By Jim Kerr
Questions about manually downshifting automatic transmissions always increase at this time of year. It may be the warm weather, with drivers looking for a little fun and performance from their cars, or drivers towing utility trailers for spring yard work, or campers going on summer holiday trips. There can be many reasons for manually shifting your automatic transmission and some of them will increase your safety.
Most automatic transmissions use planetary gearsets to provide different gear ratios. A planetary gear set has a centre “sun” gear – planet pinions that rotate around the sun gear – and an internal tooth “ring” gear around the planet pinions. These gears are always in constant mesh, which differs from a manual transmission where the gears have to be physically engaged or disengaged during shifts. The gears of an automatic never grind or clash by design, so shifting manually doesn’t increase gear wear.
By holding or driving various combinations of the planetary gears, several gear ratios and reverse motion can be selected. Modern automatics use two or sometimes three planetary gearsets in combination to achieve the four, five, six, seven or even eight speeds found in today’s vehicles.
To shift the automatic, oil is directed from the control valve body to the clutches. Each clutch is made up of several discs. Half have internal splines and the other half have external splines. The splines are connected to the transmission case, shafts or planetary gearset components. When oil pressure is applied to a clutch, the plates are forced together and either hold or drive gearset components to give the different gears. During a shift, there is some wear on the clutch plates, as they slip slightly during application until the full oil pressure is applied but this wear is minimal unless done during very hard acceleration.
Manual downshifts are slightly different than automatic downshifts. During a manual downshift, additional clutches are applied to provide engine braking. Many transmissions drive in some gears through one-way roller or sprag clutches. These one-way clutches can freewheel during deceleration, but by applying an additional clutch, the one-way clutch is prevented from freewheeling so the engine can help slow the vehicle along with the brakes.