Abstract: An additive pump system for fire-fighting vehicles is automatically engaged when the water pump is engaged. Upon engagement of the water pump, the water pump communicates with a hydraulic pump to engage the hydraulic pump. Engagement of the hydraulic pump results in a rise in hydraulic pressure. A pressure switch is tripped when a pressure of a pre-determined set point is reached. The pressure switch communicates with a controller to indicate that sufficient pressure has been obtained. In response to the receiving the signal of sufficient hydraulic pressure, the controller signals the additive pump system to engage the additive pump system.