A tell-tale (or idiot light, especially in North America[1]) is an indicator of the status or malfunction of a system within a motor vehicle. Tell-tales consist of a light bulb or an LED which lights up a symbol or text legend. The "idiot light" terminology arises from popular frustration with automakers' use of tell-tales 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 a tell-tale, which usually illuminates only once a fault has already occurred—thus providing no advance warning or details of the malfunction's extent.
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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.[2] In Canada, the analogous Canada Motor Vehicle Safety Standard 101 applies.[3] In Europe and throughout most of the rest of the world, ECE Regulations specify various types of tell-tales.
A tell-tale can convey different kinds of information. It may light or blink to indicate a failure (as of oil pressure, engine temperature control, charging current, etc.); lighting and blinking may indicate progression from warning to failure indication. It may light to alert the need for specific service after a certain amount of time or distance has elapsed. It may light to indicate a condition of readiness. A "closed-circuit tell-tale" lights to indicate a device or system has been switched on, while an "operating tell-tale" lights to indicate a device or system is operating correctly.[4]
Some tell-tales indicate whether critical vehicle systems are running and operating normally. To test the lamps, many tell-tales will illuminate when the vehicle ignition is turned on but before the engine is started, so as to prove that they are working.
The oil pressure tell-tale lights when engine oil pressure drops below a predetermined level. This tell-tale may 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.[5]
The engine temperature tell-tale is usually installed singly, but has less commonly been installed in pairs. A pair of lights indicate insufficient (cold) and excessive (hot) engine temperature.[6] A single light usually indicates only an engine overheat condition.
A "malfunction indicator lamp" (MIL) or "check engine light" (CEL) illuminates, usually in red, amber, or yellow, to indicate malfunction or service required in the computerized engine management system. It is found on the instrument console of most automobiles. In the United States an MIL with specified functions is required by environmental protection regulations.[7]
On vehicles equipped with OBD-II, the light has two stages: it burns steadily to indicate a minor fault such as a loose gas cap or malfunction with the vehicle's emissions controls, and it flashes repeatedly when a severe misfire is detected to alert the driver to the potential of severe engine or catalyst damage. The MIL may present a legend such as check engine, power limited, engine, or service engine soon; or it may present a pictogram of an engine.
Some vehicles have a MIL that illuminates at predetermined distance intervals without regard to actual condition or status of any system or device. Volvo, for example, used a light labeled lambda or Λ, "Lambda Sond" being another name for oxygen sensor. This was done to remind the driver to change the oxygen sensor.
Some vehicles built by Chrysler between 1973 and 1990 and some Toyota-built vehicles have a similar mileage-indicator light presenting various legends such as check egr or maint reqd; once the indicated inspection or service is completed, the light can be reset and will come on again after the next pre-determined distance has passed.
A tell-tale may be installed to indicate that the vehicle's alternator or generator is not properly charging the battery, which would eventually under this condition become discharged. This light may present a pictogram of a battery, or it may present the legend amp or batt or gen.
Some vehicles had a single indicator labeled trouble or engine; this was not a Check Engine Light, but a catch-all warning light to indicate trouble serious enough to portend an imminent breakdown. Early 1980s Fords, such as the LTD Crown Victoria without the instrument option had only this light, which indicated low oil pressure, overheating and carburetion problems without distinguishing among them. This usage of the "engine" light was discontinued in the mid-1980s to prevent confusion with the MIL.
Many tell-tales indicate that a device or system has been switched on, or is operating.
US, Canadian, and international European regulations require tell-tales to indicate to the driver that various vehicle lighting functions are operating. Such functions include the high beam Headlamps, the turn signals, the daytime running lights, the front fog lamps, and the rear fog lamps.
Vehicles equipped with a backglass defogger have a tell-tale to indicate when the system is switched on and operating.
All vehicles have a red tell-tale to indicate that the parking brake is engaged or that there is a fault with the vehicle's braking system.[8] This tell-tale in most cases bears the ISO symbols for a parking brake and for a braking system emergency. In the past, US regulations required this tell-tale to bear the legend brake.[9]
Vehicles with Antilock braking system (ABS) will have a lamp to indicate a detected failure.[10]
As automobiles became increasingly complex, more dashboard indicator lights have been added for the status of safety and convenience systems. Lights may indicate many condictions including:
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