August 6, 2003

When to shift an automatic transmission manually
by Jim Kerr

For many drivers, automatic transmissions are something that we put in drive and forget about until we are ready to stop the vehicle. However, summer is travelling season and trips through the mountains or taking that trailer to the lake puts extra load on our vehicles. Sometimes we need to think about manually shifting our automatic transmissions.

When to shift an automatic transmission manually is dependent upon the driver's knowledge of what is happening with the vehicle. There are two reasons for shifting it manually: to control engine speed and power for performance during acceleration, and to provide engine braking during deceleration. Let's look at accelerating first.

During acceleration, transmission automatic shifting is based on two variables - engine load and vehicle speed. The lighter the load, the sooner the transmission will upshift. The heavier the load, the longer it takes before the upshift occurs. Higher vehicle speed causes the transmission to upshift while low vehicle speeds keep it in a lower gear. In the past, only two mechanical inputs were used to shift transmissions - throttle opening (for load) and governor pressure (located on the output shaft for vehicle speed). Today, almost all automatic transmissions are computer controlled so there are many inputs used to determine shift points, but all of these inputs still basically signal engine load and vehicle speed to the computer.

By manually shifting the transmission during acceleration, a driver can keep the transmission in a lower gear with higher engine speeds for more power. At the drag strip, drivers shift manually at very high rpm for maximum power. Pulling a trailer, a driver might want to keep the transmission in a lower gear even though the throttle is not wide open so the vehicle is more responsive and for better control of the trailer.

Drag racers never downshift an automatic transmission manually, but there are many reasons for other drivers to do it. Engine braking is one of the major ones. High gear in many automatic transmissions is an overdrive range. During deceleration, the drive wheels are transferring the torque back through the transmission to the engine. Because the engine is running so slowly in high gear, there is very little engine braking effect occurring. This places a higher load on the vehicle brakes.

Many automatic transmissions use overrunning or one-way mechanical clutches to transmit power through the transmission. These devices make upshifts smooth because only one clutch has to apply during a shift instead of one applying and another releasing (the overrunning clutch releases automatically). The disadvantage of overrunning clutches is that they can only transfer torque one way so they provide no engine braking. When a driver places the gearshift in a lower gear, extra clutches are applied inside the transmission that lock the overrunning clutches so engine braking is possible. The gear reduction added to the engine braking can assist in slowing the vehicle and prevent brakes from overheating on long downhill stretches.

Another time to manually downshift is on long uphill climbs. Keeping the transmission in high gear can cause the engine to lug and it may start to overheat. Use a lower gear selection to aid engine power. It can also save your automatic transmission. Many automatics will not downshift until the throttle is pressed nearly to the floor but before the transmission downshifts, the torque converter has already unlocked to provide a hydraulic multiplication of engine power.

When a torque converter is unlocked (a large clutch plate inside the converter releases the input and output of the converter), the internal slippage quickly causes the oil temperature to climb. In just a few seconds of heavy uphill pulling, the oil temperature can climb enough to break down the oil lubricating qualities. This is the reason vehicles that haul heavy loads or pull trailers need the automatic oil changed more often. Save your transmission when driving uphill with a load by manually downshifting it as soon as you feel the torque converter unlock. If you can't feel it, you should be able to see it occur by watching a tachometer. Engine speed will increase about 300 to 500 rpm when it unlocks.

Finally, there is another reason for shifting the gears manually. It can be fun! Many manufacturers are adding steering wheel paddles or buttons that allow drivers to upshift and downshift just like Formula One drivers for that sporty feel.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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