Pontiac Fiero 1988 Service Manual page 770

HYDRAULIC DIAGNOSIS 125C-29
DETENT DOWNSHIFTS
(Valves In Second Gear Position)
FORWARD CLUTCH - APPLIED INTERMEDIATE BAND - APPLIED
While operating at speeds below approximately 65 mph (105 km/h), a forced or detent 3-2
downshift
is possible by depressing the accelerator pedal fully. This will position the throttle valve (T.V.)
plunger to allow shift T.V. oil to enter the detent passage. This oil, called detent oil, is then routed
to the following:
1. 2-3 Throttle Valve
2. 1-2 Throttle Valve
3. RNDI and Detent Checkball (6)
Detent oil from the T.V. plunger flows to the 2-3 throttle valve. Detent and part throttle (P.T.) oil pressure
combined with the 2-3 spring force will close the 2-3 shift valve against governor oil and allow direct
clutch oil through two orifices to exhaust at the 2-3 shift valve.
At high vehicle speeds, above approximately 50 mph (80 km/h), governor oil acting on the 3-2 control
valve will close it. Now the exhausting direct clutch accumulator oil from the intermediate servo will
seat the direct clutch accumulator checkball (2) and flow through another orifice controlling the
intermediate band apply for a smooth 3-2 shift at high speed.
A detent 2-1 downshift can be accomplished at speeds below approximately 30 mph (48 km/h),
because detent oil pressure and the 1-2 spring force acting on the 1-2 throttle valve will close the
1-2 shift valve, shifting the transmission to First Gear.
Detent oil acts on the RNDI and detent checkball (6) but will not unseat it because RNDI oil is at
a higher pressure.
JH0025-125C
Figure 26 Detent Downshifts - Valves in Second Gear Position