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
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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
In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...
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
An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...
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.
Automotive tell-tales are regulated by automobile safety standards worldwide. In the United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, National Highway Traffic Safety Administration
The National Highway Traffic Safety Administration is an agency of the Executive Branch of the U.S. government, part of the Department of Transportation...
Federal Motor Vehicle Safety Standard 101 includes tell-tales in its specifications for vehicle controls and displays. In Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, the analogous Canada Motor Vehicle Safety Standard 101
Transport Canada is the department within the government of Canada which is responsible for developing regulations, policies and services of transportation in Canada. It is part of the Transportation, Infrastructure and Communities portfolio...
applies. In Europe
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
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.
Vehicle running condition
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 or a pictogram of an oil can. Some cars illuminate the light red or yellow, depending on the action needed to be taken.
The engine temperature tell-tale is usually installed singly, but has less commonly been installed in pairs. A pair of lights indicate insufficient and excessive engine temperature. A single light usually indicates only an engine overheat condition.
Malfunction indicator (check engine)
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
An engine control unit is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters an internal combustion engine needs to keep running...
engine management system
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
. 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.
On vehicles equipped with OBD-II
On-Board Diagnostics, or OBD, in an automotive context, is a generic term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or a repair technician access to state of health information for various vehicle sub-systems...
, 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 , , , or ; 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
AB Volvo is a Swedish builder of commercial vehicles, including trucks, buses and construction equipment. Volvo also supplies marine and industrial drive systems, aerospace components and financial services...
, for example, used a light labeled or , "Lambda Sond" being another name for oxygen sensor
An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It was developed by the Robert Bosch GmbH company during the late 1960s under the supervision of Dr. Günter Bauman...
. 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 or ; 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 or or .
Some vehicles had a single indicator labeled or ; 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 Headlamp
A headlamp is a lamp, usually attached to the front of a vehicle such as a car or a motorcycle, with the purpose of illuminating the road ahead during periods of low visibility, such as darkness or precipitation. Headlamp performance has steadily improved throughout the automobile age, spurred by...
s, the turn signals, the daytime running lights
A daytime running lamp is an automotive lighting device on the front of a roadgoing motor vehicle, installed in pairs, automatically switched on when the vehicle is moving forward, emitting white, yellow, or amber light to increase the conspicuity of the vehicle during daylight...
, the front fog lamps, and the rear fog lamps.
Vehicles equipped with a backglass defogger
A defogger, demister, or defroster is a system to clear condensation and thaw frost from the windshield, backglass, and/or side windows of a motor vehicle...
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
A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....
. 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 .
Vehicles with Antilock braking system (ABS) will have a lamp to indicate a detected failure.
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:
- Traction control anti-skid system operative
- Low fuel
- Water in fuel (diesel engines)
- Low windshield washer fluid
- Air-bag system malfunction detected
- Driver or passenger seat-belt not buckled(along with a buzzer)
- Door open, or trunk lid open
- Anti-theft alarm active
- Speed radar detected
- Modes of the heating/air conditioning system (defrost, recirculation, and others)
- Automatic transmission fault
- Tire pressure low.
- Cruise Control on