Overdrive Range, 4-3 Downshift: Notes
WARNING: This page is about a different car, the 2009 GMC Yukon and 2009 Chevrolet Tahoe. However, it is still accessible from the selected car via links, so may be relevant.
When the transmission is operating in fourth gear, a forced 4-3 downshift occurs if there is a significant increase in throttle position. At minimum throttle, the vehicle speed decreases gradually (coastdown) and the PCM commands a 4-3 downshift. The PCM also initiates a forced 4-3 downshift when the throttle position remains constant but engine load is increased, such as driving up a steep incline. To achieve a 4-3 downshift, the PCM de-energizes the 1-2 shift solenoid valve and the following changes occur to the transmission's electrical and hydraulic systems: