Ford Mustang Owner's Manual page 164

Driving
Your vehicle's gearshift is console-mounted on
the floor. The Transmission Control Switch (TCS)
is located on the gearshift handle. The
Transmission Control Indicator Light (O/D OFF)
is located on the instrument panel. You can put
the gearshift lever in any of the several
positions.
The OVERDRIVE OFF light is off (not
illuminated) during normal vehicle operation.
This allows the transmission to upshift and
downshift from 1st through 4th gears. When the
TCS is pressed, the Transmission Control
Indicator Light will illuminate (O/D OFF) on the
instrument panel. When this O/D OFF is
illuminated the transmission will upshift and
downshift from 1st through 3rd gears.
If the O/D OFF light does not come "on" when
the TCS is depressed or if the light "FLASHES"
when you are driving, have your vehicle
serviced at the first opportunity. If this condition
persists, damage could occur to the transmission.
To return the transmission to normal operation
(1st through 4th gear) depress the TCS marked
"O/D" 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 O/D light will not be
illuminated.
When to use D (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.
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