Method of preheating an internal combustion engine of the diesel, or similar, type

To rapidly heat glow plugs having positive temperature coefficient characteristics, or glow plugs in circuit with a positive temperature coefficient resistor, without overloading connecting relay contacts, a current limiting resistor is placed in circuit with the glow plugs upon first connection thereof to a supply source. After elapse of a first time interval of about 0.1 to 0.3, maximally 1 second, a second switch is controlled to short-circuit the current limiting resistor to permit full voltage to be applied to the glow plugs for rapid heating thereof, during a second time interval, for example between about 2 to 6 seconds, the positive temperature characteristics of the circuit self-limiting current through the resistors to thereby protect circuit switches and relay contacts.