Understanding
your dashboard gauges
by Charles Ofria
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
The most used of the gauges. The speedometer is 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. Some newer 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. Another
cause for inaccurate speed readings
is 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 the
new transmission.

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
above.
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. 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.