Ford Econoline/Club Wagon E-150 Owner's Manual page 109

If you drive up or down a steep hill, your
vehicle may momentarily slow down or speed
up, even though the speed control is on. This is
normal.
NOTE: If your speed increases above your set
speed while driving in j (Overdrive)
on a downhill grade, you may want to
depress the transmission control switch
located on the shift lever to turn off
overdrive to reduce vehicle speed.
Speed control cannot reduce the vehicle
speed if it goes above your set speed
on a downhill grade. For the best fuel
economy during normal driving
conditions, leave the shift select in j
(Overdrive), or resume as soon as
practical.
Do not use the speed control in heavy
traffic or on roads that are winding,
slippery, or unpaved.
In mountainous areas, at higher elevations, or
when pulling a trailer, the speed control may
not be able to maintain the preset speed with
the transmission in j (Overdrive).
Do not shift the transmission into N
(Neutral) with the speed control on.
RWARNING
RWARNING
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