Modern cars have a large amount of electrical equipment which can go wrong
and so need checking.
One way of checking the electrical
is to use a simple test lamp
connected between the circuit live wires and earth, but this method only
indicates if there is an electrical supply to the particular point you are
A more accurate way of checking circuits is to use a test
indicate the level of voltage reaching the component and also check the
of the circuit or component.
You can buy meters designed especially for car applications at accessory
shops. The most useful type are those known as multi-meters which, as the name
suggests, provide a number of different functions for checking car
used on cars is
(DC) and multi-meters can check
current, voltage and resistance readings. They may also include other settings
Always remember to zero the meter before each test, particularly when
measuring low resistances.
Do not use a moving needle test meter for checking electronic components
or you could overload and damage them. Instead, use a digital meter.
Using a multimeter
You can use a multi-meter for checking voltage, current and resistance. Some also allow you to check the dwell angle and engine speed. Always remember to connect the meter probe correctly.
by connecting the meter to the two
posts. Test resistance in the HT circuit by probing both ends of the lead.
or dynamo current output using a meter connected across a
wire. Test voltage to the
or any other circuit by connecting one side of the meter to the circuit and the other side to earth.
Taking a reading
When using a multi-meter the first thing is to make sure that you connect
the meter leads the right way round. This depends on the
of your car.
If your car uses a negative earth system you should attach the lead marked
negative or (-) to the body. If your car uses a
earth, the lead marked
positive or (+) is connected to the car body. Check in your car handbook for
the polarity of your car.
Make sure that the appropriate lead makes good contact and that there is no
rust or paint in the contact area to
the meter readings. Clean the
connection if necessary with wet-or-dry paper.
When working in the engine bay it is best to connect the lead to the
Before checking other circuits it is a good idea to make a check on the
battery itself to make sure it is performing properly.
Set the meter to the appropriate
), then connect the meter
leads across the battery terminals (not the lead connections). You should get a
reading of between 11 volts (low
) and slightly over 12 volts (full
charge), depending on the battery's state of charge.
If the reading is less than 10 volts, suspect a fault in one of the battery
. Move the earth lead to a point on the car body and take the voltage
reading again. It should be the same as the first reading. A lower reading
means there is a poor contact between the earth lead and the car body or
Repeat the process, this time connecting one meter lead to the earth
terminal and the other to the live lead connection at the
low reading here indicates a poor connection between the live battery terminal
and the starter
If any low readings are found on the battery leads, correct them now before
checking other circuits on the car. Clean up any suspected dirty or loose
connections and test the voltages again. When the tests all give the same
reading as that across the battery you can then use the reading as a reference
for readings made on the other circuits.
Many instruments are supplied with power that passes through a voltage
stabilizer. If several instruments are showing erratic readings it could be due
to a fault in the stabilizer. To check the voltage stabilizer, connect the
meter to the stabilizer output terminal and
The meter should read around 10 volts although it might pulsate slightly
because of the regulator. Any lower or higher means the stabilizer needs
Sender unit check
The petrol tank sender unit uses a variable
, which you can check
for continuity by using the resistance scale on the multi-meter.
Disconnect the wire to the sender unit and connect the meter between its
terminal and a suitable earth point. If the sender unit circuit is complete,
there should be a definite reading on the meter. To make a complete check make
individual readings with the tank full, half full and empty.
The three readings should fall in progression with more or less equal gaps
between them. If two of the readings are very close together it is likely that
some of the resistor tracks have shorted out, giving false readings on the
When checking the low-tension circuit remember the contact
have to be closed to complete the circuit.
If the coil uses a
the voltage at the input terminal will
be lower (usually around 6 to 8 volts) due to the action of the resistor. To
check the coil starting voltage, connect a lead between the coil points
terminal and earth. Operate the starter briefly to
the ballast resistor.
The value should be around 12 volts. Remove the lead.
If the value did not rise to this level there is a fault in the low-tension
circuit or the solenoid terminal connections.
You can check the points by measuring any
across them. Connect
the meter between the points terminals on the coil and earth.
With the contacts closed, turn the meter to read on the low volt scale. The
reading should ideally be in the zero to 0.5 volts range. Any more than 0.5
volts indicates the points are faulty.
Turn the meter to the high volt scale and open the points. The voltage
should be the same as that on the input side of the coil.
A zero reading may be because of a fault with the
and you can
check this by disconnecting the distributor lead. If the reading is still zero
there is a fault in the coil, but if it rises there is a fault with the
You can use the resistance setting of the meter to check for problems with
high-tension leads. If the car has an
misfire, you may be able to
trace it to one of the leads.
Find out which type of lead is fitted.
leads have a resistance in the
range. Copper-cored leads have a very low resistance but may
be fitted with resistive plug caps for radio suppression and these have a
resistance of around 10,000 ohms.
Disconnect each lead from the
and hold the
meter lead probes to the central core at each end. Make sure the meter reads
To check the HT lead
hold one probe to the central core of the
lead and the other to the insulating plastic. If the lead is in good condition
there should be no movement on the meter scale.
Measuring the current output from a dynamo or alternator is more difficult
with a multi-meter because the current levels produced by the
great to be directly handled by most meters.
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If your car's instrument panel includes an
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