That just means there is a diagnostic fault code stored in the Transmission Computer. Almost any code in the Transmission Computer will set the 700 code in the Engine Computer and turn on the Check Engine light. That is because if the transmission slips or sticks in the wrong gear, or waits too long to up-shift, it can have an adverse affect on emissions. Any code that could affect emissions is required to turn the Check Engine light on. There are a lot of other codes that do not turn the light on.
Code readers used by the auto parts stores that read them for you for free usually do not access anything other than the Engine Computers so you might have to visit a mechanic with a scanner that can read all the codes.
Be aware too that if there's a charging system problem, system voltage will be low. There are dozens of computers on your car and they will start to do weird things. In the case of the transmission, it is electronically-controlled with electrically-operated solenoids. When they try to run on low voltage, they will not open and close properly. That alone can be detected as a problem and set a fault code. The computer can also set false codes. If the problem is serious enough, the transmission will default to "limp mode" where it stays in second gear. That is to allow you to drive it slowly to a repair shop without needing a tow truck. Don't try to drive at highway speeds with it stuck in second gear.
The anti-lock brake system is a perfect example of what is happening with the computers. It has to cycle numerous solenoids on and off very quickly to prevent skids and the computer knows it can't do that properly with low system voltage. In response it turns the system off and turns the warning light on to let you know.
The first thing to do is check the charging system. With intermittent problems like you described, that testing must be done while the problem is occurring, meaning the "battery" light is on while the engine is running. With it running, measure the battery voltage. It must be between 13.75 and 14.75 volts. Expect it to be low. If it is, that's where the two voltages on the two smaller wires on the back of the alternator will be helpful to know.
Tuesday, February 5th, 2013 AT 4:36 AM