In my years as a Ford technician (with a degree in Automotive Technology to boot) I have never seen so much misinformation, misconceptions, and downright BS surrounding any automotive subject as I have with the mysterious and majical "overdrive." Transmissions with overdrive have been around in numbers since the late 1970's but a majority of the public still refuses to believe that overdrive is nothing more than another gear ratio available from the transmission. It has nothing to do with special trans fluid, torque converters, or the transmission fairies.
In the old days we had auto transmissions that had three speeds, which were derived from a single planetary gearset. A simple planetary gearset will give you three speeds: A low or first gear, an intermediate or second gear, and direct drive. In first gear the output shaft spins slower than the input shaft, which is driven by the torque converter, due to the gear ratio provided by the planetary set. Thus, first gear is UNDERDRIVEN: Output is slower than input. Second gear is the same, except the ratio is slightly changed to provide more output speed, but it is still UNDERDRIVEN. Now, third gear is different in that the planetary members are locked together and perform no torque multiplication. The output speed of the transmission is exactly the same as the input speed, that is, it is in DIRECT DRIVE. In DIRECT, if the engine is spinning at 3,000 rpm, so is the output shaft of the transmission.
When the need for improved fuel economy became an issue, engineers had to find a way to slow the engine down without reducing normal cruising speeds. This could be accomplished by changing the axle ratio of the vehicle, but this change would also result in much slower acceleration and decreased towing capacity. The final solution was to add an "extra" gear to the transmission, one that would allow the output speed of the transmission to be higher than the input speed. Therefore, the transmission would be OVERDRIVEN. This is where the term "overdrive" comes from: it is simply the mode that the transmission is operating in. Overdrive is accomplished by either adding a second planetary gearset to the trans or by using a "Ravigneaux" gearset (try Google.) This trend continues today as five and six speed automatic transmissions are common, many of which have TWO overdrive speeds.
So, why is there a switch for turning overdrive off? The same gear mechanism that slows the engine down also drastically reduces the amount of torque that can be delivered to the wheels. Under a heavy load at freeway speeds, the engine may not produce enough power to maintain road speed for that gear and will downshift to the next lower gear. The rpms will rise and power increases. As you accelerate up to speed, the transmission or powertrain computer will sense the reduction in throttle as you try to maintain your target speed. Since you are letting off the throttle, the transmission will shift back into top gear (overdrive.) With the engine slowed down again, it won't produce the needed power to maintain speed, so you push farther down on the throttle and the transmission will again downshift to get the engine back up to speed. This upshift/downshift cycle continues and each shift builds up heat due to friction in the trans. Heat is bad for a transmission so to prevent these damaging shifts engineers give us a way to lockout overdrive either by a switch or a separate gearshift position. Of course, this makes the engine rev much higher and your fuel mileage will plummet.
You'll notice nowhere above is there any mention of the torque converter. Why? Overdrive has nothing to do with a lock-up torque converter. As a matter of fact, Ford's first major overdrive transmission put into production, the AOD (Automatic OverDrive,) didn't even HAVE a lock-up torque converter. Ford's prolific Super Duty trucks with the 7.3L have a four-speed auto with fourth-gear overdrive but will lock their torque converter in second, third, and fourth gear.
The simple fact is that unless your transmission is "hunting" for gears (repeatedly shifting up and down while trying to maintain a set speed) there is no reason to turn off or lock out overdrive, regardless if you are towing or not. I have yet to see an owner's manual that fails to explain this in this manner.
So please, before perpetuating the "Freemason's guide to the secret Overdrive" do a little research and find out how overdrive works from a shop manual and help me put an end to overdrive slander.
Jay
