System Pre-Fill

WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2004 GMC Yukon XL, 2004 GMC Yukon, 2004 Chevrolet Tahoe, 2004 Chevrolet Suburban, and 2004 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.

This vehicle is equipped with a 4-wheel disc brake system. Disc brake calipers are designed so that when hydraulic pressure is not being applied, the caliper piston lip seal causes the piston to retract, creating measurable clearance between the brake pads and the rotor. Since a small amount of brake fluid must be delivered to the calipers before any actual braking occurs, the VSES uses system pre-fill to prevent delayed brake application and enhance system performance. If the EBCM determines that a brake application is likely to be needed, the ABS pump motor runs momentarily to take up any clearances between the brake pads and the rotor. By monitoring the master cylinder pressure sensor feedback signal, the EBCM can determine when the brake pads are contacting the rotor. The EBCM then holds this small amount of pressure in the system.

A VSES brake application may or may not occur after pre-fill is complete. If the EBCM determines that a brake application is no longer pending, the pre-fill pressure is released and the VSES system returns to the normal, monitoring state.

The reason that we must understand system pre-fill is that pre-fill may lead to customer concerns. Any time the ABS pump motor is active, the motor draws a large amount of current, and may cause the vehicle lighting systems to dim noticeably. When ABS activity occurs, most drivers understand that this activity is the cause of noises and dimming lights. Likewise, when an actual VSES event occurs, the Stability System Active message is displayed, which helps drivers understand why these other conditions occur. Since pre-fill is not an actual VSES event, but preparation for a pending event, no message is displayed. Also, system noise during pre-fill is very minimal. A customer may become concerned with what is perceived to be an electrical problem, due to the intermittent dimming lights, when, in fact, no malfunction exists and the condition is normal.