1997 Buick Skylark At times hard shift

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DESCRIPTION
The 4T60-E transaxle uses 2 electric solenoids to control transaxle upshifts and downshifts. See Fig. 1 . Each solenoid is turned on or off by the Powertrain Control Module (PCM). PCM also includes on-board self-diagnostics. This helps identify which parts or circuits may need further testing.
OPERATION
Each solenoid either holds hydraulic pressure (solenoid on) or releases hydraulic pressure (solenoid off). This action controls shift valves inside valve body. By switching one or both solenoids on or off, different combinations of clutches, sprags and bands are operated. See CLUTCH & BAND APPLICATION CHART (4T60-E) under ELECTRONIC TESTING.
LIMP-IN MODE
If sensor input signals are missing or inadequate for transmission operation, PCM will output preset operating signals to transmission. This mode will keep vehicle operational and allow it to be driven with reduced transmission function and performance, to a repair facility. Malfunction Indicator Light (MIL) will light if malfunction occurs. Vehicle should not be driven for extended periods in limp-in mode.
COMPONENT DESCRIPTION
PCM
On "N" body models, PCM is located at right side of instrument panel, behind glove box. On all models, PCM utilizes 2 different colored 80-pin connectors. See Fig. 2 and Fig. 3 . PCM controls ignition, fuel and emission devices related to engine and transaxle upshifts and downshifts.
PCM receives electronic signals from sensors and switches. These signals help PCM determine when to operate various relays and solenoids related to engine and transaxle control.
Sensors & Switches
PCM controls upshifts and downshifts based on coolant temperature (or transaxle fluid temperature), throttle position, transaxle range switch position, vehicle speed sensor and brake pedal switch. System also includes several other switches and sensors which are used for engine control. Testing for these components is covered in SYSTEM/COMPONENT TESTS in ENGINE PERFORMANCE section.
Solenoids
Transaxle is shifted up or down by 2 electric solenoids. Both solenoids are located on valve body. Ignition power is supplied to each solenoid by fused circuit. See WIRING DIAGRAMS .
Solenoid "A" controls hydraulic pressure to 1-2 and 3-4 shift valves. Solenoid "B" controls hydraulic pressure to 3-2 and 4-3 shift valves.

Self-Diagnostics
PCM constantly monitors all electrical circuits. If PCM detects circuit problems or sensors out of range, it will record a Diagnostic Trouble Code (DTC). If problem continues for a predetermined time, Malfunction Indicator Light (MIL) will glow.
If MIL is on all the time, DTC(s) are currently being detected. If MIL is off, but PCM had detected a circuit or sensor problem, DTC(s) will be stored in computer memory.

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