4EAT: NEW DESIGN FOR '99

First introduced in the 1987 Subaru XT, the four-speed automatic transmission (4EAT) is a complex engineering accomplishment that has proven to be reliable, efficient and fun to drive. Available in all Subaru vehicles sold in the U.S. today, the 4EAT uses a combination of sophisticated hydraulic components controlled by an even more sophisticated electronic control unit known as the Transmission Control Unit (TCU).

Like many automatics, the Subaru four-speed automatic uses a combination of hydraulic clutches, one-way clutches and planetary gear sets for its operation.

For 1999, extensive modifications to the 4EAT further increase its efficiency by reducing friction loss and by working even more closely with the engine management system to enhance fuel economy.

TCU BASICS
A basic understanding of the TCU is necessary to fully appreciate the design enhancements to the 4EAT. The TCU works in harmony with the engine control module (ECM) for predictable, sure shifting. It also helps to ensure that the vehicle is always in the right gear at the right time.

A great deal of engineering effort has been put into how the transmission shifts. "Shift shock," the jolting movement experienced during gear changes, receives extra attention to make sure that drivers get a firm up and down shift without undo fuss. To help reduce shift shock, the TCU alters the way the transmission shifts based on loads and driver demand. For example, say you shift from either Park or Neutral into Drive or Reverse. In those instances, when a gear is selected and engine speed is above 1,500 rpm, fuel to two cylinders is momentarily cut and the engine ignition timing is momentarily retarded to reduce the driveline shock.

The TCU further reduces shift shock while the vehicle is being driven. Based on preprogrammed variables, the TCU signals the engine's computer to reduce engine torque (it retards ignition timing momentarily) to minimize the occurrence of shift shock.

CONTINUOUS SMOOTH PERFORMANCE
While under way, the TCU continually monitors how the vehicle is being driven and applies it to the transmission's learning control system. By monitoring engine air intake it can determine load demands and operating altitude. It can then alter shift schedules (software maps). Up shifts can be delayed; down shifts can occur sooner or a specific gear can be selected as in the case of Anti-Lock Brake operation. This system also compensates for wear of the internal clutch's friction materials and works to reduce shift shock caused by that wear. Using a sophisticated formula, the TCU compares target clutch engagement times to actual engagement times. The TCU then changes the timing of the solenoids that allow gear engagement to occur.

Other significant inputs to the TCU include a throttle position sensor (which measures how far the gas pedal is depressed and the rate at which it's pressed), an engine temperature sensor and a gear lever position sensor.

Combined, these inputs help decide which software map is to be used to determine when to lock up the torque converter and which gear is to be selected.

What sets 4EAT apart from the rest is how All-Wheel Drive is incorporated within the transmission. A future issue of Drive will explain how the TCU controls the Subaru AWD system.