Automatic transmission is all about the gearbox

The torque converter eliminates the hassles of a clutch and allows smooth, strong pull-away from rest, but the soul of an automatic transmission lies in the gearbox mounted just behind it.

The gears found in an auto-shift box differ from the gearwheels in a constant-mesh manual box. In the latter, helical gears (gears with teeth cut at an angle to the axis of rotation), mounted on parallel shafts, are pairwise in constant mesh.

The gears on the output shaft can spin freely until the appropriate one is locked onto the shaft by a sliding collar which is splined to the shaft and moved towards the gear by means of the gear lever. Once a gearwheel is locked on to the output shaft, power can be transmitted through that gear and the one on the other shaft in mesh with it, thus providing one of the ratios of the gearbox.

If you think that sounds complicated, wait until you look inside an auto-shift gearbox.

Instead of helical gears in constant mesh, you find there planetary gearsets.

A planetary gearset is a wondrous thing. Understanding its workings will keep even a technically astute person out of mischief for a good few days. Suffice it to say that one planetary gearset can provide different ratios of input speed to output speed, and if you link two or more such gearsets together in tandem so that the output of one serves as the input of the next, you are able to provide a wide range of gear ratios.

This is what you find in an automatic gearbox. Planetary gearsets working in series, together with multiplate friction clutches running in transmission fluid, and brake bands that clamp around rotating parts to stop them from spinning when necessary.

All these components are packed tightly together inside the gearbox housing. Shifting from one gearbox ratio to another is done by stopping certain parts of gearsets from spinning by means of the brake bands, while releasing the bands around others, and locking together parts of the same or different gearsets by engaging the appropriate clutches.

These actions are performed by hydraulic servos which on older gearboxes were controlled by a valve body. As on-board computers became more sophisticated, more and more of the valve body's tasks were offloaded to them.

Today the engine computer, or a separate transmission computer linked to the engine computer, has taken over all the control logic from the valve body. Solenoids, turned on and off by the computer, now control shift patterns, rather than the spring-loaded valves in the valve body. Some of the latest systems can sense whether the driver is in a relaxed or urgent mood and adjust the shift points accordingly.

Planetary gearboxes, incidentally, were used on cars long before the arrival of automatic transmissions.

Henry Ford's Model T (1908 - 1927) had a planetary box with two forward speeds, neutral and reverse. It provided clutchless gearshifts (the Model T had no conventional clutch) by means of two foot pedals operated by the driver. The idea of automatic control of gearshifting, however, was still a pipe dream when Henry designed his Tin Lizzie.

Maintenance of automatic transmissions is mainly a matter of keeping an eye on the transmission fluid. This fluid has a number of important tasks: it must transfer torque in the torque converter, lubricate the gears and bearings in the gearbox, carry away the heat generated by friction, and apply hydraulic force to the servos.

The biggest enemy of any ATF is excessive heat. Next week we shall see how to detect when transmission fluid has overheated. We shall also look at a few other maintenance and repair jobs on autoboxes.