2002M3Drew

I had an interesting experience while driving my 911 the other day to an autocross. Cruising at a steady (indicated) 80 MPH on the highway, I shuffled my foot a bit and realized that the engine note didn't change. I realized that the throttle was still depressed even though my foot was now off the gas, and the car was continuing on steadily at 80. Hmmm. Could my long lost Cruise Control which has never worked in the 4 years I've owned the car all of a sudden come to life? I played with that staulk, and no change. I pressed the brake, hoping to take it out of whatever mode it had jumped into, and nothing..the brakes just worked against the motor, but the motor kept pulling. I briefly pressed the clutch, and the motor wanted to wind up, so I let that back out quickly. What I had was a stuck throttle, apparently.

Thankfully, I had plenty of service lane to work with. I got the car over there, made sure I had lots of clear road in front of me, and turned the key just enough to kill the motor but not lock the steering. Whew.

Once stopped, I popped the rear deck to see what was amiss. Sure enough, the black cable running around the outside and connecting to the throttle linkage was pulled out a bit. This is the cruise control linkage. It connects a servo unit on the drivers side of the engine bay to the throttle linkage. When the cable pulls, it opens the throttle. But why was mine stuck out?

As a brief background on how the cruise works in these cars, the cruise control is an entirely separate device from the motor and other electronics. It really seemd like an "add on" device. When the driver selects a speed, it electronically "remembers" that speed, and then compares the set speed with the speed the car is traveling at. Based on that, it uses a cable hooked up to a servo unit to adjust the speed. The cable reaches out around the rear of the motor, and connects to the throttle at the same point the throttle rod comes up from underneath the motor. It is like a push-pull action...when the throttle linkage is moved to open by the rod pushing up, or will naturally push the wire back the other way, outward. Similarly, if the cruise were working, and it went to accelerate the motor, it would pull on that cable, which would also pull that rod that is linked to the gas pedal up. The two controls are connected, but work in opposition to one another.

Further inspection later revealed that there is a plastic or nylon threaded coupler that feeds into a holder near the throttle. Apparently, through years of stress from the throttle linkage and cruise cable being moved back and forth. that coupler is stressed, and can snap. When you mash the throttle, the cruise control wire is forced back. Years of this action must stress that coupler, and eventually lead to its failure. One that coupler breaks and pushes out with the wire, it can't re-seat itself in the housing, and it hangs the throttle open by keeping the cruise control wire out. It can hang open at varying degrees, too, all depending on your throttle position when it sticks open.

Therefore, the failure had nothing to do with the operation of the cruise control unit, but rather a failure of the threaded couple that simply holds this wire. I think this is a failure that can easily occur with age and normal stress on any 911 equipped with cruise control. It's pretty spooky when you think about it, and even more so when you really analyze the parts and see how easily that wire can hang up.

I did a search on this, and I saw a couple of instances where a car has been stuck with the cable out and the throttle open (most recent was a white 911 in 10/07), but I have not seen any analysis of the cruise cable coupler.

The choices to minimize the chances of this happening are to either check and possible replace that coupler, or remove the wire altogether (and go without cruise control).

Here is what the cable looks like seated in the housing:




Here is what it looks like when the coupler breaks and it comes out of the housing:



I hope this helps someone avoid the same problem, and potentially avoid a serious accident! I was lucky this happened on a wide open highway at 7 AM on a Sunday. Imagine I was in city traffic, or at Watkins Glen in the esses, or anywhere else that doesn't have the room to allow for a calm, planned stop.

theiceman

great write up / Glad your okay .. that must have een a little scary

Steve Weiner-Rennsport Systems

Nice work Drew,...

Those situations are a sphincter-tightener,......

psychoideas

Thanks for the info Drew. Glad it was uneventful.

I've seen after market Cuise Controls that use a small "chain" to pull the throttle open. That way when the cruise is off and you use the foot pedal, the chain just goes slack and can't push anything out of where it's supposed to be.

jester911

This happened to me not once but twice not long after I bought my car. The first time I didn't figure it out and thought it to be some type of anomaly. The second time I found it and clipped the cable using some side cutting pliers. My throttle was stuck around 4k rpm both times. I would never trust the cruise on these cars for that reason.

88911coupe

Very useful. I have noticed that my cruise control does not look like most of the pictures I've seen but I also ran into an odd problem. I had the cruise on, which appeared to be working fine, but came onto some traffice so touched the brakes as I depressed the clutch at which point the car immediately redlned! I re engaged the clutch and mashed the brakes but it still was on, the only way to turn it off was the stalk which worked fine. I've heard this happening to others and it would seem to be a flaw in the design. Once it's back on the road I may try it again but in a controlled situation.

gerrygug

Thanks for the heads up on this safety issue. I have never been a big fan of cruise control. What would be the easiest way to disable the cruise control so this would not happen but still be able to put things back to stock in the event the car were sold?

Peter Zimmermann

As is the case with just about everything, part failure is always a possibility. Regarding the CC cable on SCs, that failure is often caused by incorrect/incomplete cable routing/installation. The pictures below show correct installation, which is crucial on cars that still have their A/C components in place (if the CC cable isn't routed correctly and zip-tied to the air hose, it can become trapped between the air filter housing and the A/C condenser under the lid). When the cable is routed and tied correctly, as pictured, it forms a smooth arc which removes stress at the upper anchor, and avoids the issue of becoming trapped. Below pictures are my '82 SC.

Peter Zimmermann

gerry: Remove the cable. If your car is CIS, look at the top picture in my post #8. In the lower left corner you can see a small metal bracket held in place by two Phillips-head screws. Remove those screws and you can pull the cable out of its bracket. At the other end (remove the black, plastic air hose to gain access) remove three screws where the cable disappears beneath a small cover. With the cover loose you will see how to unclip the cable.

Bob Rouleau

This thread would be even more useful if you would specify which year and model you are driving. A 911 is a pretty broad term these days

ron mcatee

I had the same problem a couple of years ago on my 88 Carrera, only I was in traffic. My quick fix was to wrap a piece of wire around the cable, where it separated, twisted it with pliers to make sure it would come loose and anchor it around the bracket where it went into the threaded cable.

My permanent fix was to get an old cable housing and by combining the old and new ones, made a cable from hell. I also made a bracket with a clamp that prevents the cable from flexing and breaking again. Scared the hell out me, but quick thinking/reaction saved me. Once I got the engine shut off, I tapped the accelerator and luckily it reseated itself. I guess I was lucky.

2002M3Drew

Mine is a 1983 SC US Model

Amber Gramps

My 3.2 has this same cruise control part and looks like it could go at any time. The adjustable nut for the cruise was used to bump up the idle and upon resetting the idle I see that the plastic that holds the nut on the metal bracket is broken. I could probebly get the same condition. I am sure it's a ten cent part...

sagolfer

I have had my accelerator cable stick open for just a second on hard acceleration twice. I have not been able to recreate it since. After reading this, I went out to my car, opened the engine cover and pulled the throttle back all the way (engine not running of course) and, sure enough, my CC cable connector was broken and the throttle stuck open. Here's a pic.




Thanks for the thread.

86 Targa

Amber Gramps

Pete, I think we have found a reason for book #9. This could save lives. Thanks Drew, I'm adding this to the list of to do's.