Torque Convertors
by Jim Kerr

Electronics are amazing. They enabled this article to be filed from the middle of a scenic mountain pass. Electronics have also made amazing improvements in our automobiles. Automatic transmissions are almost all electronically controlled now. They last longer, react to driver's needs better, and can even adapt for wear or driving style. Unfortunately, on my travels I have met several others who have experienced automatic transmission problems. Most of these problems occur when climbing steep mountain passes or pulling heavy loads, and the torque converter is the cause of most failures.

A torque converter couples the engine to the actual transmission. One part bolts to the engine, while another part drives the transmission gearbox. A series of vanes on each part direct transmission fluid from the engine side to the transmission side of the converter, causing the transmission input shaft to turn. Think of two fans facing each other - one fan is turned on and the moving air causes the second fan's blades to rotate. A torque converter is more complex, because the shape of the vanes actually multiplies the torque of the engine more than double, but it operates on similar principles.

When a vehicle comes to a stop, the vehicle's brakes keep the transmission from turning and the engine is not going fast enough to cause enough fluid flow to drive the transmission side of the converter. This slipping action is needed when stopping, but some also occurs when travelling down the highway; between 3 and 5% slippage. This not only wastes fuel, but the slipping also creates a lot of heat. In fact, almost all the heat in an automatic transmission is caused by the slippage in the torque converter.

To improve fuel economy and reduce slippage, almost all automatic transmissions now use a computer-controlled "lock-up" converter. A large plate with a clutch lining bonded to it is splined to one half of the converter. When fluid flow is applied to the back side of the plate, it moves forward and touches the engine side of the converter, locking the two halves together. Now the transmission is driven directly from the engine, with no heat building slippage occurring.

Under normal driving conditions, most drivers place the gearshift in Drive or high gear and leave it there. The electronics looks after all the shifting and torque converter lockup operation. However, when pulling loads or climbing steep hills, the computer will unlock the converter if necessary for more torque. If a driver steps on the gas hard enough, the transmission may shift down, but often it will stay in high gear and let the torque multiplication of the converter provide the extra needed power. This is when transmission-destroying heat builds up.

Under high torque multiplication conditions, heat can build up high enough to destroy the lubricating qualities of the transmission fluid in only a few seconds. Even under partial load conditions, the fluid temperature increases enough to shorten fluid life. Most light duty trucks and passenger car maintenance schedules show that changing transmission fluid is not required for as high as 160,000 km, but look at the suggested schedule for high load or towing conditions and the fluid may need changing at as low as 20,000 km.

So the trick is to keep the converter locked, or reduce the slippage in it as much as possible. The best way to do this is shift to a lower gear. The mechanical advantage of a lower gear multiplies the torque, so the converter slips less. Some transmission control computers will also lock up the converter in lower gears, for even less heat build up.

Drivers with manual transmission equipped vehicles wouldn't dream of shifting into high gear and leaving it there for all driving conditions. Neither should automatic transmission owners. Steep hills and heavy loads may even require shifting down to first or second gear, but read your owner's manual first; some transmissions will start in second gear if shifted there, so it is better to start in first and then shift manually to second.

Another advantage of using lower gears is that they provide engine braking on downhill slopes. Allowing the engine to provide some of the braking force reduces the load on the brakes and allows a more controlled decent.

Don't invite transmission troubles on the road. If you are pulling loads or climbing hills, it is better to get there slowly in a lower gear than push hard and arrive late because transmission repairs are needed.