Knowing when to shift an automatic transmission manually can extend engine life. Click image to enlarge
By Jim Kerr
I just returned from a camping trip in the mountains, and as enjoyable as the outdoors is, the traffic on the highways was frustrating. Keeping safety first, there was nothing to do but sit back and relax as we waited for vehicles slowly climbing hills or inching down steep grades. Often, the smell of overheated transmission oil or the stench of burning brake pads would make me pull out and find a parking spot where I could enjoy fresh air. The smell was coming from vehicles where drivers were not shifting their automatic transmissions manually.
You don’t have to be on a mountain grade before shifting an automatic transmission manually becomes a good idea. Even on flat roads, if you are towing a trailer into a strong wind, it can be a good idea to shift down. So how do you know when it is time to select that lower gear?
Transmissions are put in vehicles to assist the engine. When you start out, low gear in the transmission allows the engine to increase rpm’s quickly to a point where it produces more torque. Unless your engine is supercharged or turbocharged, maximum torque is often in the 2000 to 4000 rpm range. Larger displacement engines will produce more torque and at lower rpm, while small four-cylinder engines need more rpms to produce maximum torque. This can vary due to the design of the engine camshaft and the application, but generally, smaller engines need more rpm to produce enough torque to pull a load up a grade. Selecting a lower gear allows the engine rpm’s to increase, as well as mechanically multiplying the amount of torque the engine does put out.
Many vehicles come with transmissions that have one or two overdrive gears. This allows the engine to operate at lower rpm for better fuel economy, but if there is too much load on the engine, it will start to lug. Lugging is when the engine doesn’t want to accelerate freely. It is accompanied by sluggish throttle response, sometimes a pinging or rattle sound from the engine and increased engine temperatures. Lugging also places high mechanical loads on engine bearings and piston skirts. Lug an engine too much and you will create engine problems. You may have experienced lugging in vehicles with manual transmissions if you are driving in too high a gear at low speeds. The proper thing to do is gear down.
The same applies to automatic transmissions. If the engine rpm are low and the throttle response is sluggish, then it is time to gear down. You may even notice that it takes less throttle to keep the vehicle moving after you select a lower gear, which usually indicates you are also getting better fuel economy. Engines under high load use a lot of fuel. Selecting a lower gear reduces the load and fuel consumption.
In automatic transmissions, a torque converter is used to multiply engine torque. Modern transmissions use a lock-up converter, which uses a clutch disc to “lock” the engine to the transmission input shaft for improved fuel economy under light loads. If loads increase, the computer unlocks the converter so it can multiply torque. Unfortunately, there is a lot of heat created in the torque converter when it is multiplying the torque, so doing this over a long time such as a long uphill grade or driving into a strong headwind while towing a trailer can overheat the transmission.
Shifting down a gear will provide a mechanical advantage so the torque converter doesn’t have to work so hard. In many automatic transmissions, the torque converter can also lock up in lower gears if load is low enough, so heat is reduced greatly.
Many vehicles I saw in the mountains were trying to climb grades with transmission shifters in Drive position. Yes, the transmission will automatically shift down but only when speeds are getting low and the accelerator is almost all the way to the floor. By that time, there is a lot of heat generated in the transmission oil. It is better to select a lower gear and let the engine rev a little faster.
Downhill grades won’t overheat transmissions but they are hard on brakes. Placing an automatic transmission in a lower gear will engage additional clutches in the transmission so that the engine can supply some compression braking to assist the wheel brakes. Using the engine and transmission to slow the vehicle increases vehicle safety by reducing heat in the regular brakes.
Going uphill, downhill, or into a strong wind, if you are loaded or towing a trailer, selecting a lower transmission gear will make the drive more enjoyable, economical and safer.
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).