In the nine years I've owned my '98 Dakota, 3.9L 2WD, AT, I had never
turned the OD off. Last week, while pulling a fairly light trailer in
a hilly area I decided to turn it off (manual says to in order to cut
down on the transmission hunting). When I turned it off, I noticed
that it stopped the shift to OD but also into the fourth range. Is
that right? Is that what it is supposed to do?
shifts three times indicating four gear ranges, and within about 5
seconds of shifting into the fourth range if there is no load other
than cruising, the RPM's drop again.
I thought that was the overdrive. What happens at that point? Is
that another gear range or something else associated with the fourth
That last RPM drop is the torque converter locking up. You still get
converter lock-up in 3rd with overdrive disengaged.
When not locked, the torque converter provides a fluid coupling between the
engine and the transmission. The engine is turning one half of the torque
converter directly (it's bolted to the output shaft of the engine). The
other half is driven by the fluid that the first half is churning around.
This means that the engine is always turning faster than the input shaft of
the transmission. This is why, when you're cruising at around 30MPH or so,
and you nail the gas (but not enough to cause a downshift), the engine RPMs
rise quickly, but your speed doesn't immediately increase.
Now, when the computer feels like it (several conditions have to be met...
speed, throttle position, temperature, etc.), it will engage a clutch inside
that torque converter, which will mechanically lock both halves of the
torque converter together. Now the engine is effectively DIRECTLY coupled
to the transmission (like in a manual transmission), and any change in
engine speed has an immediate and direct effect on the vehicle's speed.
When the converter is locked, you can vary your throttle from a little to a
lot, and you'll notice the engine RPMs don't change. Let completely off the
throttle, or mash it to the floor, and the computer will unlock the
converter. Tap on the brake pedal, and the computer will unlock the
You want to operate with your converter locked as much as possible. It's
more efficient (the whole reason companies developed a lock-up converter in
the first place), and it creates less heat inside the transmission (the
transmission fluid heats up quickest when the converter is unlocked, due to
the friction of the fluid sliding across the blades inside the converter).
This is exactly why you're instructed to disable overdrive when towing.
It's not that 4th gear is worse than 3rd - it's to eliminate the constant
locking/unlocking of the converter, which causes heat, which kills
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