Automatic Transmission

An automatic gearbox has complex gear sets in them. The gear sets consist of sun, planet and ring gears. The gear ratios are controlled by the bands braking the ring gear forcing the planet gears to drive the output shaft or it releases the ring gear and allows it to drive the output shaft. The bands are controlled by pressurised hydraulic fluid that is pump through the gearbox. The same fluid is also used to lubricate the moving parts in the gearbox like the bearings. Older gear boxes would control the gear selection using mechanical shift valves  linked to the kick down switch that is connected to the trottle and the governor. Newer cars use mostly clutch packs instead of the the bands and the fluid is directed to them using solenoids that are controlled by the ECU according the the engine speed, the speed of the vehicle and the load on the engine (the amount of vacuum in the intake manifold.

We took an old rear wheel drive automatic gear box apart, it had two bands that would lock the ring gears according to which gear is selected. The bands were controlled by an actuator piston that would push the two ends of the bands together locking down on the ring gear. The governor was located on the output shaft and the oil pump was located on the input shaft. The oil pump in the gearbox is a crescent type pump.

Before removing and stripping down the gearbox it is good practice to take out the dip stick to check the colour and smell of the transmission fluid. The colour and smell can help a lot with finding the problem. If the fluid smells burnt it means that the clutch pack or the bands are slipping causing them to burn. If the fluid is milky it means it has been contaminated by water. If the engine flares between gear changes it means that the transmission oil is low or the clutch pack is worn down or the bands need to be adjusted, it could also be because the oil pump is worn out and it is not supplying the correct amount of pressure or there is a blockage in the fluid passages.

 Bottom of the gearbox with its sump off

 Park locking mechanism

 Outside of the oil pump

 Governor, the governor controls the oil pressure using centrifugal forces

Axle, gears and the oil pump

This is a diagram to show how the oil pump works

The inside of the oil pump from our gearbox looked like the one in the picture above