Tell-tale (automotive)

An idiot light , sometimes "tell-tale", is an indicator of malfunction of a system within a motor vehicle by an illuminated symbol or text legend.

The "idiot light" terminology arises from popular frustration with automakers' use of lights for crucial functions which could previously be monitored by gauges, so a troublesome condition could be detected and corrected early. Such early detection of problems with, for example, engine temperature or oil pressure or charging system operation is not possible via an idiot light, which lights only when a fault has already occurred – thus providing no advance warnings or details of the malfunction's extent. The Hudson automobile company was the first to use lights instead of gauges for oil pressure and the voltmeter, starting in the mid-1930s.

Automotive tell-tales are regulated by automobile safety standards worldwide. In the United States, National Highway Traffic Safety Administration Federal Motor Vehicle Safety Standard 101 includes tell-tales in its specifications for vehicle controls and displays. In Canada, the analogous Canada Motor Vehicle Safety Standard 101 applies. In Europe and throughout most of the rest of the world, ECE Regulations specify various types of tell-tales.

Different tell-tales can convey different kinds of information. One type lights or blinks to indicate a failure (as of oil pressure, engine temperature control, charging current, etc.); lighting and blinking indicate progression from warning to failure indication. Another type lights to alert the need for specific service after a certain amount of time or distance has elapsed (e.g., to change the oil).

Colour may also communicate information about the nature of the tell-tale, for example red may signify that the vehicle cannot continue driving (e.g. oil pressure).) Many older vehicles used schemes which were specific to the manufacturer, e.g. some British Fords of the 1960's used green for oil pressure warning, while many other makes of the same era used amber for oil pressure.

The oil pressure idiot light lights when engine oil pressure drops below a predetermined level. The light normally appears when the vehicle ignition is turned on, but the engine is not running, as it detects no oil pressure due to the oil pump not operating (the oil pump is powered off the crankshaft, so only operates when the engine is running). It normally turns off a few seconds after engine start as the oil pump starts working and building up oil pressure. This tell-tale can bear the legend oil or a pictogram of an oil can. Some cars illuminate the light red or yellow, depending on the action needed to be taken.

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