for however long the white plunger was not extending.

PP does not have these 96461330100 in stock. Did find one online and should have it in about 10 days as its “special order” part. The switches Pelican has are the upgraded 96461330101 which look different but could surely be spliced in.

Removed the brake light switch. The plunger was definitely not working smoothly. Catching at any point in its travel just as it is in the pic above. I could tap it 5-6 times with a screwdriver then it would pop up and show continuity on the DVM. Finally figured nothing lost by taking it apart as it was useless as is.



Pried up a couple of the white tabs on the switch cap and it separated from the lower body. The plunger motion was course and dragging even with lots of spray lube. I played with it a bit and the plunder basically fell out.

Unfortunate I did not take a pic, but the white plastic plunger has a lip around the bottom to keep it attached to the lower body. The lip on mine was in poor shape, bent and missing part way around. I layed the plunger on a file a removed the ruined plastic lip.


Snapped it all back together and it works perfectly.

Put it back in the car. After checking the clutch pedal micro switch (it was ok) I went for a drive. Cruise control now disengages at a touch of the brake pedal. And I have reliable brake lights again, which is nice.

Should have caught it when I had the pedal cluster on the work bench a month ago.

[Funracer]

Last thing, then I will shut up. Page 59 in the owners manual:


The first paragraph is a huge understatement at 75 mph.

After I fixed everything today I tried to replicate this at 30 mph with CC engaged. What catches you is there is a delay before the engine revs. Meaning you put the car in neutral and the rpm drops nearly to idle, just like it normally would when you have your foot off the gas. Next instant the revs increase really fast as the cc tries to maintain whatever speed it was set to. With no resistance from the power train it comes on fast. This time I was ready, and flicked it off with the CC stalk.

Sure caught me by surprise on the expressway.

[A930Rocket]

Wow. What a story!

Glad you got home, and figured out what it was.

[Funracer]

Could have gone worse. And better if I had thought to use the cc stalk to turn it off. Read lots of threads here where peoples could not turn the cc on. None where someone could not turn it off.

Embarrassed too that I likely did not have brake lights for quite a while. No one likes that.

[brianlay]

Sounds like you may still have an issue.

There should be a switch on the clutch that disables the CC momentarily while you shift, at least that's how my 86 works.

My manual also has the warning about selecting Neutral above 30 mph and over-revving. I think what it's saying is don't select neutral without using the clutch or it will over-rev. That makes sense to me, never tried it!

I think you should confirm that the clutch switch is working correctly.

[pmax]

Porsche 964 & 993 Technical Forum - Pelican Parts Forums

[Funracer]

Originally Posted by brianlay

Sounds like you may still have an issue.

There should be a switch on the clutch that disables the CC momentarily while you shift, at least that's how my 86 works.

My manual also has the warning about selecting Neutral above 30 mph and over-revving. I think what it's saying is don't select neutral without using the clutch or it will over-rev. That makes sense to me, never tried it!

I think you should confirm that the clutch switch is working correctly.

It will rev even if you use the clutch. The pause allows you time to shift to the next gear. If you do not put it in gear during the pause, it will over rev to the limit after the pause. Off the cuff the pause is about 1 second give or take. See my post #3. My clutch switch is working.

[Funracer]

Originally Posted by pmax

Porsche 964 & 993 Technical Forum - Pelican Parts Forums

I debated putting it there. A lot more eyeballs here. Reference the post before yours from someone with an 86. Could affect anyone with an older cruise control.

Thanks