Automatic Transmission Valve Bodies

The valve body is the automatic transmission’s hydraulic control center. Automatic transmissions use planetary gearsets for all forward gears and reverse. Clutches and bands, applied by hydraulic pressure, are the components used to control the planetary members – the sun gear, outer ring gear, and the planetary gears in between – to shift gears. Transmission fluid that is pressurized by the transmission oil pump flows to the valve body and the pressurized fluid is then directed by the valve body to the appropriate clutches and bands. The valve body in your transmission or related valve body components may be failing if you’re experiencing delayed shifts, hard shifts, improper shift timing, shift flares, or noise. If one or more parts prove faulty, we have the OE quality replacement valve bodies & components to restore transmission performance.

The valve body is a structure containing valves and numerous fluid passageways that is bolted to the underside of the transmission case, just above the fluid pan. It may have multiple sections and is usually installed to the transmission with a separator plate and gasket. Several types of valves are used in valve bodies including check balls, poppet valves and spool valves. Check balls seat on the valve body and separator plate where they allow or block fluid flow. Poppet valves are balls or flat discs with stems that guide opening and closing, and like check balls they also allow or block fluid flow. Spool valves are the most familiar type of valve. Spool valves consist of a stem with large circular sections called lands that fit into bores in the valve body, separated by stemmed areas called valleys. When the spool valve is activated by fluid pressure, spring pressure or linkage, the lands block fluid flow and the valleys allow it.

A typical valve body has a manual valve and valves for enacting specific gear changes, as well as pressure control valves, and depending on model year application may also have other valves including a throttle valve, and a valve for the TCC (Torque Converter Clutch). The manual valve is controlled by the driver via the shift lever and directs fluid pressure to the appropriate clutches and bands when the transmission is placed in Park, Drive, Neutral or Reverse. For example, when the transmission is placed in Drive, fluid is directed to the clutch that controls first gear. For an automatic transmission to shift at the proper time and with the necessary firmness, the load on the engine must be communicated to the transmission. On modern computer controlled vehicles, the transfer of engine load information takes place electronically, but on older vehicles a vacuum modulator or TV (Throttle Valve) cable is used to signal engine load to a throttle valve, and the governor and throttle valve control hydraulic pressure in the transmission.

On older, non-electronically controlled transmissions, the transmission pump supplies fluid to the governor and throttle valve, which in turn direct fluid to the shift valves in the valve body. The transmission output shaft drives the governor, which builds pressure directly in relation to vehicle speed. Under light acceleration, the throttle valve does not have much effect on the shift valve and governor-controlled shifts are earlier and softer. Under hard acceleration, when engine load is high, the throttle valve will act against the shift valve to force governor pressure higher. This delays the shift and supplies higher pressure to the transmission bands and clutches, giving them the clamping power needed to control members of the transmission’s planetary gearset and enact firm, hard shifts. The throttle valve detects engine load via a vacuum modulator or TV cable.

On electronically controlled transmissions, shift timing and firmness are controlled by a computer. Depending on the vehicle, model year and system, the transmission may be controlled by a PCM (Powertrain Control Module), which controls both the engine and transmission, or by a dedicated TCM (Transmission Control Module). A network of engine and transmission sensors and switches provides input to the computer about operating conditions including engine load, temperature, and vehicle speed. The computer uses this input, along with internal programming, to manipulate fluid pressure and flow via electric solenoids in the valve body. There are pressure control solenoids, shift solenoids that direct fluid pressure to the appropriate clutches or bands, and a solenoid that directs fluid pressure to engage the TCC. A solenoid contains a coil of wire with an iron plunger inside. When current is applied to the solenoid it becomes an electromagnet and the plunger changes position. When the plunger moves it opens or closes a valve, allowing or preventing fluid flow.

When shift valves are actuated, pressurized fluid is sent to pistons in clutch packs or servos that apply bands to shift gears. Accumulators are devices in valve bodies that are designed to affect shift feel. An accumulator contains a piston and spring in a bore that is connected at both ends to fluid pressure. When fluid pressure from a shift valve is first applied to a clutch or band, the pressure forces the piston down against the spring, thereby absorbing some of the pressure and preventing a harsh shift. However, under hard acceleration, when there is greater engine torque and more pressure is needed to apply the clutch or band, throttle pressure acts on the opposite end of the piston, preventing easy accumulator piston movement and resulting in a firmer shift.

When the symptoms described in the first paragraph above are experienced on vehicles with computer controlled transmissions, they may be accompanied by a CHECK ENGINE or other malfunction indicator light illuminating on the dash. A scan tool should be connected to the vehicle’s diagnostic link connector to check for diagnostic trouble codes. The cause of the transmission problem may not be in the hydraulic circuits, but with a sensor, actuator, controller or some other part of the electronic control system. Pressure testing can reveal defects in the valves and valve body. If a valve body problem is suspected, it should be disassembled and all components thoroughly inspected for wear, which could be caused by extensive use, poor maintenance or a combination of the two. If the transmission fluid has not been changed regularly, or the transmission has overheated or there is a problem in another part of the transmission, the fluid could have reduced lubricating capabilities and there may be a buildup of varnish and contaminants in the fluid, which can cause sticking or worn valves.

No matter what you’re working on, we have the replacement automatic transmission valve body & components needed to restore long-lasting performance and durability to your transmission. You’ll also find performance components in our store. What can a performance automatic transmission valve body do to your vehicle that a stock one cannot? The main thing is that it quickens up gear changes and modifies the shift points when the gear shift happens. It also makes your driving much smoother with gear shifting that is crisper and more powerful. An upgraded valve body won't just improve your overall driving experience, it will also add more durability to your automatic transmission.