The data buss is a pair of wires used to transmit signals between all the computer modules on the vehicle. Both wires have approximately 6 volts on them supplied by the body computer. If either wire has a break, (open circuit), is shorted to ground, or if both are shorted together, the signals will be lost. The various computers will stop receiving information and won't know what to do. The hand-held diagnostic computer that plugs in under the steering column also communicates on the data buss. It will not work either when there is a buss problem. An entire diagnostic manual was produced to troubleshoot this problem. Sorry to say, but this is the price you pay for buying a car with a whole bunch of over-complicated, unnecessary, unreliable computers. There are no other choices either. Every car today is way over-engineered.
Although a lot of safeguards are built in, any computer on the car has the potential to short out the data buss. Your mechanic will run around unplugging each computer one by one until the car runs. In your case, he only has to read the message in the odometer as he tries different computers. Logic would dictate the anti-lock brake computer would be the first one to try since you have a warning light on. In addition, there is the engine, transmission, auto-load leveling, air bag, remote keyless entry, vehicle theft and security system, electronic vehicle information center, instrument cluster, front power module, radio, and heater / ac computers. The body computer could be the problem too.
Be thankful you don't have a GM product. With the push of one button on the hand-held scanner, all the computers on the car are locked electrically to the body computer. You will never know it was done, and the car will run fine, until the day the body computer needs to be replaced. Since all the other modules are now programmed to match the body computer, they are worthless to thieves, and to salvage yards because they won't work on other cars, and they can not be reprogrammed. Means you can't buy a good used one; you are tied to buying it from the dealer and having it programmed for your car. A very expensive repair. The funny thing is, ... If the body computer must be replaced and all the other computers are locked to the old body computer, you must replace every computer on the car and have them all programmed. Do you have any idea the cost of up to 46 computers and the time to program each one over an internet connection? Gonna be a lot of crabby people who were tricked into buying them with their "free" government rebates. My fear is there is going to be a lot of bad word-of-mouth publicity for GM cars and trucks, and they will have an even harder time trying to sell them in the future.
Luckily Chrysler hasn't pulled that trick on unsuspecting customers yet, but there's no denying cars today are very unnecessarily complicated. To help your mechanic save time, (and your money), be as observant as possible of other clues. For example, when the problem occurs, does it only prevent the engine from starting, or will the engine die while you're driving? Does the "Check Engine" light come on while driving? If it does, do not disconnect the battery. Diagnostic fault codes will have been memorized that will provide clues to your mechanic. Disconnecting the battery erases those codes. Will the engine start immediately after cycling the ignition switch off and back on, or must you wait a while before the engine will start?
As far as the ABS light, it should turn on when you first turn on the ignition switch, then go off after a few seconds when it's done performing a series of self tests. If it stays on without blinking off for half a second, there is a problem with the computer or one of the power feeds. If it blinks off, but comes right back on before you move the car, the computer detected a problem with one of the wheel speed sensors or a problem in the hydraulic control unit or its sensors. If the light goes off and stays off until you drive from a few hundred feet to a mile or more, there is a mechanical problem with one of the tone rings by the wheels. Those observations will also help speed up your mechanic's diagnosis.
Fuses will not be the problem because once they blow, that circuit is dead. It won't magically start working later. Intermittent problems like this are definitely not something a do-it-yourselfer should monkey with. Everything you try could add another variable into the equation. Unplugging even one connector could affect half a dozen sensors and set a whole bunch of fault codes in the various computers. Your mechanic will likely waste valuable time following those clues to non-existent problems. Intermittent electrical problems are hard enough to diagnose. Since troubleshooting can only be done when the problem is occurring, the memorized fault codes will provide clues about what was happening when the problem last occurred.
Although this might turn out to be an expensive problem, you will likely get the most efficient repair from the dealership mechanics since they will be very familiar with your car. There are a lot of good mechanics out there at independent repair shops, but they don't specialize in any one brand or system. Dealership people specialize in certain areas and the mechanic assigned to your car will have already worked on many others, possibly some with the same problem you're having.
Wednesday, December 2nd, 2009 AT 1:39 AM