The minimum number of gauges on a passenger car dashboard are the speedometer and the
fuel gauge. The most common additional gauge is the temperature gauge followed by the
tachometer, voltmeter and oil pressure gauge. If your car does not have a temperature
gauge, oil pressure gauge or charging system gauge, then you will have a warning light for
these functions.
The most common configuration in today's family car is: Speedometer, Tachometer, Fuel
& Temperature.
Typical instrument panel (this one is from a 2004 Ford
Taurus)
Note: To find out more about the gauges
on your car, the best source of information is your owner's manual.
Speedometer In the past, the most used of the gauges. The speedometer was usually driven by
a cable that spins inside a flexible tube. The cable is connected on one
side to the speedometer, and on the other side to the speedometer gear inside
the transmission. Today, just about all vehicles have eliminated the cable and use
an electronic sensor to measure wheel speed and send the signal to an
electronically driven speedometer.
The accuracy of the speedometer can
be affected by the size of the tires. If the tires are larger in diameter than
original equipment, the speedometer will read that you are going slower then
you actually are. On older vehicles, another cause for inaccurate speed readings
was an
improper speedometer gear inside the transmission. This can sometimes
happen after a replacement transmission has been installed. Most good
transmission shops are aware of this and will make sure that the correct
speedometer gear is in the new transmission.
On vehicles with electronic speedometers, the computer has settings to
for speedometer calibration when necessary, to allow a technician to adjust
for different sized tires. These calibrations usually require
specialized equipment like diagnostic scanners to do these types of
adjustments.
Fuel Gauge
Deliberately designed to be
inaccurate! After you fill up the tank, the gauge will stay on full for a long time, then
slowly drop until it reads 3/4 full. After that, it moves progressively faster until the
last quarter of a tank seems to go very quickly. This is a bit of psychological
slight-of-hand to give the impression that the car gets better gas mileage then it does,
it seems to reduce the number of complaints from new car buyers during the first few weeks
after they bought the car.
The fuel gauge shown here is probably more accurate than most. Notice the difference
between 3/4 to full and empty to 1/4.
When the needle drops below E, there is usually 1 or 2 gallons left in reserve. To find
out for sure, pull out your owners manual and find out how many gallons of gas your tank
holds, then the next time you fill up an empty tank, check how many gallons it took to
fill it. The difference is your reserve. Note: It is not a good idea
to let your tank drop below 1/4. This is because your fuel pump is
submerged in fuel at the bottom of the tank. The liquid fuel helps to
keep the fuel pump cool. If the fuel level goes too low and uncovers the
pump, the pump will run hotter than normal. If you do this often enough,
it can shorten the life of the fuel pump and eventually cause it to fail.
Temperature Gauge or warning lamp This gauge measures
the temperature of the engine coolant in degrees. When you first start the car, the gauge
will read cold. If you turn the heater on when the engine is cold, it will blow cold air.
When the gauge starts moving away from cold, you can then turn the heater on and get warm
air.
Most temperature gauges do not show degrees like the one pictured here. Instead they will
read cold, hot, and have a normal range as pictured in the dash panel at the
top of this page.
It is very important to monitor the temperature gauge to be sure that your engine is not
overheating. If you notice that the gauge is reading much hotter than it usually is and
the outside temperature is not unusually hot, have the cooling system checked as soon as
possible. Note: If the temperature gauge moves all the way
to hot, or if the temperature warning light comes on, the engine is overheating! Safely pull off the road and turn the engine off and
let it cool. An overheating engine can quickly cause serious engine damage!
Tachometer The tachometer measures how fast the engine is turning in RPM (Revolutions
Per
Minute). This information is useful if your car has a standard shift transmission and you
want to shift at the optimum RPM for best fuel economy or best acceleration. One of the
least used gauges on a car with an automatic transmission. You should never race your
engine so fast that the tach moves into the red zone as this can cause engine damage. Some
engines are protected by the engine computer from going into the red zone.
Usually, the tachometer shows single digit markings like 1, 2, 3 etc.
Somewhere, you will also see an indicator that says RPM x 1000.
This means that you multiply the reading by 1000 to get the actual RPM, so if
the needle is pointing to 2, the engine is running at 2000 RPM.
Oil Pressure Gauge or warning lamp Measures engine oil pressure in pounds per square inch. Oil pressure is just as
important to an engine as blood pressure is to a person. If
you run an engine with no oil pressure even for less then a minute, you can easily destroy
it. Most cars have an oil lamp that lights when oil pressure is
dangerously low. If it comes on while you're driving, stop the vehicle as soon as is
safely possible and shut off the engine. Then, check the oil level and add oil as
necessary.
Charging system gauge or warning lamp The charging system is what provides the electrical current for your vehicle.
Without a charging system, your battery will soon be depleted and your vehicle will shut
down. The charging system gauge or warning lamp monitors the health of this system so that
you have a warning of a problem before you get stuck.
When a charging problem is indicated, you can still drive a short distance to find help
unlike an oil pressure or coolant temperature problem which can cause serious engine
damage if you continue to drive. The worst that can happen is that you get stuck in a bad
location.
A charging system warning lamp is a poor indicator of problems in that there are many
charging problems that it will not recognize. If it does light while you are driving, it
usually means the charging system is not working at all. The most common cause is a broken
alternator belt.
There are two types of gauges used to monitor charging systems: a voltmeter which measures
system voltage and an ammeter which measures amperage going out of, or
coming into the battery. Most modern cars that have gauges
use a voltmeter because it is a much better indicator of charging system health. A
voltmeter is usually the first tool a technician uses when checking out a charging system
A modern
automobile has a 12 volt electrical system. A fully charged battery will read about 12.5
volts when the engine is not running. When the engine is running, the charging system
takes over so that the voltmeter will read 14 to 14.5 volts and should stay there unless
there is a heavy load on the electrical system such as wipers, lights, heater and rear
defogger all operating together while the engine is idling at which time the voltage may
drop. If the voltage drops below 12.5, it means that the battery is providing some of the
current. You may notice that your dash lights dim at this point. If this happens for an
extended period, the battery will run down and may not have enough of a charge to start
the car after shutting it off. This should never happen with a healthy charging system
because as soon as you step on the gas, the charging system will recharge the battery. If
the voltage is constantly below 14 volts, you should have the system checked. If the
voltage ever goes above 15 volts, there is a problem with the voltage regulator. Have the
system checked as soon as possible as this "overcharging" condition can cause
damage to your electrical system.
If you think of
electricity as water, voltage is like water pressure, whereas amperage is like the volume
of water. If you increase pressure, then more water will flow through a given size pipe,
but if you increase the size of the pipe, more water will flow at a lower pressure. An
ammeter will read from a negative amperage when the battery is providing most of the
current thereby depleting itself, to a positive amperage if most of the current is coming
from the charging system. If the battery is fully charged and there is minimal electrical
demand, then the ammeter should read close to zero, but should always be on the positive
side of zero. It is normal for the ammeter to read a high positive amperage in order to
recharge the battery after starting, but it should taper off in a few minutes. If it
continues to read more than 10 or 20 amps even though the lights, wipers and other
electrical devices are turned off, you may have a weak battery and should have it checked.
Notice: The information on this site is
not intended as a substitute for the advice of a professional who is
qualified to examine, diagnose and repair your vehicle.
Deliberately designed to be
inaccurate! After you fill up the tank, the gauge will stay on full for a long time, then
slowly drop until it reads 3/4 full. After that, it moves progressively faster until the
last quarter of a tank seems to go very quickly. This is a bit of psychological
slight-of-hand to give the impression that the car gets better gas mileage then it does,
it seems to reduce the number of complaints from new car buyers during the first few weeks
after they bought the car.
This gauge measures
the temperature of the engine coolant in degrees. When you first start the car, the gauge
will read cold. If you turn the heater on when the engine is cold, it will blow cold air.
When the gauge starts moving away from cold, you can then turn the heater on and get warm
air.
A modern
automobile has a 12 volt electrical system. A fully charged battery will read about 12.5
volts when the engine is not running. When the engine is running, the charging system
takes over so that the voltmeter will read 14 to 14.5 volts and should stay there unless
there is a heavy load on the electrical system such as wipers, lights, heater and rear
defogger all operating together while the engine is idling at which time the voltage may
drop. If the voltage drops below 12.5, it means that the battery is providing some of the
current. You may notice that your dash lights dim at this point. If this happens for an
extended period, the battery will run down and may not have enough of a charge to start
the car after shutting it off. This should never happen with a healthy charging system
because as soon as you step on the gas, the charging system will recharge the battery. If
the voltage is constantly below 14 volts, you should have the system checked. If the
voltage ever goes above 15 volts, there is a problem with the voltage regulator. Have the
system checked as soon as possible as this "overcharging" condition can cause
damage to your electrical system.
If you think of
electricity as water, voltage is like water pressure, whereas amperage is like the volume
of water. If you increase pressure, then more water will flow through a given size pipe,
but if you increase the size of the pipe, more water will flow at a lower pressure. An
ammeter will read from a negative amperage when the battery is providing most of the
current thereby depleting itself, to a positive amperage if most of the current is coming
from the charging system. If the battery is fully charged and there is minimal electrical
demand, then the ammeter should read close to zero, but should always be on the positive
side of zero. It is normal for the ammeter to read a high positive amperage in order to
recharge the battery after starting, but it should taper off in a few minutes. If it
continues to read more than 10 or 20 amps even though the lights, wipers and other
electrical devices are turned off, you may have a weak battery and should have it checked.