So something happened the other day that made it pretty interesting not to die all of a sudden.
My car has now decided when the cruise control is turned on, the accelerator must suddenly be slammed to the floor. Park, reverse, drive, doesn’t matter.
The servo seems to obviously work, and looking at a diagram, there appears to be a speed control gizmo somewhere… Nobody seems to know where it is but even the Buick dealership down the street says, “Oh yeah, thats whats wrong.” But they have no idea where the thing is. All the wiring seems to just go into a big taped up loom and disappears into 1992 somewhere under the hood.
I know its not exactly a complicated car, but does anyone have any ideas what it could be or where to go about finding the defective “speed control gizmo”?
So just to be clear, by “turn the cruise control on” do you mean the throttle floors the instant you turn the CC on (and the dash light comes on) or it does it only when you try to set a speed? And if you step on the brakes, does that un-floor the accelerator or do you have to actually turn the switch off?
I believe the way this era of GM cruise control system works is that the servo is vacuum-powered (it’s the big bellows-y thing that’s actually connected to the throttle) but it has an electric solenoid that’s controlled by the cruise control module, which is a nondescript little box that’s under the hood somewhere.
The most common failure mode is you get a vacuum leak and so the servo can’t pull the throttle open enough, but the opposite where it’s always pulling the throttle wide open is pretty baffling. If it happens the instant you turn the CC on, I think it almost has to be the module. If it only happens when you set it, it could be the module or the switch or possibly even the vehicle speed sensor, although you should have a check engine light if that were the case.
One of the mechanics should be able to tell you where it’s located but aftermarket cruise controls use a magnet on the drive shaft and a pickup that senses it as it rotates by. It gives the computer an RPM count so it has a reference point. If the magnet fell off or the sensor wire got yanked off by driving over debris the computer has no way of knowing what speed to stop at.
If it’s surging widely up and down that’s a different story. then you have a vacuum leak. All that assumes it uses a vacuum motor to pull the throttle open.
If it does this in Park, when there’s no speed signal to deal with, I’m thinking it has to be a faulty module. The module is under the dash, a few inches to the right of the top of the brake pedal lever, on the dash support bracket at the right side of the steering column. I’m told it might be integral with the ECM (main computer) on some models with certain engines.
Could this not be the cruise control at all? My Saturn had an intake leak and it would go into a very high idle because of it. There must be an idle step motor of some kind. The computer must have been designed to take this into account because it would lower engine RPM when I shifted into drive. otherwise it would have slammed into 1st with all the torque built up.
Translucent Daydream, is this happening when you turn the cruise control on and then hit the set button or are you having a high idle problem?
Its OBD1, so I don’t know if it would throw a CEL, but it is not on. I wouldn’t be surprised from a vacuum leak, but I have sprayed brake cleaner around under the hood and I can’t get it to stutter any. The vacuum booster for the brakes works just fine, and the vents change position normally as well.
If its in park even, if you just slide the switch on the control arm to the “ON” position, the servo will start pulling the throttle wide open. If I hit the brake, it will cut off the wide open throttle signal and drop it, until I let off the brake again.
I don’t have a “CRUISE” light at all, so when you activate it, it just comes on.
Nobody knows where this module is though. I looked around everywhere under the dash inside the car and under the hood, its undiscovered so far.
I looked up the part online, it looks like its about the size of a deck of cards with SPEED CONTROL MODULE written on a label on the outside of it… I will look under there and report back.
Its factory cruise so it should be somewhere under there, somewhere…
It idles normally, a bit rough sometimes but it isn’t a particularly high idle. It only has about 119k on it. I sprayed brake cleaner around the intake manifold gasket on both heads, and it didn’t really stutter, but I wouldn’t be surprised if it had a small leak there. From what I understand, its prone to leaking because its a steel block and heads with an aluminum intake.
Having some sort of indicator to say the cruise control is activated is required by federal law, so if this car came with cruise control it should have a cruise control light. I suppose it not coming on is another line of evidence that it’s the module.
I believe this car is fortunately just a bit too old to be part of the horrible 90’s GM V6 intake manifold saga.
It sounds like you’ve eliminated what it isn’t. Gary-T gave the location of the modulein post 6. if it were me the next step would be to check the connection to see if there is any corrosion with the connections and also if you can get a junk yard replacement.
But before you go nuts with this consider what your goal is. If you just want cruise control then maybe you should look at aftermarket kits. It may be cheaper and involve less time than chasing ghosts. It’s a solid afternoon installing one.
I am going to look for this module either tonight or in the morning and report back. I was doing some reading and I found out that the modules had a design flaw that sort of sounds like what happened to all those red ring of death XBox 360s, where the components on the circuit board would lift up and come loose from their connections.
I have a big ass soldering gun so if I get it off, I will eyeball it really good. And take pictures.
