The way this works is the input speed sensor tells the Electronic Automatic Transmission Controller (EATX), how fast the input shaft is turning, and the output speed sensor tells it how fast the final drive is turning. Essentially that is engine speed and wheel speed. The EATX uses that information to determine if slippage is taking place in any clutch pack and it sends that information to the Body Computer which sends it to the instrument cluster where it is interpreted. The cluster runs the speedometer which is a "stepper" motor. That is not a regular motor. It is four coils of wire that are pulsed by the computer to position the magnet in the middle. The pointer is attached to that magnet.
When slippage occurs in a clutch pack, or when there is a sensor or other electrical problem, a fault code is set, then the system goes to "limp-in" mode. That's where it stays in second gear so you can drive slowly to a repair shop without needing a tow truck.
The speedometer problem can be caused by any of the three computers, but the Body Computer and instrument cluster can't send the transmission system into limp-in. Based on the multiple symptoms, I'm pretty sure the speedometer will work once the transmission problem is solved.
Here are a few generalizations. If the speedometer DOES work, the two speed sensors are working. If it goes to second gear as soon as you shift into drive, suspect a sensor, pressure switch, or other electrical problem. If it starts out in first gear, then goes back to second gear during or right after an up-shift, suspect clutch slippage.
As for the codes, there are two wires that travel between all of the car's computers. They take turns transmitting information on them and reading information from other computers. It's possible the EATX isn't transmitting any information, but I've never run into that problem. Replacing that computer should be pretty straight forward. It should have one large 60-pin connector. Before you replace it though, be sure to check all the fuses.
Wednesday, March 30th, 2011 AT 4:26 AM