Cruise control accelerates
Thread Starter
Instructor
Joined: Dec 2007
Posts: 101
Likes: 0
Cruise control accelerates
Sorry to keep pestering about this problem, but it seems to be fairly common - but with no common solution. I can not find any vacuum leaks, and the electrical connections to the servos are good. The problem is intermittent. Sometimes the cruise works fine - occcasionally it starts to accelerate, and neither the brake pedal nor the switch will disengage the system. Tapping the throttle cures the problem temporarily, but it often re-occurs even if I don't reengage the cruise!!??? I would appreciate anyone who has had this problem responding. PS - It seems to me that a vacuum leak would either cause deceleration or disengagement rather than acceleration - right?
Le Mans Master
Joined: May 1999
Posts: 6,517
Likes: 1,304
From: Windermere FL
I have had a couple of rare and scary instances where the car just starts to accelerate by itself for no apparent reason. However in all cases, taping the brake stops it, as does turning off the CC switch.
I am wondering if the throttle cable is kinked or bent or not operating smoothly - or if your throttle body is being locked opened somehow. Disconnect the cable to the throttle body from the cruise control and eliminate the TB and accelerator pedal cable first.
Maybe you want to leave this cable disconnected till you figure it out!
I am wondering if the throttle cable is kinked or bent or not operating smoothly - or if your throttle body is being locked opened somehow. Disconnect the cable to the throttle body from the cruise control and eliminate the TB and accelerator pedal cable first.
Maybe you want to leave this cable disconnected till you figure it out!
Thread Starter
Instructor
Joined: Dec 2007
Posts: 101
Likes: 0
Thanks for the response. Cruise cable actually goes into a black box on the left fender, as does the throttle pedal cable. Single cable exits to the throttle body. Cables seem to move freely, but I can not totally rule that out. I will disconnect and plug the vacuum input to the cruise servos for now. I sort of suspect that the "accelerate" servo may sometimes be sticking "on" after it initially receives vacuum to activate it - but I do not know how to check this out - hate to spend $200 for a new cruise servo box without knowing for sure that it is the problem.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
I wrote the procedure up for you in your other post. Did you ever try to do it. You don’t have to buy parts, it will give you a direction as to what is wrong. If you feel you can't do the procedure, maybe we can figure something else out.
If you say it still accelerates even with the main switch turned to off, remove the electrical connector to the servo. If it still accelerates, then the servo has taken on its own personality. If that is the case its probably bad.
http://forums.corvetteforum.com/c4-t...-problems.html
If you say it still accelerates even with the main switch turned to off, remove the electrical connector to the servo. If it still accelerates, then the servo has taken on its own personality. If that is the case its probably bad.
http://forums.corvetteforum.com/c4-t...-problems.html
Thread Starter
Instructor
Joined: Dec 2007
Posts: 101
Likes: 0
I confess that I have not done the procedure yet - I need to get a second dvm. I am pretty sure that the servos are getting initial voltage - if I start the car with the cruise switch off, I can hear them "click" when I turn the switch on. I have not had it accelerate if I start the car with the cruise switch off, and never turn it on - only if I turn the cruise on, experience the problem, and then turn it off without stopping the engine.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
Don’t let not having 2 DVMs slow you down. You can monitor in regard to my procedure pin E of the servo, vacuum valve relay. When you first hit the set button, the relay gets energized for a second, the car speed should stabilize and keep the same speed. Under normal operation in order to accelerate, the relay has to be energized again. In order to hold speed it might be a very short pulse of voltage. If the car starts accelerating and there is no voltage there, then the relay or servo motor is malfunctioning. If there is a steady state voltage there and it is accelerating when it should not be, then it may be coming from the cruise module.
You would think when you step on the brake pedal it would vent the system and kill the acceleration by deactivating the servo motor. Never the less with out taking some data to find out what is going on you will be guessing all day long. You have an odd problem and you need to know what, and what not are affecting certain points of the system.
You would think when you step on the brake pedal it would vent the system and kill the acceleration by deactivating the servo motor. Never the less with out taking some data to find out what is going on you will be guessing all day long. You have an odd problem and you need to know what, and what not are affecting certain points of the system.
Thread Starter
Instructor
Joined: Dec 2007
Posts: 101
Likes: 0
Thanks. Sounds like a plan! I will have to leave the DVM hooked up for a while since I can not force the problem to occur. It does "seem" to occur more frequently after driving a while on a hot day, stopping for a few minutes (or hour or so) and then starting again.
