Transmission Control Systems: Description and Operation

The Mitsubishi W4A33 is a fully automatic four-speed electronically controlled transaxle. This model is not equipped with a lock up torque converter.


The Electronic Control Module receives inputs from the following:

- Pulse Generator "A"
- Pulse Generator "B"
- Coolant Temperature Sensor
- Oil Temperature Sensor
- Throttle Position Sensor
- Idle Motor Position Switch
- Accelerator Switch
- Inhibitor Switch
- Kickdown Servo Switch

The Electronic Control Module calculates if a shift change is needed and controls the following outputs:

- Damper Clutch Control Solenoid Valve
- Pressure Control Solenoid Valve
- Shift-Control Solenoid Valve

Power Range Shift Points:

Economy Range Shift Points:

Shift Patterns
Two shift patterns are pre-stored in the control module of this transaxle. One is the power pattern (for more powerful performance), and the other is the economy pattern (for improved fuel consumption and quieter operation).

The driver can select and switch to the desired pattern by using the power/economy select switch on the center console. The solid lines shown in these shift patterns indicate up-shifts, and the broken lines indicate down-shifts. There is a difference between the shift points for up-shifts and for down-shifts so up-shifts and down-shifts will not occur frequently when driving at speeds near the shift point.

When the vehicle is stopped, there is a shift to 2nd gear in order to obtain a suitable "creeping", but when the accelerator pedal is then depressed the vehicle starts off in 1st gear.

On-Board Diagnostics
The Transmission Control Module monitors operation of the transmission control system. Some components are monitored continuously, others only during specified operating conditions. When the TCM detects a malfunction lasting longer than a specified period of time the TCM will generate an error code that can be detected at the diagnostic connector.

There are 25 diagnostic items (including normal operation) plus 7 fail safe items that will generate diagnostic codes. The malfunction codes are erased when the vehicle battery is disconnected for more than ten seconds.

If multiple malfunctions occur the diagnostic codes will be sent sequentially beginning with the lowest number code in ascending order with a 2 second pause between each code. A maximum of up to 10 fault codes can be stored in the control module's memory. If the number of stored codes exceeds 10, previously stored codes will be erased, beginning with the oldest code.

If the fail-safe system is activated, the transaxle will be locked in one forward gear while in any of the forward gear positions. This is provided so that the customer can "limp" the vehicle to a repair facility without causing further damage to the transmission.