Overdrive Range, 4-3 Downshift

WARNING: This page does not describe the selected car, but rather 13 other vehicles, including the 2005 GMC Yukon XL, 2005 GMC Sierra, 2005 GMC Savana Special, 2005 GMC Savana Camper Special, and 2005 GMC Savana. However, it is still accessible from the selected car via links, so may be relevant.

In order to obtain the 4-3 downshift, the PCM receives an input signal from the TP Sensor at increased throttle openings. This causes the PCM to de-energize the 1-2 Shift Solenoid (SS) Valve.

1-2 Shift Solenoid (1-2 SS) Valve: When the 1-2 SS valve is de-energized, 1-2 signal fluid exhausts at the solenoid valve. This allows spring force to move the 3-4 shift valve to the extreme right.
3-4 Shift Valve: The fourth clutch apply pressure is shut off and a port is opened at the valve. This exhausts the fourth clutch fluid, which exits the fourth clutch and the fourth clutch accumulator. The fourth clutch releases, shifting the transmission into third gear.
Fourth Clutch: Fluid which exhausts from the fourth clutch unseats the #10 checkball. This allows for a rapid exhausting of the fourth clutch apply fluid at the 3-4 shift valve.
Fourth Clutch Accumulator: Fluid which exhausts from the fourth clutch accumulator also exhausts through the unseated #10 check valve and exhausts at the 3-4 shift valve.

Fig 1: Overdrive Range, 4-3 Downshift

Courtesy of GENERAL MOTORS CORP.