Hi wally painter. Welcome to the forum. The first thing to do is read and record the diagnostic fault code(s) that will be stored in the transmission computer any time it has defaulted to "limp-in" mode. Input and output speed sensors have been a common problem, but so is clutch plate wear. Besides fault codes, the mechanic can read the "clutch volume index" (CVI). That is a set of four numbers that indicate the volume of fluid it takes to apply each clutch pack, and tells how much clutch plate wear has taken place.
The input and output speed sensors ARE the speed sensor. Actually, the transmission computer calculates vehicle speed based on the the information from these sensors.
When it doesn't shift, it defaults to "limp-in" mode which is second gear, to allow you to drive slowly to a repair shop without needing a tow truck. There will be at least one diagnostic fault code in the computer that will lead the mechanic to the circuit or system with the problem, not necessarily the defective part. These codes are especially useful for intermittent problems that don't act up all the time.
On older transmissions, as the plates wear, shifts become sluggish and some slippage takes place while the clutch packs are locking up. Your computer-controlled transmission computer watches for signs of that slippage. When it is detected, the system is turned off and the transmission stays locked hydraulically in second gear. To reset it you must cycle the ignition switch off and restart the engine.
The computer modifies shift points to hide the effects of that clutch wear so you continue to get nice crisp shifts like when it was new, but you don't have any warning that wear is taking place until the day it can't update any further and goes to limp-in mode.
An experienced transmission expert will be able to determine the amount of wear based on the clutch volume index. Replacing the sensors and solenoid pack take care of a lot of problems but more diagnosis is required when the problem continues. The fault codes provide a lot of information but they are only meant to get the mechanic to the circuit or system with the problem, not necessarily the defective part.
Tuesday, November 2nd, 2010 AT 8:52 PM