Chapter 7: Classroom and shop manuals Automatic Transmission

Hydraulic Circuits and Controls Classroom Manual Hydraulic Circuits and Controls

Hydraulic Circuits and Controls Classroom Manual Hydraulic Circuits and Controls

Describe the basic types of valves used in automatic transmissions. Objectives Describe design and operation of hydraulic controls used in modern transmissions. Describe the basic types of valves used in automatic transmissions. Describe various configurations of a spool valve and explain how it can be used to open and close various hydraulic circuits. Explain the role and operation of the pressure control solenoid (PC solenoid) assembly.

Describe how fluid flow and pressure can be controlled by a solenoid. Objectives (cont’d) Describe how fluid flow and pressure can be controlled by a solenoid. Explain why load sensing devices are necessary for automatic transmission efficiency. Identify the various pressures in the transmission and explain their purpose Describe the purpose of a transmission’s valve body.

Transmissions shift according to engine load and vehicle speed Introduction Transmissions shift according to engine load and vehicle speed Transmissions shift by holding and driving different members of a planetary gear set Using servos, bands, and one-way clutches These are applied by hydraulic pressure routed by the valve body

Two or more main parts including a separate and/or transfer plate Valve Bodies Directs fluid pressure to the apply devices that hold and drive the planetary members Three types of valves Disc (poppet) Check ball Spool valves Two or more main parts including a separate and/or transfer plate

Valve Bodies (cont’d)

Multiple-Part Valve Bodies Found in many Chryslers The valve body assembly is comprised of the main valve body, regulator valve body, servo body, top accumulator body, and accumulator body, all assembled together as a single assembly. GM also has a multiple-part valve body that is attached to the TCM in the oil pan, along with the input and output shaft speed sensors and the transmission range switch

Slow down hydraulic apply pressures Orifices Slow down hydraulic apply pressures Can be part of the separator plate under the valve body

Orifices (cont’d)

Check valves can be used to redirect fluid flow

Check Valves (cont’d) Check valves can be used to redirect fluid flow

Check Valves (cont’d) Click on graphic above to animate

Normally a flat disc with stem to guide valve’s operation Poppet Valve Normally a flat disc with stem to guide valve’s operation Often used to prevent back flow

Poppet Valve (cont’d)

Open or close a fluid passage Check Balls Commonly used to redirect fluid flow or to stop it from back flowing to the reservoir Open or close a fluid passage

Check Balls (cont’d)

Used to control hydraulic pressures in the valve body Spool Valves Used to control hydraulic pressures in the valve body As the valve moves, it covers and uncovers hydraulic passages

Spool Valves Click on box above to play video

Spool Valves (cont’d)

Spool Valve Operation Click on graphic above to animate

Spool valve that can have several lands and reaction areas Relay Values Spool valve that can have several lands and reaction areas Controls direction of fluid flow and does not control pressure

Relay Valves

Direct line pressure to apply devices Shift Valves Direct line pressure to apply devices Acted on one side by governor pressure Acted on other side by throttle pressure When valve moves the transmission shifts Late model transmissions use shift solenoids

Valve directs fluid to components when drive range is selected Manual Valve Spool valve operated manually within valve body by gear selector linkage Valve directs fluid to components when drive range is selected

Pressures Mainline Regulated pressure from the pump

Shifting The TCM can activate a shift solenoid to signal the transmission shift valve to move and apply the clutches Signal pressures control the direction of the line pressure to the appropriate apply device by their action on shift valves May use a speed sensor to help the TCM decide when to activate a shift solenoid

Pressure Control Valves Control line pressures and pressures used to control or signal shift points Shift pressure control solenoids can regulate the shift pressure for a particular shift Pressure regulator valves Pressure relief

Pressure Regulator Valve To prevent stalling caused by increased pressure in the transmission, pressure regulator valve is used Allows pump to produce enough pressure for transmission to function Resulting pressure is called line pressure

Pressure Regulator Valve (cont’d)

Electronic Pressure Controls The hydraulic pressure is controlled by an electronic pressure control (EPC) solenoid which is controlled by the TCM The shift pressure control solenoids can alter the pressure to a particular clutch pack to compensate for wear and to make sure the shift happens at the particular time and pressure conditions

Electronic Pressure Controls (cont’d) Pressure Switches Are used to let the TCM know if there is hydraulic pressure at a particular hydraulic circuit

Electronic Shift Timing Controls Modern transmissions use electronic sensors to determine: Engine load Temperature Throttle position Gear selector position and vehicle speed Picking the optimum shift speed and quality

Electronic Shift Timing Controls (cont’d) Vehicle Speed Sensors Used to determine shift points Load Sensors Shift timing also depends on engine load Engine load is sensed electronically by the MAP and TPS

Can be traced like electrical diagrams Oil Circuits Can be traced like electrical diagrams Individual circuit tracing can help in diagnosis They determine how these circuits work and what components are doing

Typical Shifting Controls in a Late-Model Transmission Oil Circuits (cont’d) Typical Shifting Controls in a Late-Model Transmission Uses clutch-to-clutch shifting Fluid Flow before the Shift Is mainline pressure Clutch Pressure Control Solenoid Normally open allowing fluid to pass

Clutch Transmission Fluid Pressure Switch Oil Circuits (cont’d) Clutch Transmission Fluid Pressure Switch Monitors if fluid pressure is present at the clutch apply circuit Shifting Control Valves and Solenoids The TCM commands the shift solenoid valve open. Under pressure the shift valve opens, it then applies pressure to the clutch pack

General Hydraulic System Service Shop Manual General Hydraulic System Service

Remove and install the valve body from common transmissions Objectives Remove and install the valve body from common transmissions Check valve body mating surfaces Inspect and measure valve body bores, springs, sleeves, retainers, brackets, check balls, and screens Inspect and replace valve body spacers and gaskets Check and/or adjust valve body bolt torque

Refer to service manual for specific instructions for removal Valve Body Removal Refer to service manual for specific instructions for removal If valve body is removed without removing the transmission: Always use new gaskets and seals Torque the valve body bolts Carefully note the position of parts Watch for springs and check balls that might fall out during removal

Valve Body Removal (cont’d)

Keep parts well organized Carefully clean each part Valve Body Service Keep parts well organized This is mandatory Carefully clean each part Dry with compressed air and reassemble Do not use heavy sandpaper on valves Polish with a fine abrasive only

Valve Body Service (cont’d) Check Valve to Bore Tolerances Reassembly After the valve body has been cleaned, it should be allowed to air dry Once dried, it should be soaked in clean ATF Torque valve body bolts

Summary Automatic transmission receives power from engine through the torque converter Gear ratios are selected according to engine load and vehicle speed Valve body directs fluid pressure through the transmission to shift gears Check, poppet, orifices, and spool valves are used to control hydraulic pressure Relief valves allow or prevent pressure until a particular pressure is reached

Spool valves are flow directing valves Summary (cont’d) Spool valves are flow directing valves Mainline pressure is the source of all pressures in the transmission A PM generator Electronic device that uses a magnetic pickup on the transaxle housing and a trigger wheel on the output shaft Engine load can be monitored electronically through the MAP and TP sensor