Ford 1997 Aerostar Manual page 174

When to use Drive
You will note there is not a drive position on
your gearshift indicator. However, you will find
a transmission control switch labeled
OVERDRIVE OFF located on the end of the
gearshift lever. Press this switch and the
indicator light (OFF) illuminates on the gearshift
lever. With overdrive cancelled and the
OVERDRIVE OFF illuminated, the transmission
will operate in gears one through three. This is
useful if you experience frequent
downshift/upshift conditions, when operating
with a heavy load or in hilly terrain, or if
additional engine braking is useful. However,
shift back to j (Overdrive) whenever practical
for optimum powertrain cooling and fuel
economy. This can be accomplished by pushing
the transmission control switch again, to cancel
the OVERDRIVE OFF control and light.
When starting your vehicle, the overdrive system
will automatically be in the normal overdrive
mode.
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