A/T System Description - General Operation
A/T System Description - General Operation
General Description
The automatic transmission is a transverse mounted three-shaft design, implementing an electronically controlled hydraulic circuit that provides five forward speeds and one in reverse. Engine power is transmitted through the torque converter, a combination of shafts which hold gears and clutches and a differential that transmits power to the driving wheels. 4WD equipped models also have a transfer assembly which transmits power to the rear differential.
Shaft, Gears, and Clutches
Three parallel shafts hold gears and clutches. The gears on the input shaft (mainshaft) and the secondary shaft are in constant mesh with those on the output shaft (countershaft). When specific gears are engaged by the clutches, power is transmitted through the mainshaft, to the secondary shaft, and/or the countershaft, then to the final drive gear of the differential to provide drive.
Electronic Control
Shifting and lock-up is achieved by a system of solenoid valves driven by the PCM to control ATF flow through various valves in the valve bodies to select the appropriate gears for all driving conditions.
Hydraulic Control
The valve bodies include the main valve body, the regulator valve body, and the servo valve body. They are mounted to the torque converter housing. Fluid from the regulator valve passes through the manual valve to the various control valves. All the clutches receive fluid from the internal hydraulic circuit.
Shift Control Mechanism
To shift gears, the PCM controls shift solenoid valves A, B, C, D, and E, and A/T clutch pressure control solenoid valves A, B, and C. The shift solenoid valves change the positions of the shift valves in the valve body which open and close ports to send hydraulic pressure to the appropriate clutch. A/T clutch pressure control solenoid valves A, B, and C regulate their respective pressure, and pressurize the clutches to engage them and their corresponding gears. The pressure of the A/T clutch pressure control solenoid valves also applies to the shift valves to switch the port.
Lock-Up Mechanism
The lock-up mechanism causes the input shaft (mainshaft) to rotate at the same speed as the engine crankshaft. Pressurized ATF is drained from between the torque converter cover and the torque converter clutch piston through a fluid passage, causing the torque converter clutch piston to be held against the torque converter cover. Together with the hydraulic control, the PCM optimizes the timing and degree of lock-up. The lock-up mechanism operates in D (2nd, 3rd, 4th, and 5th gears) and in D with D3 driving mode (2nd and 3rd gears).
Torque Converter
The torque converter is a fluid coupling, which allows the engine to spin independently of the transmission yet connects them together as needed. It is an assembly that consists of an impeller (pump), turbine, stator and lock-up clutch, which uses ATF to transmit engine power to the input shaft (mainshaft), and acts as a flywheel to help the engine run smoothly. During certain conditions the lock-up clutch is engaged by the PCM to mechanically connect the engine crankshaft with the input shaft (mainshaft) which improves fuel economy. Around the outside of the torque converter is a ring gear which meshes with the starter ring gear, when the engine is being started.
Gear Selection
The shift lever has six positions; P: PARK, R: REVERSE, N: NEUTRAL, D: DRIVE 1st through 5th gear, and 1st through 3rd gear with D3 driving mode, 2: SECOND, and 1: FIRST.
Starting the engine is possible only in P and N because of a neutral-safety switch.
Automatic Transmission (A/T) Gear Position Indicator
The A/T gear position indicator in the gauge control module shows which shift lever position has been selected.
Transfer Mechanism (4WD)
The transfer assembly consists of the transfer drive gear on the differential, the transfer shaft, the transfer drive gear (hypoid gear), the transfer output shaft (hypoid gear), and the companion flange. The transfer assembly is on the rear side of the transmission, beside the differential. The transfer drive gear on the differential drives the transfer shaft and the transfer drive gear (hypoid gear), and the transfer drive gear (hypoid gear) drives the transfer output shaft (hypoid gear). Power is transmitted from the transfer drive gear on the differential to the rear differential via the transfer assembly and the propeller shaft.