Engine preheating control system having automatic control of glow plug

Abstract

In an engine preheating control system, in which the temperature of glow plugs mounted in a diesel engine is held in a constant high temperature range during the cranking operation after the closure of an engine key switch through the control of power supply to the glow plugs based upon the detection of changes of the resistance of the glow plugs, a main relay for controlling the power supply from a power supply source through a current detecting resistor to the glow plugs and a sub-relay for controlling power supply through a stable preheating resistor and the detecting resistor to the glow plugs are energized and de-energized in response to first to fourth control signals. The first control signal is produced from a comparator comparing the voltage drop across the detecting resistor and a reference voltage, the second control signal is produced from a start sensitive means, the third signal represents a predetermined period of time depending upon the engine cooling water temperature and is produced through a cooling water temperature detector and a timer circuit, and the fourth signal is produced through the detector and a voltage comparator circuit through the comparison of the detected cooling water temperature and a preset temperature. After the closure of the engine key switch, a fifth control signal representing a predetermined time period depending upon the cooling water temperature is produced through the voltage comparator circuit and a timer circuit, and a display lamp is on-off controlled according to the fifth control signal.