This is the overall control system for you transmission.

It collects information from the car’s control systems such as the engine, temperature, speed, throttle position and the brakes to give road speed and braking information not to mention what gear is selected.

From this information it calculates which clutches to apply and how much pressure to use. This determines which gear ratio is used and how hard the shift is.

This achieved by applying current to solenoids inside the transmission.

If it detects a problem it will put the transmission in to safe mode, the pressure is boosted and gear shifts are restricted or the transmission will stay in one gear. This will generate a fault code that can be read by a diagnostic testing machine and will aid diagnosis

automatic transmission technical info

Automatic Transmission Technical Information

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Sucks oil from the sump of the transmission through the filter and delivers high pressure oil to the valve body.

Typically they are either a gear or vane type pump.

hydraulic pump mechatronic valve body


An intricate machined casting inside the transmission that contains valves that direct hydraulic pressure from the pump to the clutches.

It is controlled by solenoids in modern transmissions. The solenoids in turn are controlled by the transmission control module. Very old units were more simply controlled by a hydraulic governor and kickdown cable.


An electrical component used to control your transmission.

Current is applied to an internal coil which opens and closes a hydraulic valve to control and direct oil pressure.

They can simply open and close or pulse to regulate pressure.

solenoid electrical component


This takes the place of the clutch in a manual transmission.

It transmits the drive hydraulically to the input shaft of the transmission.

Most units now have a clutch within them which is applied once the car reaches cruising speed. This stops the impression of slip that the torque convertor gives and aids fuel economy, drivability and economy, known as the Lock up Clutch

TCC torque convertor


These are used apart from the obvious task of keeping the oil inside the transmission to seal high pressure oil in the hydraulic pistons.

The former uses conventional spring loaded rotating shaft seals while the latter uses rubber rings.

Internal rotating shafts typically are sealed with teflon rings.

transmission oil leak


A kind of tachometer that provides a signal to the Transmission Control Module

Typically two or three are used in a transmission. The transmission control module uses this signal to determine gear shifts and also to know if the transmission is slipping.

speed fault codes in transmission


Work in clutch packs with alternating friction and steel plates to apply load. Hydraulic pressure from the valve body actuates a piston that squeezes the clutch pack together to engage drive to a gear.

This is the part that ‘burns out’ creating slipping problems and loss of drive.

slipping clutch


When the transmission control module detects a fault, it stores it in it’s computer.

This can be accessed by a diagnostic machine and the results interpreted by the technician to diagnose the fault.

The TCM is connected to the other systems in the car to access data, so it may also record faults elsewhere on the car such as in the braking system.

This highlights the need for careful checking before taking your transmission out, the fault may be elsewhere.

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Gearboxes and Axles