Chevrolet TrailBlazer, Service Manual

Page: 408 / 677

Most automatic transmissions are electronically controlled; electrical solenoids are used to
control the hydraulic fluid. The shift solenoids are regulated by an electronic control
module. Shift timing is regulated through sensor feedback information provided to the
electronic controller.
On older transmissions there are two pressures that control the shift valves. One is the
governor pressure which is affected by vehicle speed. The other is the modulator pressure
which is affected by intake manifold vacuum or throttle position. Governor pressure rises
with an increase in vehicle speed, and modulator pressure rises as the throttle is opened
wider. By responding to these two pressures, the shift valves cause the up shift points to
be delayed with increased throttle opening to make the best use of the engine's power
output.
Older transmissions also make use of an auxiliary circuit for downshifting. This circuit may
be actuated by the throttle linkage, vacuum that actuates the modulator, or by a cable or
solenoid. It applies pressure to the downshift surface on the shift valve or valves.
The transmission modulator also governs the line pressure, used to actuate the servos. In
this way, the clutches and bands will be actuated with a force matching the torque output
of the engine.