Ford Grand Marquis Owner's Manual page 168

To return the transmission to normal operation
(1st through 4th gear) depress the TCS again.
This switch may be used to cancel overdrive any
time the vehicle is being driven. Each time your
vehicle is started, the overdrive system will
automatically be in the normal overdrive mode
and the light will not be illuminated.
When to use j (Overdrive)
Overdrive is the usual driving position for an
automatic overdrive transmission. It works the
same way Drive works, automatically upshifting
or downshifting as the vehicle speeds or slows.
However, Overdrive also shifts into a fourth
gear —an overdrive gear — when your vehicle
cruises at an appropriate consistent speed for
any length of time. This fourth gear will increase
your fuel economy when you travel at cruising
speeds.
When to use j (Overdrive) — with
Transmission Control Switch activated
(TCS)
Overdrive may not be appropriate for certain
terrain. If the transmission shifts back and forth
between Third and Fourth gears while you are
driving on hilly roads or if your vehicle does
not have adequate power for climbing hills,
press the TCS located on the shift lever.
You can depress the TCS at any speed.
When to use 2 (Second)
Use 2 (Second) for start-up on slippery roads or
to give you more engine braking to slow your
vehicle on downgrades.
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