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Auto, Car, Truck Gages
All modern cars and trucks have gages and lights to warn the driver of problems and give them information on things like fuel level and speed. Some vehicles have only gages. On some the only gage is the speedometer and fuel gage. The rest are lights. Some cars have totally electronic video displays for speedometer and all.
IS THAT LIGHT IMPORTANT?
Ideally you should never have any abnormal warning lights on your dash. The "fasten seat belt" light should be off, because everyone has their seatbelts on. Turn signal lights should blink normally and cancel when the turn is done.
What about a CHECK ENGINE LIGHT? How important is that?
A general rule of thumb: RED LIGHT: STOP!.... YELLOW LIGHT: CAUTION!
A red light means something bad has happened. Stop as soon as possible. If you ignore a red light and keep driving you may cost yourself a lot of money, or even kill yourself!
A yellow light is less urgent. The better your car runs, the less urgent the light is. For example, if your check engine light goes on when you are on vacation, and the vehicle still runs well and gets acceptable fuel mileage then you can probably finish your vacation and take it in to your regular mechanic for repair when you get back in town. A red oil pressure light on the other hand is cause to stop as soon as safely possible!
A red "brake" light can be from 3 things: parking brake left on, low brake fluid, or off center proportioning valve caused by loss of pressure in the brake system. Once again if your brake pedal is firm you might be OK, but check the fluid level and operate your parking brake a couple of times to see if the light goes off. An amber "ABS" (Antilock Brake System) light warns of a problem that won't affect you under normal conditions, but if you start skidding on a slick road your antilock brake system may not work properly. Ditto with an amber air bag light: it's no problem as long as you're not involved in an accident!
NORMAL GAGE READINGS AND WARNING LIGHT ON CONDITIONS
The two vital factors monitored by a gage or light on a vehicle are engine temperature and oil pressure. Normal temperature range is between 165 degrees F and 220 degrees F. Oil pressure should be between 20 and 80 pounds. At highway speed the engine should have 40 or more pounds oil pressure. Generally the more oil pressure the better, although too much (over 80 PSI) can cause some problems. Some cars and trucks have jus a "L" and "H" on their oil gage and no numbers. A gage like this should read at least 1/4 of the way up at idle, at least 1/2 gage range at highway speed. Temperature warning lights usually come on at 220 to 250 degrees F. Oil pressure warning lights come on if pressure goes under 6 to 10 pounds.
The alternator light (or ammeter or voltmeter if a gage) isn't as crucial as the oil pressure and temperature light, but a charging system problem can leave you on the side of the road just like an empty gas tank can. Thus nearly every vehicle ever made has had an oil pressure, temperature, and charging system light or gage, and a fuel gage. Modern cars have a light or gage for so many functions that they look like a pinball machine when you first start them up! Even so, the "vital" ones are the temperature gage or light and oil pressure gage or light.
"IDIOT LIGHTS" vs. a gage
There are a couple of different attitudes toward gages versus lights on a vehicle. Gage lovers like to know EXACTLY what their oil pressure, engine temperature, etc. are. Gage lovers know what readings are normal for their cars and check things out if the readings get too low or high. For example if you have a temperature gage you can see your car "getting warm", possibly from a coolant leak, and check into things before the car overheats severely. By the time a temperature warning light comes on a car is severely overheated. I even knew one guy with a "oil user" car who knew when to add a quart of oil by watching his oil pressure gage go down!
The advantage of the "Idiot light" is obvious: it is hard to keep track of a lot of little white gage needles on the dash while driving. People who don't know anything about cars can be confused by gages or will ignore them.
But ANY IDIOT will notice a bright red light on the dashboard!
Personally I like systems that have gages combined with a single "CHECK GAGES" red warning light. I have also seen gages that have a red warning light on the face of the gage. This gives you the best of both worlds.
BASIC GAGE AND LIGHT TYPES
A direct gage connects directly to whatever it measures. Direct temperature gages have a metal tube with a liquid or gas in a bulb at one end and the gage at the other. The bulb is inserted into the water jacket of the motor and the metal line is routed through the firewall and under the dash. A direct oil pressure gage has a line of either metal or sometimes plastic running engine oil pressure from the engine oil passages through firewall to the gage.
A few direct gages are original equipment on vehicles, especially oil gages on trucks. Most direct gages are aftermarket custom installations.
ADVANTAGES AND DISADVANTAGES OF DIRECT GAGES
A direct gage gives an instant and reliable reading. This is important especially in an oil gage. Many electric gages have a delay between a change and the gage registering the change. This was especially a problem with the older thermo-electric gages. An electric gage can give a bad reading if you have a bad battery or a charging system problem. An electric gage, like the temperature gage, only works when the vehicle is running or the key in the "run" position.
Direct gages have the edge on accuracy and reaction speed, but they are a pain to install. the oil gages they sell today come with a tiny plastic tube which you connect to your engine oil pressure journals somewhere. Trouble is, this little tube has to thread its way past hot exhaust parts and a lot of sharp sheet metal on its way under the dash. One little slip against that hot exhaust, or a one cut from sharp metal under the hood or dash, and it can leak all the oil out in seconds! (If it gets cut under the dash, it can cover your feet in oil too! You'll be walking with hot oil in your shoes!) The temperature gages have a somewhat fragile copper line usually wrapped with a steel coil with a thermal bulb on one end. once again routing can be difficult. if you break the copper line from the engine to the gage you have to replace everything: the whole gage assembly.
SPEEDOMETERSMost newer vehicles have electronic speedometers, but until the early 90's most cars had mechanical speedometers. With a direct mechanical speedometer a cable runs from the transmission or front wheel (Old VW Beetle) to the speedometer unit mounted in the dash, often called the "speedometer head " . A shaft inside the speedometer unit attaches to the cable on one end and has a magnet attached to the other. As the cable spins, the magnet spins inside a cup attached to the speedometer needle. A spring pulls the needle towards zero. As the cable spins the magnet pulls the cup around with it and moves the needle over to indicate speed. A gear on the shaft in the speedometer unit turns the odometer, measuring miles traveled.
Electric gages are by far the most common original equipment on vehicles today. An electric gage just requires a wire or two going to a sensor and connected to the dashboard. Although sensors can leak, you aren't piping oil pressure all over your car like with a direct oil pressure gage. Modern electric gages have a pretty good response time , so it isn't as big of an issue as with older thermo-electric gages.
TYPES OF ELECTRIC SENSORS, or SENDING UNITS
Most all electric gages get a varying ground provided from a sensor or sending unit. A low constant voltage is delivered to the gages via a VOLTAGE LIMITER, which is normally part of the dashboard instrument cluster and speedometer assembly, or incorporated into the fuel gage. The sensors or sending units operate the gage or light by providing a circuit to ground.
The sensors which operate electronic gages or warning lights are sometimes called "sending units". The sending units for gages and lights are different, even though either sensor will fit on the same engine. A sending unit is usually either a variable resistor or a switch to ground. It gets its ground from the engine block where it is screwed in.
A light sending unit provides its wire with either an open circuit or a closed circuit to ground: it's just a switch. The gage sending unit feeds a varying resistance to ground back to its gage.
in any case, most all sensors operate by providing circuit to ground, either a variable resistance circuit or an "on/off" circuit to ground.
One exception is an electronic speedometer sensor, usually called the vehicle speed sensor.. It has a magnet inside a wire coil and is usually linked by a gear to the output shaft of the transmission or transaxle. As the magnet spins faster, more current is produced in the wire coil. This current goes to the speedometer, where an electronic circuit moves the needle. On some vehicles this sensor provides data for the engine control computer , transmission control computer , and electronic cruise control. A digital speedometer uses the same sensor, but a computer takes the electric signal from the speed sensor, converts it to digital, and displays it on the dash.
ELECTRIC GAGE TYPES
THERMO-ELECTRIC GAGES The earlier electric gages were thermo-electric. These gages had a bi-metallic strip connected to the needle. A bi-metallic strip bends back and forth when heated or cooled. A thermo-electric gage has an electric coil which heats this bi-metallic strip, thus moving the gage needle. The problem with thermo-electric gages is that it takes a bit of time for the bi-metallic strip to heat up and move, so there is a slight time delay in the readings from these gages.
BALANCED COIL GAGES
More modern gages are balanced coil type, much like an analog voltmeter dial. A magnet is deflected by a coil, moving the needle according to the current. These gages react almost instantly so the only delay in reading is the time taken for the sensor or sending unit to react.
REPAIR, TEST GAGES AND LIGHTS
It helps to have 2 people to do this test:
Since most gages and operate by grounding the gage or light usually a test and repair is easy, needing no special tools. Disconnect the wire from the sensor or sending unit. Have an assistant BRIEFLY touch the disconnected wire to a good ground. A bent paper clip works well for "probing" an insulated terminal.
If it is a gage, it should "peg", or go all the way to one side or the other. If it is a light, it should turn on when grounded.
If the gage or light responds in this way, then your gage or light problem is probably from a bad sensor or sending unit. Repair as needed.
EXCEPTIONS TO THIS TEST
This test DOES NOT APPLY to the charging system light or ammeter or voltmeter. It also is not meant for any computer sensor or for any light powered by a computer, such as the "CHECK ENGINE" light, Air Bag Warning light, or ABS (Anti-lock brake) warning light.
Click here for some images of car gages
So is it GAGE or GAUGE?
You might not believe it, especially when I spend hours on this site giving away a lifetime of auto repair knowledge for free, but sometimes I'll get a downright nasty e-mail concerning really trivial things like spelling and grammar. So I address this issue in advance: Both spellings are acceptable, gage and gauge. Gauge has been acceptable longer, BECAUSE IT'S BRITISH SPELLING! Gage is newer and more correct phonetically, "gage" is the spelling used most often in technical literature, it's one letter shorter, AND IT'S AN AMERICAN SPELLING!.
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Auto, Car, and Truck Article List
ABS: Anti-Lock Brake Systems
ADVANCE: Car ignition timing
ALTERNATORS and Car Battery
BAD CAR DESIGNS
Bad Drivers: How NOT to drive
BATTERIES: Auto, Car or Truck
BELTS AND HOSES
BODY AND BUMPER REPAIRS
BRAKE REPAIRS: Car or Truck
Car Washing and Care
CARBURETORS:Car & Truck
CHECK ENGINE LIGHT
CLEANING: Engine Cleaning
CLUTCH REPAIRS: Car & Truck
COMPRESSION: Car Engine
COMPUTER CAR CONTROLS
CV JOINT OR CV AXLES
ELECTRIC WIRING REPAIR
ENGINES: Car & Truck
FILTERS: OIL, AIR, ETC.
FUEL AIR MIXTURE
FUEL INJECTION: Car & Truck
FUEL PUMPS: Car & Truck
GAGES AND "IDIOT LIGHTS"
GASKETS AND SEALS
GLASS: WINDOWS AND WINDSHIELDS
HEADS & HEAD GASKET
HOSES AND BELTS
"IDIOT LIGHTS" AND GAGES
IGNITION TIMING: Car & Truck
AUTO JACKS: lifting cars safely
LEAN "Car runs lean"
LIGHTS: WARNING OR "IDIOT LIGHTS"
Limp Home Mode
NO START: Car Won't Start
OIL: What's right for your car?
OIL LIGHT ON OR GAGE LOW
RADIATORS: Car and Truck
RICH: Car runs rich
SEALS AND GASKETS
SERVICE ENGINE SOON LIGHT
STARTERS: Auto, Truck
TIMING: IGNITION TIMING
TIMING BELT & TIMING CHAIN
WARNING LIGHTS OR "IDIOT LIGHTS"
Car Washing and Care
WATER PUMP REPAIR
WINDOWS AND WINDSHIELDS
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