Hydraulic Circuits


Hydraulic Circuits

GOVERNOR ASSEMBLY

The governor assembly is a very important part of the transmission control system. The purpose of the governor assembly is to sense vehicle road speed and send a fluid pressure signal to the transmission valve body to either upshift or permit the transmission to downshift. The governor assembly is located on the transmission output shaft and is influenced by increases and decreases in output shaft speed. When output shaft speed decreases, governor pressure decreases. Governor pressure decreases to a point less than throttle pressure and coil spring tension. As a result, high throttle pressure and coil spring tension force the shift valve to the downshifted position. Fluid inlet passages are now blocked by the 1-2 shift valve land, which stops fluid flow from stroking the 2-3 shift valve and preventing the engagement of the upshift planetary controls. The 1-2 shift valve remains in the downshifted position. With the transmission shift valve in this position, the transmission pulls away from a stop in first gear. As vehicle speed increases, the transmission output shaft speed increases, resulting in more governor pressure to automatically upshift the transmission.

HYDRAULIC CIRCUITS

To provide a practical review of the valve body's operation, the following sections examine the 3-speed transaxle with computer-controlled lockup torque converter that is installed in Chrysler front-wheel-drive vehicles. This transaxle and valve body are not highly sophisticated, but it is a good example of general, fundamental valve body operation and the control of various fluid pressures. The hydraulic circuits for each gear selector position is explained using flow charts, accompanied by the torque converter's lockup clutch controls and operation along with schematic diagrams of transaxle power flows. The flow charts and transaxle diagrams bring together the hydraulic controls and mechanical operation, that combined, give transaxle operation.

Preceding the explanation for each flow chart, note the gear range, torque converter mode, planetary controls engaged, approximate vehicle speed, and throttle position. Be sure to read the flow indication and pressure chart to establish the parameters for valve body and transaxle operation.

Gear Range: Neutral and Park

Gear Selector Position: N and P

Throttle Position: 0 to 10 psi (Approximately closed)

Pump pressure leaves the transmission pump and is directed to the pressure regulator valve and manual valve. At the pressure regulator valve, pump pressure is regulated to become line pressure. Line pressure enters the pressure regulator valve and leaves as converter pressure, flowing to the switch valve. The switch valve allows line pressure to enter the torque converter. Converter pressure circulates from the switch valve to fill the torque converter and returns to the switch valve to become cooling and lubrication pressure.

From the pressure regulator valve, line pressure flows to the manual valve. From the manual valve, line pressure seats #9 check ball and flows around #8 check ball to stop at the land of the closed throttle valve. Throttle pressure is low because the throttle valve is not open. Line pressure flows to the accumulator to cushion the engagement of the planetary controls when the gear selector is moved to D or R ranges. The accumulator is basically a hydraulic shock absorber designed to absorb the shock of engaging planetary controls.

In neutral, line pressure is established by the pressure regulator valve and flows to the manual valve and throttle valve.

Gear Range: D First Gear

Selector Position: D

Torque Converter Mode: Unlock

Planetary Controls Engaged: Rear Clutch; Overrruning Clutch

Approximate Speed: 8 mph

Throttle Position: Half Throttle

Pressure between the manual valve and pressure regulator valve is considered to be line pressure. This line pressure circulates to the switch valve. Since the switch valve is held in the torque converter unlocked position line pressure flows no further.

LINE PRESSURE Beginning at the first manual valve outlet #1, line pressure seats #9 check ball flows past #8 check ball to enter the throttle valve and establish throttle pressure. Line pressure also flows to the pressure regulator valve to regulate pressure. Line pressure strokes the accumulator piston against coil spring tension, cushioning the engagement of the rear clutch.

At outlet port #2 of the manual valve, line pressure charges (fills) the worm track, which engages the rear clutch and flows to the governor assembly. When the rear clutch is engaged and the governor assembly is filled with line pressure, the forward circuit is ready to drive the vehicle forward.

THROTTLE PRESSURE As line pressure passes through the valley of the throttle valve, it becomes throttle pressure. Throttle pressure circulates around the kickdown valve valley. With throttle pressure at the kickdown valve, a very quick downshift response to full-throttle operation is provided. Throttle pressure is directed to the pressure regulator throttle plug. It acts on the throttle plug, compressing the throttle plug spring. The result is that the pressure regulator valve closes the exhaust port, which results in a line pressure (boost) increase.

Throttle pressure moves to act on the spring end of the 1-2 shift valve. The throttle pressure and coil spring tension work together to hold the shift valve and governor plug in the downshifted position against governor pressure. Throttle pressure passes #5 check ball, which is acting on the spring end of the 2-3 shift valve. From the 2-3 shift valve, throttle pressure flows to hold the shuttle valve throttle plug against its stop in the valve body.

GOVERNOR PRESSURE Governor pressure developed from line pressure leaves the governor assembly terminating at the shuttle valve spool land. Governor pressure also acts on the 2-3 and 1-2 shift valve governor plugs. Because the vehicle is traveling at 8 mph, governor pressure is not strong enough to overcome throttle pressure and spring tension at the opposite end of the shift valves. Therefore, the transmission stays downshifted in drive range first gear.

CONVERTER PRESSURE From the pressure regulator valve, converter pressure is directed to the converter pressure control valve. From the pressure regulator valve, converter pressure is directed to the converter pressure control valve. From the converter pressure control valve, converter pressure flows through the switch valve valley and enters the torque converter turbine shaft to keep the lockup piston disengaged. Converter pressure entering between the impeller and turbine fills the torque converter. Converter pressure flows back to the switch valve and enters the cooler to become cooler pressure. When cooler pressure returns to the transmission, it cools and lubricates transmission parts.

Drive Range: D Second Gear

Gear Selector Position: D

Torque Converter Mode: Unlock

Planetary Controls Engaged: Rear Clutch; Front Kickdown Band

Approximate Vehicle Speed: 15 mph

Throttle Position: Half Throttle

Line pressure leaving the area between the pressure regulator valve and manual valve develops throttle pressure and torque converter control pressure.

Referring to the manual valve, line pressure leaves the #1 outlet port, seating #9 check ball. Line pressure passing #8 check ball moves to the throttle valve to become throttle valve pressure. From the #1 outlet, line pressure also flows to the pressure regulator valve, regulating line pressure. Line pressure from the same circuit flows to the accumulator, opposing line pressure and spring tension. The accumulator cushions the engagement of the intermediate band.

From the #2 manual valve outlet, fluid moves to the upshifted 1-2 shift valve. Line pressure flows to engage the rear clutch, then around the #7 check ball, through the governor screen to the governor assembly.

From the 1-2 shift valve, line pressure circulates to the shuttle valve and bypass valve through the restriction, or around the #6 check ball to operate the front servo. When the front servo piston strokes in the cylinder, the front kickdown band engages around the front clutch drum.

THROTTLE PRESSURE The throttle pedal is in the half-open position, developing throttle pressure, which is directed to the kickdown valve. Throttle pressure is also directed to the throttle plug of the pressure regulator valve. Throttle pressure moving the throttle plug forces it against the throttle plug spring tension. The action of the throttle plug removes some of the opposition to the pressure regulator valve spring, resulting in a boost in line pressure. Throttle pressure moves to the 1-2 shift valve where it is blocked by the upshifted valve's spool land. After seating the #5 check ball, throttle pressure and coil spring tension pushes the 2-3 shift valve reaction area and moves it to the downshifted position against governor pressure. Throttle pressure leaves the 2-3 shift valve to hold the shuttle valve throttle plug against its seat.

GOVERNOR PRESSURE Governor pressure leaves the rotating governor assembly and flows to the shutting valve throttle plug. Governor pressure also acts on the governor plugs of the two shift valves. Vehicle speed and governor pressure are high enough to overcome throttle valve pressure and coil spring tension at the 1-2 shift valve. Governor pressure forces the 1-2 shift valve to move against throttle pressure. Therefore, throttle pressure is blocked from acting on the shift valve reaction area.

CONVERTER PRESSURE Converter pressure flow is the same as in first gear.

Drive Range: D Third Gear

Gear Selector Position: D

Torque Converter Mode: Unlock

Planetary Controls Engaged: Rear Clutch; Front Clutch

Approximate Vehicle Speed: 25 mph

Throttle Position: Half Throttle

Drive range third gear is covered in two parts. The first explains the hydraulic operation to shift the transmission into third gear. The second part introduces the sensors and controls affiliated with engine computer operation and torque converter lockup clutch control. (To this point, in drive range first and second gears, the torque converter clutch has been unlocked.)

LINE PRESSURE Line pressure between the manual and pressure regulator valves is directed to the throttle valve and switch valve. Start at the #1 manual valve outlet where line pressure seats #9 check ball and circulates to the throttle valve. With the throttle valve open, throttle pressure is developed. In this circuit, pump pressure flows to the pressure regulator valve to develop line pressure, which, in this circuit, also operates the accumulator.

From the #2 manual valve outlet, line pressure charges the forward circuit, engages the rear clutch, and enters the governor assembly to produce governor pressure.

Line pressure also flows from the #2 manual valve outlet to circulate around the valley of the upshifted 1-2 shift valve and then around the valley of the 2-3 shift valve to the restriction above the shuttle valve. This restriction biases line pressure to the shuttle and by-pass valves. Line pressure leaves the shuttle and by-pass valve area and moves to the release side of the front servo and the front clutch. The feed line to the front clutch has a restriction that causes pressure to build on the release side of the front servo, which disengages the front kickdown band. Line pressure flowing through the front clutch feed restriction engages the front clutch for third gear direct drive operation.

THROTTLE PRESSURE With throttle valve open, throttle pressure flows to the kickdown valve, throttle plug of the pressure regulator valve, 1-2 and 2-3 shift valves, and the shuttle valve throttle plug. At the 2-3 shift valve, throttle pressure and coil spring tension are opposed by increasing governor pressure.

GOVERNOR PRESSURE Governor pressure leaves the governor to move the shuttle valve, opening line pressure circuits. The shuttle valve's movement buries the coil spring in the hollow shuttle valve throttle plug. The governor pressure has moved the 1-2 and 2-3 shift valves to the upshifted position, directing line pressure to engage the front clutch and hold the front servo released during third gear operation.

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Torque Converter Controls and Pressure

The lockup torque converter clutch assembly is controlled by the engine computer. The engine computer energizes the torque converter relay, which sends 12 volts to the lockup solenoid. When the engine computer receives electronic signals from the different sensors confirming the requirements for lockup have been met, lockup clutch engagement begins.

These sensors include an engine coolant sensor, vehicle speed sensor, engine vacuum sensor, and throttle position sensor.

Lockup Relay

The lockup relay is energized when the engine computer grounds the circuit. The lockup relay sends 12 volts to energize the lockup solenoid.

Lockup Solenoid

A solenoid is a device capable of converting electrical energy into mechanical force. When applied to the automatic transaxle lockup solenoid, the check ball is held off its seat by fluid pressure. The unseated check ball prevents line pressure from building until it is high enough to move the switch valve against switch valve spring tension.

When the lockup solenoid is electrically energized by a signal from the engine computer and lockup relay, the check ball is seated by the lockup solenoid, stopping the exhaust of line pressure. Line pressure builds up on the reaction area of the switch valve and moves it against spring tension to begin lockup engagement.

Drive Range: D Third Gear

Gear Selector Position: D

Torque Converter Mode: Locked

Planetary Controls Engaged: Rear Clutch; Front Clutch

Approximate Vehicle Speed: 40 mph

Throttle Position: Half Throttle

In drive range third gear lockup, the transaxle operates in the same manner as third gear unlock. The focus of attention is on the torque converter lockup clutch control system.

Lockup Clutch Engagement

The coolant sensor reports to the computer that the engine has reached a temperature of at least 150 degree F. The vehicle speed sensor located on the speedometer cable sends an electronic signal to the computer, which reports that vehicle speed is above 40 mph. Since the vehicle is traveling at engagement speed, the throttle must be open (driver's foot on throttle). The vacuum transducer reports to the computer via electronic signal that engine vacuum is above 4 inches and within 22 inches. Based on these inputs, the computer energizes the clutch relay and lockup solenoid to move the solenoid check ball on its seat. The lockup solenoid check ball stops the lockup solenoid from exhausting line pressure. The increasing line pressure forces the switch valve to move against the coil's spring tension. Line pressure from the switch valve is directed to the pump drive hub and stator support to fill the torque converter with fluid. Fluid in the torque converter during lockup operation resides there to become the cooling and lubricating pressure. Line pressure flows from the impeller and turbine to fill the space behind the torque converter clutch piston and force engagement.

Drive Range: Reverse Gear

Gear Selector Position: R

Torque Converter Mode: Unlock

Planetary Controls Engaged: Low and Reverse Rear Band; Front Clutch

Approximate Vehicle Speed: 5 mph

Throttle Position: Part Throttle

Line pressure from a manual valve outlet not used before circulates through a bypass around the manual valve valley. Line pressure circulates to the low and reverse servo and front clutch. During the process of engaging the low and reverse servo and front clutch, the #4 and #3 check balls are seated by line pressure.

LINE PRESSURE Line pressure from between the pressure regulator and manual valve circulates around the pressure regulator valve. After flowing through a restriction to seat #8 check ball, line pressure enters the throttle valve, which produces line-to-throttle pressure. With the throttle valve open, throttle pressure charges the kickdown valve and strokes the throttle plug to its extreme left position at the pressure regulator valve. Line pressure from the manual valve does not flow to the pressure regulator valve to oppose spring tension. The pressure regulator valve coil spring pushes the pressure regulator valve over to close the exhaust port. Line pressure builds to approximately 200 to 300 psi. You may wonder why line pressure must be increased so much in reverse gear. The planetary control units engaged in reverse are the front clutch and the low and reverse gear band. The front clutch, unlike the rear clutch, does not have a belleville spring. The belleville spring multiplies clutch piston apply force on the clutch pack, which reduces possible slippage. Therefore, to keep the front clutch from slipping when moving the vehicle from a stop, reverse line pressure is increased. The concept of increasing line pressure in reverse is common in many automatic transaxles and transmissions.

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