Transmission Control Systems: Description and Operation

GENERAL DESCRIPTION

The AW-4 is a four-speed, electronically controlled automatic transmission. Running gear consists of a lockup torque converter, oil pump, three planetary gear sets, clutch and brake units and hydraulic accumulators.

A valve body with electrical solenoids and a transmission computer unit (TCU) are used to control shifting. The valve body solenoids are controlled by signals from the TCU. Signal sequence is determined by vehicle speed and throttle position.

Fourth gear is an overdrive range with a ratio of 0.75:1. First, second, third and reverse gear are conventional ranges. Third gear ratio is 1:1. A separate planetary gear set provides overdrive operation in fourth gear.

Cables are used for shifting and throttle pressure control. A neutral safety switch permits engine starting in Park and Neutral range only.



ELECTRONIC CONTROLS

The AW-4 is electronically controlled in the forward gear ranges. The controls consist of the TCU, valve body solenoids and sensors that monitor vehicle speed, throttle opening, shift lever position and brake pedal application.


ON-BOARD DIAGNOSTICS

Some circuits are tested continuously; others are checked only under certain conditions.

Each circuit monitored by the AW-4 TCU has a corresponding fault message assigned to it. Refer to "Powertrain Management/Computers and Control Systems/Testing and Inspection/Procedures" on page 3 for a list and definition of the fault messages.

^ If the on-board diagnostic system senses that one of the circuits is malfunctioning, it stores the corresponding fault message in memory.

^ If the malfunction goes away after the fault is stored, the fault message will be erased after 75 key cycles.