Band Apply Solenoid

The Band Apply solenoid is a normally open solenoid that controls the brake band apply and the release. This is accomplished by the TCM varying the solenoid's duty cycle (percent time energized) in relation to vehicle operating conditions and the various TCM input signals. The brake band is always applied in the 1st and 2nd gears.

Power to the solenoid is provided by the TCM via the Transmission Power Relay Output circuit. The Band Apply solenoid control circuit has a constant operating frequency of 32 Hz (cycles per second). The solenoid movement is controlled by varying the duty cycle (on-time) of this signal. With the solenoid being normally open, a greater duty cycle drives the solenoid farther closed causing less fluid flow through the solenoid (closed with respect to D32/1-2 fluid entering the valve and servo apply fluid circuit). The Hydraulic properties are as follows:
- Brake Band Applied - When the band is applied, the electrical path to ground for the solenoid is always open and the duty cycle is 0%. Therefore, the current does not flow through the coil in the solenoid and the solenoid is always off. With the solenoid off, the solenoid spring force holds the ball away from the D32/1-2 fluid inlet port and against the exhaust seat in the solenoid. This allows the D32/1-2 fluid to flow through the inlet port, past the ball, and into the two passages leading to the solenoid valve. This D32/1-2 fluid pressure moves the valve completely to one side. With the valve in this position, D32/1-2 fluid flows through the valve and enters the servo apply fluid circuit, tightening the band.

- Brake Band Application Rate - If the solenoid remained off and fully open (0 percent duty cycle) during the band apply, the servo apply fluid pressure would increase too rapidly and create a harsh shift. Therefore, in order to control the band apply rate, the solenoid's duty cycle is increased from 0 percent. As the TCM increases the duty cycle which creates a magnetic field that magnetizes the center pole, The magnetized pole repels the ball against the spring force, seating the ball against the inlet port. This allows D32/1-2 fluid from the end of the valve to exhaust past the ball and through the solenoid.

Without D32/1-2 fluid pressure at the end of the valve, the fluid dynamics acting on the valve shift the valve completely to one side. This blocks the D32/1-2 fluid from entering the valve and supplying the servo apply fluid circuit. A higher percentage duty cycle increases the current flowing through the solenoid, thereby increasing the coil's magnetic field This causes a slower increase in the servo apply fluid pressure and slower application of the brake band.

- Brake Band Release - The solenoid state during the band release depends on the vehicle speed and the gear selector lever position. During a shift from a forward Drive Range to Park, Reverse, or Neutral, or a 2-3 upshift at speeds above approximately 20 km/h (13 mph), the TCM operates the solenoid at a 0 percent duty cycle (fully open). This allows excess servo apply fluid pressure to exhaust quickly through the solenoid, thereby releasing the band quickly.

During a 2-3 upshift at low speed, below approximately 20 km/h (13 mph), the TCM operates the solenoid at 100 percent duty cycle. With a 100 percent duty cycle the solenoid valve is completely closed, and blocks exhausting servo apply fluid from entering the valve and the D32/1-2 fluid circuit. This forces the exhausting servo apply fluid pressure through a small orifice. Orificing this fluid slows the exhaust and creates a slower band release. At low speeds a slow band release takes place in order to prevent a harsh release feel.

Approximately every 15 seconds the TCM pulses the band apply solenoid to either a maximum or minimum duty cycle. These pulses prevent possible contamination from sticking the solenoid valve or plunger in any given position.