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21 - 112
AUTOMATIC TRANSMISSION
•
T h e standard valve body is modified b y adding sev-
e r a l new components t o a lower housing (Figs. 2 and
3). The new components include: a n overdrive sole-
noid, a lockup solenoid (A-500 only), a 3-4 shift
valve, a 3-4 t i m i n g valve, a 3-4 accumulator, and a
3-4 s h u t t l e valve.
I n
f o u r t h
gear overdrive,
t h e A-500
converter
lockup solenoid, lockup valve, a n d lockup t i m i n g
v a l v e are actuated. A t lockup, t h e converter t u r b i n e
is locked t o t h e converter housing b y t h e converter
lockup clutch.
Converter lockup i n overdrive f o u r t h gear i s con-
t r o l l e d b y sensor i n p u t s t o t h e engine controller. I n
t h i r d gear above 25 m p h , sensor i n p u t s to t h e con-
t r o l l e r t h a t determine lockup a n d s h i f t t i m i n g are:
coolant temperature (verifies t e m p e r a t u r e m i n i m u m
of 60° F ) , engine speed, vehicle speed, t h r o t t l e posi-
t i o n and manifold v a c u u m t h r o u g h t h e map sensor.
The c o l u m n gear shift m e c h a n i s m provides t h e
same s h i f t positions used w i t h t h r e e speed automatic
transmissions (P-R-N-D-2-1). T h e s h i f t i n t o overdrive
f o u r t h gear range occurs o n l y after t h e transmission
3-4 ACCUMULATOR PISTON
3-4 ACCUMULATOR HOUSING
LOWER VALVE
BODY HOUSING
3-4 TIMING
VALVE
END
COVER
PLATE
OVERDRIVE
SOLENOID
(BLACK WIRE)
PLUG
SPRING
RETAINER
LOCKUP TIMING
VALVE
LOCKUP SOLENOID
(WHITE WIRE)
STEEL
SEPARATOR PLATE
J9021-143
Fig. 2 A-500 Valve
OVERDRIVE
CONTROL AND
OPERATION
F o u r t h gear overdrive i s electronically controlled
and h y d r a u l i c a l l y activated. A variety o f sensor i n -
puts are supplied to t h e engine controller w h i c h op-
erates
t h e valve
body
solenoids.
T h e solenoids
energize a n d close a vent allowing a 3-4 upshift. The
engine electronic module also controls operation of
the lockup torque converter using m a n y of the same
sensor inputs.
Closed t h r o t t l e 3-4 upshifts occur a t 25-28 m p h , re-
gardless o f axle ratio. Closed t h r o t t l e 4-3 downshifts
occur a t approximately 25 m p h , regardless o f axle r a -
tio.
A 3-4 u p s h i f t w i l l n o t occur i f t h r o t t l e opening is
greater t h a n approximately 70 percent.
Body Lower Housing
has completed the shift i n t o D t h i r d gear range. T h e
shift t o overdrive f o u r t h gear occurs automatically
and is determined b y sensor i n p u t t o the engine con-
t r o l l e r .
The overdrive control s w i t c h i s located i n t h e i n -
s t r u m e n t panel. I n t h e O n position, automatic shifts
i n t o f o u r t h gear overdrive w i l l occur. I n the O f f posi-
t i o n , t h e s w i t c h overrides t h e electronic control mod-
ule p r e v e n t i n g a shift to f o u r t h gear.
The control switch has a n i n d i c a t o r l i g h t t h a t i l l u -
minates w h e n overdrive i s t u r n e d off. T h e switch
also resets when the i g n i t i o n k e y is t u r n e d to the O f f
position so the overdrive feature is restored.
F a u l t codes are employed t o help diagnose elec-
tronic components t h a t operate t h e overdrive
u n i t
and lockup converter.
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