Yup. That's exactly what I said, ... At least in my mind. The speed control system is run by the Engine Computer which has a lot of self-test capabilities. You can test every switch input and every servo output circuit manually, one at a time, but you would need the service manual to show wire colors and connector pin numbers, you'd have to know every part of the circuit, and you'd have to know each acceptable test result. That takes way too much time but it was the only option many years ago. Many years ago these systems weren't nearly as complex, and popping on a new part as a test was not a terribly expensive troubleshooting method. Today, randomly throwing parts at a problem in hopes one will stick and fix it is the least effective and most costly way to diagnose something. There aren't many parts in speed control circuits, but for many other things we often read about all the parts people bought to "try" to solve a problem, and they don't see that they spent way more on those parts than if they had just had the problem diagnosed and repaired by a mechanic.
Okay, that said, that's why I suggested the DRB3, because it speeds things up so much and takes a lot of guesswork out of it. Now, in this case, my story about the "reason for last cutout" might not apply here in solving this one. Depending on the state of the car when you read that description, you're almost certain to find the expected or normal condition. If you plug in the scanner and then turn on the ignition switch, the reason WILL be "ignition switch power lost" or something to that effect. If you leave the engine running after driving to a shop, the reason WILL be "vehicle speed below minimum threshold".
Besides that "reason for last cutout" aid, there are "actuator tests", input / output states", and diagnostic fault codes. I already mentioned inputs and outputs. That will show whether the Engine Computer thinks the brake switch is pressed, a button on the steering wheel is pressed, and vehicle speed is high enough to allow speed control operation. All you have to do is look down that list for anything that doesn't appear to be correct. There can be some confusion with "on" and "off" because some switches are turned on when they're released and off when they're pressed, so the DRB3 displays them as "pressed" or "released".
Actuator tests on the speed control system are only allowed by this scanner with the engine not running. They don't want you suddenly sailing off down the highway from pressing a button! I did this so little that I don't remember all the details, but I believe you can select the dump valve, vent valve, or vacuum valve in the servo, and the computer will cycle them on and off about once per second. In this case you can hear them click, but the idea is if one of them is dead, you can stick a test light or voltmeter anywhere along that wire and look for where voltage is lost. If voltage is present all the way up to the servo, that just leaves that solenoid in the servo as the suspect. The only way you could do that type of testing without the scanner would be to run alongside the car while a helper drove it at least 30 mph! The scanner provides a whole new dimension in troubleshooting methods and is why mechanics can diagnose some of these problems so quickly.
I know you didn't find any stored diagnostic fault codes, but for the benefit of others who might be researching a similar problem, the Check Engine light must only be turned on when a fault code is related to something that could potentially adversely affect emissions. Many people think there are no codes in memory if the Check Engine light isn't on. On Chryslers you can read the code numbers yourself very easily by turning the ignition switch between "off" and "run" three times within five seconds without ever cranking the engine, then leave it in the run position. '95 and older models will flash two-digit codes with the Check Engine light. '96 and newer models will display them in the odometer display.
Even without the scanner there are a lot of things we can figure out by your dandy observations of how the "Cruise" light acts. First, we know the on / off switch and all of the circuitry are working. The "clock spring" is a wound-up ribbon cable in a plastic housing under the steering wheel. It is a fairly high-failure item on any brand of car. Most have individual circuits for the speed control, horn, and air bag, but all of the speed control switches use a total of just two wires. They switch in different value resistors, and the resulting different voltages are interpreted by the Engine Computer to determine which switch you pressed. I only had one switch assembly that had a crack after a crash and caused half of the speed control switches to not work. Other than that, we can assume that since the "cruise" light turns on when you press the on / off switch, the switch assembly, clock spring, and wiring to the computer are all okay.
Next, three things can make that light turn off on its own. Switches can deteriorate with age and lose their nice springy feel. I used to see that all the time with '80s model radios but it isn't very common today. A defect with the on / off switch such as that deteriorated rubber contact, deformed plastic, or anything else that causes it to momentarily turn on would cause the light to turn off. This can't be the cause in your car because at some point the light would also turn ON by itself.
Next, with so many miles of wires on today's cars, finding some that are rubbed through and grounding to the body or to each other is quite common. Road vibration will make those shorts act up while driving, but there is one major clue to both of things. That is they are random and can happen anytime. Also, they always start out being intermittent, act up just once in a while, and get progressively worse over time. You found that light always turns off after ten seconds and can be predicted to do so every time. That only leaves the Engine Computer as the cause of shutting the system down, but we still have to determine why the computer is not happy with some self-test.
Whatever problem the computer detected, we can assume it is a "hard" fault, meaning it exists all the time. If the problem was intermittent, the computer wouldn't shut the system down until it occurred. Without the scanner display to look at, the things I can think of that could cause hard faults include an open solenoid coil in the servo, an open, (break) in one of the four wires going to the servo, a dead circuit inside the computer that turns on one of those three solenoids, a problem with the cruise control part of the brake light switch or its wiring, or the computer is seeing an impossible voltage from the switch assembly, as in two switches are pressed at the same time, or it thinks they are.
I'm not aware of the computer being able to test for proper operation of the solenoid-controlled valves in the servo, and I've never seen a listing for such a fault code. If, for example, the vent valve was plugged or sticking, it is supposed to be closed, and the vacuum valve opens, for the system to increase speed. Once more than the desired speed is reached and the computer thinks it opened the vent valve, if the response is speed keeps on increasing, it knows there's a problem and will shut the system down. The thing is though, that could simply be due to you pressing the gas pedal.
We know it's not a switch problem because the light is turned on in response to you pressing the on / off switch. The switch and wiring have to be okay. If the light turns off before you start moving the car, we know it's not due to a mechanical problem with any of the valves in the servo. All that leaves is an electrical problem with the servo or brake light switch. You might try holding the brake pedal up with your toes, then see if the system engages. If it does, the switch has a pitted contact or is out-of-adjustment.
The only other indirect input is for road speed so the computer knows when it has to tug or release the servo's throttle cable. There are two speed sensors connected to the Transmission Computer. Since all the computers share information between them, a digital signal relays road speed to the Engine Computer. We know we don't even have to consider a problem in that circuit. If there was such a problem, the transmission would default to "limp mode" and stay in second gear, and the speedometer would stay on "0".
If you're near central WI, I have a DRB3. You can also buy them on the internet from the manufacturer and on eBay, but they're very expensive and not a good investment for the average do-it-yourselfer. You'd have to save a lot of repair bills to justify owning your own scanner. The only plus side is with additional plug-in cards, which I also have, it will work on emissions-related systems on any brand of car or light truck sold in the U.S. After the '95 models, and it can work on all the way back to '83 Chrysler models. Normally aftermarket scanners, which get better every year, work on all domestic models or large groups of import models, but they never do nearly as many things as the manufacturer's equipment. Manufacturer's equipment does everything you could possibly want it to, but most only work on their own brand. One of the main reasons shop's labor rates are so high is to help pay for these multiple pieces of equipment they're constantly buying, and for the expensive updates multiple times per year.
You might be able to buy a DRB3 through the dealer's parts department yet, if they're still available. I bought mine about five years ago through the dealer I used to work for. By around 2004 the industry switched to a new type of computer language and the current crop of scanners couldn't be modified or updated to communicate with those vehicles. That means, as with everything else, the DRB3 will be obsolete soon and you might be able to find them at auctions or on eBay for a better price. I still have a couple DRB2s that I can use on my '88 and '95 Grand Caravans, but the DRB3 with the plug-in card is more fun.
Thursday, January 12th, 2012 AT 1:09 AM