Transmission System Description and Operation

Transaxle

The general design of the Saturn S-Series automatic transaxle is a parallel shaft arrangement where the entire transaxle is located in line and directly behind the engine. This arrangement is basically the same as most front wheel drive transaxles. The transaxle provides 4 speeds forward and 1 reverse.

The transaxle is a fully automatic unit consisting of four multiple disc clutches, a four element torque converter with a lock up clutch, 1st gear sprag clutch, and a servo actuated dog clutch.

The most significant departure from the traditional automatic transaxle comes from the use of electronic controls. This unit utilizes five electrohydraulic solenoids in conjunction with a powertrain control module (PCM) and its sensors to control shift timing, shift feel, and provide on-board diagnostics.

Transaxle Shifts

Transaxle shifts are controlled by the manual valve and the PCM through the hydraulic solenoids.

The manual valve controls oil direction to provide 1st  gear. When the transaxle has been shifted into D4 or D3, the 1st clutch is applied and the 1st sprag locks the 1st driven gear to the output shaft. The output shaft is driven counterclockwise by the 1st  driven gear, causing the differential to turn clockwise in the forward direction. At the same time, 1st oil pressure is directed to the Forward/Reverse servo to assure the dog clutch is moved, locking 2nd driven gear to the output shaft.

At a predetermined point, based on vehicle speed, throttle position, and temperature, an upshift to 2nd will occur when the PCM opens the circuit to shift solenoid 2. When the circuit is opened, the solenoid is turned off electrically and turned on hydraulically, flowing oil to apply the 2nd/Reverse clutch. As the clutch applies, the ratio change causes the 1st gear sprag to overrun or freewheel and the shift to 2nd is complete.

When the transaxle shifts to 3rd, shift solenoid 3 is turned OFF applying oil pressure to the 3rd clutch. Shift solenoid 3 is then turned ON, exhausting the 2nd/Reverse clutch as the 3rd clutch is applied. The 3rd to 4th shift is accomplished in the same manner.

The torque converter clutch (TCC) is applied in 1st, 2nd, 3rd, and 4th gears. The speed at which the TCC will apply in 1st or 2nd is based on vehicle speed and tailored by throttle position, engine temperature, and transaxle temperature. Once applied, the TCC will stay applied until vehicle speed is low enough in 2nd gear for the TCC to release. The TCC will also release when the brake pedal is depressed in 2nd gear at low engine speeds or when the transaxle downshifts into 1st gear.

Reverse is controlled by the manual valve. When reverse is selected, the manual valve directs oil to the Forward/Reverse servo. The servo moves the Forward/Reverse dog clutch to the reverse position, releasing the 2nd driven gear and locking the reverse driven gear to the output shaft. As the dog clutch is engaged, the servo allows oil flow to shift solenoid 2. Shift solenoid 2 is turned OFF allowing the clutch to engage driving the 2nd/Reverse gear. The 2nd/Reverse gear turns the reverse idler gear driving the reverse driven gear clockwise, which in turn, provides reverse.

Transaxle Operation

	INPUT SHAFT TURNING	OUTPUT SHAFT TURNING	REV IDLER TURNING
In Park	Yes	No	No
In Drive Vehicle Moving	Yes	Yes	Yes
In Drive Vehicle Stopped	No	No	No
In Neutral	Yes	No	No
In Reverse Vehicle Moving	Yes	Yes	Yes

The following conditions apply when the input shaft only is moving, no clutches applied:

The following conditions apply when the output shaft is moving:

COLD TEMPERATURE OPERATION

Under cold operating conditions the high viscosity of automatic transaxle fluid (ATF) can result in sluggish operation of the hydraulic controls of the automatic transaxle. Because of this, when the transaxle fluid temperature is below -13°C (9°F) the PCM will control the transaxle using only 1st and 3rd gears. Once the transaxle fluid temperature is above -12°C (10°F), the PCM will control the transaxle using the standard shift patterns.

Uphill Feature : While traveling up certain grade hills, the transaxle will not upshift to a higher gear if that gear cannot maintain speed on the grade. Even if a driver completely lifts off the throttle, the transaxle will not upshift thus preventing unnecessary shifting. After reaching the top of the hill, the normal gear shift schedule will resume operation.

Downhill Feature : If traveling down steep grades, the PCM may automatically select a lower gear. This system assists engine braking, reduces transaxle shifting, and reduces braking required while going down a hill.

Traction Control : Under certain slip conditions, the PCM may perform an upshift to help reduce slip.