Transmission Control Module

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Transmission Control Module


The general design of the Saturn 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 manual transaxles. The transaxle provides four speeds forward and one 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 actuators in conjunction with a powertrain control module (PCM) and its sensors to control shift timing, shift feel and provide on-board diagnostics.

Actuator Solenoids


The transaxle control actuators are electrohydraulic solenoid valves consisting of a housing, valve body, sliding armature and electromagnetic coil. They are a three port design with a pressure supply port, a pressure outlet or control pressure port, and an exhaust port.

The five actuators have normally open valves. When no voltage is applied to the terminals a return spring holds the valve open and the actuator will allow oil to flow from the supply port to the outlet pressure port. When current flows through the coil, the magnetic field is energized pulling the sliding armature against the return spring When the armature is in this position, the valve is closed, the pressure supply port is blocked and the control port is connected to the exhaust port.

An actuator may be completely energized (closed), de-energized (open), or pulse width modulated (PWM). When it is being modulated, the valve opens and closes up to 70 times per second. This allows a percentage of the oil pressure available at the supply port to pass to the control port. The actual percentage is determined by an electrically controlled duty cycle from the PCM. This is the ratio of the length of time the valve remains open (pulse width) to the total length of time of each cycle (one On and Off cycle).

These actuators are the:

   * Main line presure (MLP) actuator
   * Torque converter clutch (TCC)
   * 2nd/reverse clutch actuator
   * 3rd clutch actuator
   * 4th clutch actuator


The actuators are located in the transaxle valve body.

Transaxle Shifts

Normal Operation

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

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 the 2nd/Reverse actuator. When the circuit is opened, the actuator 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, the 3rd actuator is turned off applying oil pressure to the 3rd clutch. The 2nd/Reverse actuator 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 (1993 and 1994 model year vehicles), 2nd, 3rd, and 4th gears. The speed at which the TCC will apply in 1st (1993 and 1994 model year vehicles), 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 the 2nd/Reverse actuator. The 2nd/Reverse actuator 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.

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/TC will control the transaxle using only 1st and 3rd gears. Once the transaxle fluid temperature is above -12°C (10°F) the PCM/TC will control the transaxle using the standard shift patterns.

Design Changes

1992 - Throughout the database there are references to 1st and 2nd design. This refers to a change that was made during the 1992 model year. 1st design applies to vehicles with VIN prior to NZ200001 and VIN NZ200001 through NZ205625. This change was implemented to prohibit the movement of the Forward/Reverse servo piston toward reverse if the vehicle is traveling forward greater than 3 mph. The components that changed are case assembly, FWD/REV servo piston and spring, valve body including manual valve, spacer plates, gaskets and PCM. Refer to part number references as these components are not interchangeable.

1993 and 1994 - Several Saturn exclusive features were added:

  1. 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.

  2. Downhill Feature - If traveling down steep grades, the PCM/TC may automatically select a lower gear. 

This system assists engine braking, reduces transaxle shifting, and reduces braking required while going down a hill.

  3. Traction Control - Under certain slip conditions, the PCM/EC may instruct the PCM/TC to do an upshift to help reduce slip.