Precharge Pump

The TCS and VSES are dependent on the precharge pump to assist the ABS pump in building hydraulic pressure to perform brake pressure application. This is because the passages from the master cylinder reservoir to the ABS pump inlet are restrictive, and do not supply adequate fluid to the ABS pump. The precharge pump is used mostly at the beginning of brake pressure applications since once some pressure is built, the ABS pump is usually able to continue building pressure and maintain the application. The precharge pump inlet is fed by a flexible hose which draws fluid directly from a port on the master cylinder reservoir. When the precharge pump is activated, fluid is pumped into the combination valve, slightly pressurizing the front and rear brake circuits between the master cylinder and the BPMV. It is important to note that during this time, the EBCM must isolate of the all wheels except for those to which the brake pressure application is directed. The portion of the pressurized fluid that is not drawn into the ABS pump inlet, passes through the bypass ports in the master cylinder and returns to the reservoir. Some of the pressurized fluid is supplied to the ABS pump inlet when the EBCM commands ON a supply valve in the BPMV. Since the fluid at the ABS pump inlet is slightly pressurized, the ABS pump is able to function with higher efficiency. The precharge pump, however, is not always used during TCS or VSES events due to the fact that the precharge pump motor must be protected from overheating. During a long series of brake pressure applications, the precharge pump may be disabled by the EBCM for up to 12 seconds. Although the driver may be able to detect a change in the vehicle stability during this time, the TCS and VSES do remain functional. No indication is given to the driver if the precharge pump is disabled due to overheating.