We are a team of ASE certified mechanics that have created this guide to help
you save money while doing the job yourself, or at least see what you are paying
for when having the job done at a shop. Using a voltmeter is easy to use once you
know how, this device is inexpensive and is available on Amazon starting at $20.00
(US). This guide outlines how a voltmeter is used in automotive repair applications.
What Does it Do?
A voltmeter is used for advanced checking of electrical circuits compared to
the even easier to use
test light which
checks for power and ground. The voltmeter can confirm the amount of voltage present
(either AC or DC), or the ohm's resistance in a circuit or device.
When Do I Need it?
check engine, battery or an
warning light comes on it is telling you an electrical error has occurred. This
is where a voltmeter comes in handy by testing supply voltage, sensor resistance
values, circuit resistance and resistance to circuit ground. These readings help
confirm a problem so repairs can be made.
Let Jump In!
Checking supply voltage, this is used when a system voltage needs to be
tested, let's say the fuel pump is not working and you want to see if you are
getting 12 volts at the pump before you try removing the pump from the tank.
After finding the feed wire color using a wiring diagram, set the voltmeter
on DC voltage. Connect the ground lead of the voltmeter and use the positive
probe to pierce the wire, turn the key to the on position and the voltmeter
will display the system voltage. If wiring diagrams are needed you can
ask our experts to supply you with the information
(free), or consult an online resource such as AllData DIY (pay). After testing
seal the pierced wire with silicone rubber and electrical tape. Advanced voltmeters
will "self detect" AC or DC voltage which we show in the images below.
When an engine will not crank
over one of the first tests is to see if the battery has a sufficient surface
charge. This is easily done using a voltmeter, again set the meter to DC voltage
and connect the negative and positive leads to their respective terminals, the
voltage should be between 12.2 and 12.4 telling you the battery has sufficient
surface voltage. To further test the
battery, a load test need to
be performed, this can be done by cranking the engine over and observing the
voltage, it should stay above 10.4 volts, if the voltage drops off quickly the battery is
down on charge or the battery
is bad and needs to be replaced.
When a battery warning light comes on while driving it means there is a
problem with the charging system. In this case the
alternator output will need
to be tested, this is done by starting the engine and allowing it to idle.
With the voltmeter set to DC voltage, connect the leads to their respective
terminals, the voltage should be between 13.7 and 14.2 telling you the alternator
is working. If battery surface voltage is observed the
alternator is probably bad
or there is a blown fuse or
The ohms setting is handy when testing sensors such as an engine coolant,
oxygen, crankshaft and camshaft sensors. Switch the voltmeter selector to Ohms
to start testing.
Each sensor will have an acceptable Ohms reading which can confirm the sensor’s
performance. This reading can be obtained by asking
one of our experts (free), in this case we are testing a coolant sensor.
This reading is subject to temperature which is how the sensor works. You can
compare the old sensor to the new sensor readings to confirm the failure, the
sensor should be at a similar ambient temperature for accurate results.
When testing coil based items such as a
fuel injector or
ignition coil a voltmeter
comes in handy. With the meter set on Ohms, attach the meter leads to either
side of the terminals, the meter will display the resistance reading of the
coil windings. If the reading is too low it means the internal coil windings
are shorting out. If the readings are too high the coil has an open circuit,
again a new part works well for comparison.
If an engine is not running
a good place to start is to see if the
computer system is
powered up. This is done by checking the system output voltage, remove one of
the ignition coil leads and turn the ignition key on. With the negative lead
grounded, probe the coil wiring connector to monitor the system voltage. If
none is present begin by checking
the fuses and
main power relays.
To test voltage of generation devices such as a
or ABS wheel speed sensor
switch the voltmeter to millivolts, connect the leads directly to the sensor
connector. In the example below we use alligator clips which are supplied with
the voltmeter in most cases. Once connected, spin the reluctor (bearing hub)
or crank the engine over to check for voltage. If no voltage is present the
sensor is bad.
A voltmeter works great for testing the PCM output signals such as ignition
coils or injectors. Connect the voltmeter and switch it to DC voltage, (if the
meter doesn't auto detect). Then crank the engine over, the voltmeter should
swing (pulse) between 0 and 12 volts letting you know the computer is working
and sending signals to the coils and injectors.