The glow plug relay control signal is used to control a relay to cycle the glow plugs for assisting engine start-up. Glow plugs will be operated at low ambient temperature and/or high altitude. Engine oil temperature, battery voltage, and barometric pressure are used to control the glow plug relay. At colder temperatures, the plugs are on longer. At high battery voltage the duty cycle is reduced to prevent plug damage. For low barometric pressure, the ON time is extended to assist start/drive-away.