Pressure Regulator Valve

Pressure Regulator Valve


When a spool valve cycles back and forth between the open and the exhaust positions, it is called a regulating valve. An automatic transmission uses a positive displacement oil pump. As the pump delivers, fluid pressure rises uncontrollably until the pump stalls from extremely high pressure. This condition is aggravated by high engine speeds that drive the pump faster to deliver more fluid. To prevent this stalling, the pump has a pressure regulator valve located in the valve body that maintains a basic fluid pressure. Fluid pressure flows from the pump to the pressure regulator valve, which is held in position by coil spring tension. Pressure regulator valve movement to the exhaust position is controlled by calibrated coil spring tension.

The three steps of pressure regulation operation are charging the torque converter, exhausting fluid pressure, and establishing a balanced condition.

When fluid pressure reaches normal baseline pressure (usually about 60 psi), fluid enters at the top of the regulator valve, forcing it down against coil spring tension. The outlet port opens, which permits fluid flow to charge (fill) the torque converter. The torque converter pressure regulator valve is usually a spring-loaded spool type. Torque converter pressure is developed from line pressure at the pressure regulator valve. The torque converter pressure regulator valve has several spools and lands so when different oil pressures are applied, converter pressure varies in proportion to torque requirements. Converter pressure transmits torque and keeps transmission fluid circulating into and out of the torque converter, which reduces the formation of air bubbles and aids cooling. The exception to this is when the torque converter is operating in the lockup mode.

In the second stage, the pressure regulator valve is forced down against spring tension by fluid pressure to uncover the exhaust port. Fluid pressure not needed for transmission operation is exhausted into the pump inlet circuit.

With fluid pressure acting at one end and coil spring tension at the opposite end, the pressure regulator valve takes a balanced position. While the coil spring tension controls pressure, the valve adjusts itself automatically to the pressure, which forces it downward. The pressure regulator valve's new position might have one of the spool's lands partially covering the inlet port.

Increasing Pressure

There are times in the automatic transmission's operation when fluid pressure must be increased above its baseline pressure. This increase is needed to hold bands and clutches more tightly and to raise the point at which shifting takes place. Increasing pressure above normal line pressures allows operational load flexibility, such as when towing trailers.

There are two methods used to monitor vehicle and engine load: vacuum modulation and throttle pressure. A vacuum modulator measures fluctuating engine vacuum to sense the load placed on the engine and drivetrain. The modulator is normally a small canister that is threaded or push-fit into the transmission housing. A rubber tube connects the canister to the intake manifold of the engine. The canister itself has two chambers divided by a diaphragm. The chamber closest to the transmission housing is open to atmospheric pressure, and the second chamber is closed to atmospheric pressure. The closed chamber contains a coil spring trapped between the diaphragm and the end of the canister. The open chamber holds a pushrod connecting the diaphragm to the modulator valve.

Vacuum Modulator Operation

When a vehicle is placed under a heavy load, the driver opens the throttle. This reduces vacuum in the intake manifold and the closed chamber or the vacuum modulator. With reduced vacuum, the coil spring tension releases to move the diaphragm, pushrod, and modulator valve. The modulator valve opens, allowing line pressure to flow through the modulator valve to the booster valve at the pressure regulator valve. The modulator pressure acting on the booster valve assists the pressure regulator valve coil spring in pushing the regulator valve up against the line pressure at the top of the pressure regulator valve. Line pressure continues to increase until the pressure on the regulator valve overcomes the pressure regulator valve spring tension and modulator (auxiliary) pressure. The pressure regulator valve opens the exhaust port at a new boosted line pressure. This new pressure holds planetary controls tightly to resist slippage and raise automatic shift points.

Throttle Pressure

Vacuum modulators are found on older vehicles. Newer vehicles use throttle valve pressure to increase line pressure. Throttle valve pressure develops when line pressure passes through the throttle valve valley to become throttle pressure. Throttle pedal movement is carried through the throttle linkage to control the operation of the throttle valve.

When the pedal is depressed (opened), the throttle valve opens to produce throttle pressure, which is directed to the pressure regulator throttle plug. It helps the pressure regulator valve spring hold the pressure regulator valve in position to close the exhaust port. This results in increased pressure.

When the pedal is released (closed), the throttle valve partially closes. This decreases throttle pressure at the throttle valve plug, resulting in a reduction of line pressure.

Relay Valve

The direction of line pressure flow is directed by a relay valve. A relay valve is a spool-type valve with several spools, lands, and reaction areas. It is held in one position in the valve body core by coil spring tension, auxiliary fluid pressure, or a mechanical force. Auxiliary fluid pressure or mechanical force can oppose coil spring tension to move the relay valve to a new position. In the new position, the valley of the relay valve aligns with interconnecting ports. Fluid pressure flows from an inlet port across the relay valve valley to the outlet port. When a relay valve is in one position, it blocks fluid flow. When moved to an alternative position, fluid is directed through to the outlet port.

Shift Valve One application of the relay valve is a shift valve. A shift valve usually operates in one of two positions - either downshifted or upshifted. In operation, throttle pressure, which is high, is acting on one reaction area of shift valve. Throttle pressure and light coil spring tension act and hold the valve in the downshifted position. In this position, fluid pressure is blocked from flowing to one of the planetary gear controls.

As vehicle speed increases, a device on the transmission output shaft, called the governor, develops a pressure, called governor pressure. Governor pressure is directed to the opposite reaction area of the shift valve. With increasing vehicle speed, governor pressure increases to overcome coil spring tension and throttle pressure. Governor pressure moves the shift valve to the upshifted position. In this position the inlet and outlet ports are in the same valley area of the shift valve. Fluid pressure flows across the shift valve valley, out of the outlet port, and through the connecting worm tracks to engage the planetary controls for the next higher gear.

If the vehicle operator pushes down on the throttle pedal, the throttle valve opens wider and throttle pressure increases. Throttle pressure is higher than governor pressure. The high throttle pressure and coil spring tension force the shift valve to move to the downshifted position against governor pressure. The transmission automatically downshifts to the next lower gear. The shift valve blocks fluid pressure at the inlet port, which prevents the planetary controls from operating for an upshift. The automatic downshifting of the transmission is often referred to as a passing gear or kickdown. A kickdown valve is often used to generate additional kickdown pressure from line pressure. This ensures that the downshift is made quickly and positively.

Manual Valve

The manual valve is a spool valve operated manually by the vehicle operator and gear selector linkage. When the operator selects the gear position, the gear selector linkage positions the manual valve in the valve body. The manual valve directs line pressure to the correct combination of circuits, which produces the range desired for the driver's requirements. For example, if the drive range is selected, the gear selector linkage is placed in a position that allows fluid pressure at the inlet port to flow across the manual valve valley passing through the outlet port to charge the forward circuit. Control areas of the forward circuit are the governor, rear clutch, and accumulator. The various transmission controls and circuitry that drive the vehicle forward make up the forward circuit. If the gear selector is moved to the reverse position, the manual valve is moved to open the reverse inlet and outlet ports to charge the rear servo and front clutch and move the vehicle in reverse.

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