47RH (A-618) 4-Speed

PURPOSE
Fourth gear overdrive range is electronically controlled and hydraulically activated. Various sensor inputs are supplied to the powertrain control module (PCM) 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.

OPERATION
The overdrive clutch is applied in fourth gear only. The direct clutch is applied in all ranges except fourth gear.

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 switch must be in the ON position to transmit signals to the solenoid. A 3-4 upshift occurs only when the overdrive solenoid is energized by an electrical signal from the PCM.

The solenoid is energized upon receiving a signal. This causes the check ball to close the vent port. Closing the vent port allows line pressure from the 2-3 shift valve to act directly upon the 3-4 upshift valve.

Line pressure acting on the 3-4 shift valve overcomes valve spring tension moving the valve to the upshift position. This action exposes the feed passages to the 3-4 timing valve, 3-4 shuttle valve, 3-4 accumulator and ultimately to the overdrive piston.

Line pressure through the timing and shuttle valves move the overdrive piston into simultaneous contact with the overdrive clutch and the direct clutch hub.

The overdrive clutch is engaged and the direct clutch is disengaged simultaneously to complete the 3-4 upshift.

The boost valve provides increased fluid apply pressure to the overdrive clutch during the 3-4 upshift and during fourth gear operation. The amount of boost is determined by throttle opening and line pressure.

The overdrive piston engages the overdrive clutch by pressing directly against the clutch pressure plate. The overdrive clutch also disengages the direct clutch during 3-4 upshifts. As fluid pressure extends the overdrive piston, the piston contacts the direct clutch hub pressing it rearward. This action compresses the direct clutch spring relieving spring load on the clutch pack. The clutch is disengaged once spring load is relieved.

The 3-4 accumulator cushions overdrive clutch engagement to smooth the transition into fourth gear. The accumulator is charged at the same time as apply pressure acts against the overdrive piston.

Converter clutch engagement in overdrive fourth gear is controlled by switch and sensor inputs to the PCM. In third gear above 25 mph, inputs that determine clutch engagement and shift timing are:

- transmission fluid temperature
- engine speed
- vehicle speed
- throttle position
- manifold vacuum
- overdrive off (control) switch position