Glowplug indicator light

In older generation diesel-engine vehicles, unlike in a gasoline-engine vehicle, the operator did not simply turn the key to the "start" position and have the engine immediately start. Instead, the operator turned the key to the "on" position for a long duration; the glowplug relay switches the glowplugs on, and a light (see picture at right) on the instrument cluster illuminates. This process is called "pre-heating" or "glowing". According to Bosch: "Older engines used a glow period of up to 21 seconds whereas more modern engines use around a 6 to 8 second heat period and provide after glow at a reduced voltage"