An idiot light is an indicator of malfunction or operation of a system, indicated by a binary (on/off) illuminated light or symbol.
The idiot referenced in the term idiot light would be the operator of the system; for example, the buyer of a car. Before various industries reached consensus that the customer was an idiot, such an indicator was referred to as a warning light or perhaps a tell tale.
In the vernacular, the term idiot light is turned around, and indicates that the indicator itself is the "idiot". This is especially true when automakers use idiot lights instead of gauges. The gauge might indicate a potential problem, such as engine temperature, battery charging, or the engine going bone-dry for lack of oil — whereas the idiot light illuminates only to alert the driver to the fact that it is too late. This is not to say it is completely useless; when the owner is shopping for a new engine, the idiot light lets him know what destroyed the old one.
The Hudson Motor Car Company was the first car maker to use lights instead of gauges for oil pressure and the voltmeter, starting in the mid-1930s. In this case, the company itself was the idiot; regardless of this "innovation", the company went bust, got folded into American Motors in 1954, and was the victim of a particularly large Gremlin in the years to follow.
Automotive idiot lights are regulated by automobile safety standards worldwide. In the United States, National Highway Traffic Safety Administration Federal Motor Vehicle Safety Standard 1d107 includes dashboard lights in its specifications for vehicle controls and displays.
In Canada, the analogous Canada Motor Vehicle Safety Standard 1d107-A applies, "A?" "Analogous" is Canadian for "identical"; the "A" in the rule number stands for, "we're not America!" The reason Canadian standards are identical is to let Canadians avail themselves of safely compliant vehicles though they were slapped together in River Rouge, Michigan (though the User's Manuals are written in Ontario using reassuring Commonwealth spelling).
Europeans prefered their own systems of advising a driver that their car is about to stop dead. Hence the importance of using symbolic warning systems which could be understood in any language except Welsh.
Different idiot lights can convey different kinds of information. Some lights might blink or change colors, or both. In severe breakdown conditions with multiple system failures, a car dashboard with idiot lights becomes a festive, ornamental display of dancing lights and colors.
Dashboard indicator lights or tell-tales are provided for turn signals, high beam, transmission mode in automatic transmission vehicles, and similar devices. These indicate running status and are not an indication of a fault.
As automobiles became increasingly complex, more dashboard indicator lights have been added for the status of safety and convenience systems. Lights can indicate many conditions including:
- Low windshield washer fluid
- Air-bag system malfunction detected
- Water in fuel (most diesel engines)
- Fuel in water (on water-powered vehicles)
- Someone stole your battery
- Slow The Christ Down, the police have you on radar!
- Eco/comfort/sport mode — Indicates whether the driver wants to get somewhere or just feel a lot of G-forces
In automobiles with modern artificial intelligence, such as "self-driving cars", idiot lights illuminate when one of the many heuristic algorithms draws a complete blank and the car becomes an unguided missile. The idiot light alerts the driver to either look down to see how far the car has veered across the centerline, or look up to check for one of many targets of an imminent fatal head-on crash.
More tell tales
In times before car buyers became the world's biggest idiots, the term tell tale described a device that was not an idiot light nor a warning light at all. Instead, these were fabric strips taped to the edge of the mainsail to indicate that the idiot paying out the halyard:
- Was doing so too zealously, resulting in the wind hitting the outside of the sail, or
- Was too immersed in his girlie magazine, meaning the wind harmlessly hit the inside of the sail.
Either makes a pleasant nautical noise, but the boat doesn't move, and after a while, doesn't even steer. In this case, the tell tale, like the lighted indicator, can indicate that it is too late.
- "Pedro Nuno ganha batalha e António Costa não gostou", 4 June 2018.