It may not be the PCM, but it could be. You are looking at around 200 bucks to replace it. To be honest with you, I've never heard of a PCM causing this problem, but it sounds like the PCM is not controlling the transition from cold to hot tranny.
If you're so inclined, you may want to pull the pan and look around. Take a read of the following info from FSM.
TRANSMISSION FLUID TEMPERATURE
Transmission fluid temperature readings are supplied
to the transmission control module (in the PCM on a 96-98) by the thermistor.
The temperature readings are used to control
engagement of the fourth gear overdrive clutch, the
converter clutch, and governor pressure. Normal
resistance value for the thermistor at room temperature
is approximately 1000 ohms.
The PCM prevents engagement of the converter
clutch and overdrive clutch, when fluid temperature
is below approximately 10°C (50°F).
If fluid temperature exceeds 126°C (260°F), the
PCM causes a 4-3 downshift and engage the converter
clutch. Engagement is according to the third
gear converter clutch engagement schedule.
The overdrive OFF lamp in the instrument panel
illuminates when the shift back to third occurs. The
transmission will not allow fourth gear operation
until fluid temperature decreases to approximately
The thermistor is part of the governor pressure
sensor assembly and is immersed in transmission
fluid at all times.
3-4 SHIFT SEQUENCE
The overdrive clutch is applied in fourth gear only.
The direct clutch is applied in all ranges except
fourth gear. Fourth gear overdrive range is electronically
controlled and hydraulically activated. Various
sensor inputs are supplied to the powertrain control
module to operate the overdrive solenoid on the valve
body. The solenoid contains a check ball that opens
and closes a vent port in the 3-4 shift valve feed passage.
The overdrive solenoid (and check ball) are not
energized in first, second, third, or reverse gear. The
vent port remains open, diverting line pressure from
the 2-3 shift valve away from the 3-4 shift valve. The
overdrive control switch must be in the ON position
to transmit overdrive status to the PCM. A 3-4
upshift occurs only when the overdrive solenoid is
energized by the PCM. The PCM energizes the overdrive
solenoid during the 3-4 upshift. This causes the
solenoid check ball to close the vent port allowing
line pressure from the 2-3 shift valve to act directly
on the 3-4 upshift valve. Line pressure on the 3-4
shift valve overcomes valve spring pressure moving
the valve to the upshift position. This action exposes
the feed passages to the 3-4 timing valve, 3-4 quick
fill valve, 3-4 accumulator, and ultimately to the
overdrive piston. Line pressure through the timing
valve moves the overdrive piston into contact with
the overdrive clutch. The direct clutch is disengaged
before the overdrive clutch is engaged. The boost
valve provides increased fluid apply pressure to the
overdrive clutch during 3-4 upshifts, and when accelerating
in fourth gear. The 3-4 accumulator cushions
overdrive clutch engagement to smooth 3-4 upshifts.
The accumulator is charged at the same time as
apply pressure acts against the overdrive piston.
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