The Subaru 4EAT transmission is a four speed, microprocessor-controlled transmission. Changes in your diagnostic approach, due to these electronic transmission controls, are required.
The Subaru 4EAT is a four speed microprocessor-controlled transmission that was installed in many 1987.5-98 Subaru vehicles. It is not a three speed transmission with overdrive. It features a lock-up torque converter, which locks up in all forward gears except 1st.
The shift quadrant has been designed in accordance with the four forward speeds. In P, R, or N, there aren’t any special features. With the selector in D, the transmission shifts through all four gears. With the selector in 3, the transmission shifts 1, 2, 3. When the selector is in the 2 position, the transgear is computer selected to prevent the engine from over-revving.
Manual 1st gear is only activated when the 1- HOLD button is depressed and the shifter is in manually selected 2. This will provide engine braking. The transmission will upshift through 2nd and 3rd if necessary, in order to prevent the engine from over-revving.
The 1-HOLD indicator is displayed on the com- bination meter when the button is activated. When the computer overrides the 1-HOLD gear selection the display will change.
An enhanced version of the 4EAT was introduced with the 1990 Legacy. Although similar in design to the earlier 4EAT, the shift quadrant is different. The Legacy 4EAT has a seven-position quadrant: P-R-N- D-3-2-1. The 1-HOLD button has been deleted and a manual button has been added.
When the selector is in 3rd range, manual switch ON, the transmission will start in 2nd gear and shift to 3rd. In 2nd range manual, the transmission starts and stays in 2nd gear, but will upshift to 3rd gear at 6500 RPM to prevent damage to the engine. In 1st range manual the transmission stays in 1st gear and also will upshift to second at 6100 RPM to prevent dam- age to the engine. Additionally, on all wheel drive (AWD) vehicles, the TCU applies a more aggressive AWD map when the selector is in the 1st position, manual switch ON or OFF. These changes result in improved driveability on low traction road surfaces.
In 1995 the manual button was deleted. When 3 is selected, the transmission shifts through1st, 2nd, and 3rd.When 2 is selected, the transmission shifts 1st and 2nd. When 1st is selected, the transmission stays in 1st. On 1995 through 1998 model year vehicles, the TCU controlled upshift logic was replaced by a fuel cut logic.
The 1993 Impreza was introduced with fuel cut logic, never having an auto up shift logic. In a transmission equipped for AWD the TCU utilizes a program which continually changes the degree of AWD based upon vehicle operating condition(s).
1992 through 1997 SVX used the same shift logic as the 1990-94 Legacy. The Manual button was retained until production of the vehicle was discontinued in 1997. The TCU monitors various engine and vehicle inputs, i.e., throttle position and vehicle speed, etc. It also controls the electronic shift solenoids in the transmission.
The TCU monitors various engine and vehicle inputs, i.e., throttle position and vehicle speed, etc. It also controls the electronic shift solenoids in the transmission.
Electronic Control System Overview
The electronic control system consists of various inputs (sensors) and outputs (lights and solenoids) in addition to the Transmission Control Unit (TCU).
This is the second generation of Subaru automatic transmission. In addition to being smoother and quieter, it is designed to help maximize fuel economy while providing performance.
It monitors the engine and transmission performance conditions, the driver’s demands and the vehicle speed.
Transmission Control Unit
The TCU is a highly sophisticated microprocessor with a self-diagnostic long-term memory. It also has a failsafe function, which maintains driveability in case of a major electrical component failure.
In a transmission equipped for AWD the TCU utilizes a program which continually changes the degree of AWD based upon vehicle operating condition(s).
The TCU controls shifting and line pressure in addition to the lock-up torque converter and the MPT clutch.
TCU Inputs
- Throttle sensor/idle switch
- Vehicle speed sensor #1
- Vehicle speed sensor #2
- Tachometer signal
- Inhibitor switch
- Cruise control signal
- ATF temperature sensor
- Ignition/battery voltage
- 1-HOLD switch
- Forced front wheel drive (FWD)
The throttle sensor/idle switch is basically electrical throttle pressure. The load signal affects: shifting, line pressure and lock-up. The closed throttle input affects the lock-up release mode as well as smooth downshifting into 2nd gear. It also causes a reduction in the pressure.
Vehicle speed sensor #1 is mounted to the transmission and is basically electrical governor pressure. It is used to detect vehicle speed and it affects shift points, lock-up and line pressure.
In FWD transmissions, the speed sensor reads parking gear rotation at the front output shaft. In AWD transmissions, it senses the transfer clutch drum rotation at the rear output shaft.
Vehicle speed sensor #2 is built into the combination meter. In FWD units, it is used as a back up for speed sensor #1. In AWD units, it is used as the front output shaft speed sensor.
An electric speedometer system was first introduced on the SVX in 1992. It was later added to the1995 Legacy, 1996 Impreza and 1998 Forester. The system uses a Magnetic
Resistance Effect (MRE) speed sensor driven by a con-ventional speedometer drive gear system. The speed sensor, which generates four pulses per revolution, is located on the front differential housing.
The Speedometer Driving Unit (SDU) receiving pulses from the MRE sensor processes the signal send-ing the information to the transmission control unit.
The TCU compares the speed signal from the front output shaft with the signal from the rear output shaft (sensor #1). The speed differential helps the TCU determine the degree of AWD (along with other inputs).
The tachometer signal affects the shift points at kick-down. The TCU uses the signal to prevent the engine from over-revving.
Note: The TCU will override the inhibitor switch, if necessary, to prevent the engine from over-revving.
The cruise control signal tells the TCU of cruise control activation. This allows for a wider operating range in 4th gear unless a large speed differential exists from the set speed in which case the transmission may downshift. This improves fuel economy.
The ATF temperature sensor is located on the lower valve body next to duty solenoid B. When the ATF is cold, the TCU won’t allow an upshift into 4th gear. The object is to warm the engine quickly for lower emissions. It is more sophisticated than the KDLH system and less objectionable for the consumer.
When the ATF is hot (AWD only), the TCU shifts the transmission as if in the POWER mode. This push-es the shift points higher which allows the engine to run faster. The oil pump then circulates ATF through the oil cooler more quickly so as not to overheat the engine coolant.
The TCU also monitors system voltage in order to correctly interpret the inputs and alter the control of the outputs. For example, the system is designed for 12-volt operation. When running, however, most vehicles have other than 12 volts available.
The 1-HOLD switch is located aft of the shift quadrant. When activated, it creates a forced 1st gear.
Note: The TCU will shift 2nd to 3rd, if necessary, to prevent the engine from over-revving.
The FWD fuse changes the driving mode from AWD to FWD. The FWD fuse is located on the left front shock tower. It is activated by inserting the spare fuse into the under hood connector. The FWD light on the combina-tion meter verifies that the vehicle is in FWD.
The Legacy FWD fuse is located on the right strut tower. The SVX and Forester fuse is located in the main fuse box.
Legacy TCU Inputs
The Legacy fuel system ECM, beginning with the 1990 model year, sends new inputs to the TCU for line pressure control. It networks the MPFI ECU RPM signal and altitude compensation inputs. This provides additional line pressure control for high altitude compensation to reduce shift shock. ABS system inputs turn OFF the over-running clutch when ABS is active and fixes the duty ratio of the MPT to mostly FWD.
Maintenance Precautions
Before jacking up one or two wheels for maintenance with the engine running or before running the vehicle on a chassis dynamometer, the electronic AWD engagement system must be disengaged by installing the spare fuse (15A) of the fuse box into the FWD connector located under the hood. Failure to do so could result in movement of vehicle.
TCU Outputs
There are two types of outputs: solenoid controls and light controls. The solenoids control shifting, line pres-sure, lock-up and AWD.
The light controls indicate operating conditions to the driver. They indicate the POWER mode, manually selected 1st or 2nd gear, or hot ATF (AWD only).
On 1990 and later Legacy models, the light controls indicate hot ATF (FWD and AWD), gears 3 -2 – 1, MAN-UAL mode and POWER mode.
Shift solenoids #1 and #2 are located on the upper valve body. The TCU induces ON/OFF conditions, which regulate the shifting of the forward gears.
When a shift solenoid is ON, it passes pilot pressure to shift valve A and/or shift valve B. The valve(s) will then shift, feeding the appropriate controlling member circuits (high clutch, band, etc.).
Note: Pilot pressure is nothing more than a pressure held at a constant value.
When a shift solenoid is OFF, the affected shift valve will move to its static position due to spring pressure. The appropriate controlling member circuit will than be fed (high clutch, band, etc.).
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