How To Checking The Electrics With A Multimeter

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Modern cars have a large amount of electrical equipment which may fail then need checking. One way of checking the electrical circuits is to use an easy test lamp connected between the circuit live wires and earth, but this method only indicates if there’s an electrical supply to the particular point you’re checking.

A more accurate way of checking circuits is to use a test meter which can indicate the extent of voltage reaching the component and also check the resistance of the circuit or component.

Multi-meters

You can buy meters designed especially for car applications at accessory shops. the most useful type are those referred to as multi-meters which, as the name suggests, provide a number of different functions for checking car electrics. The current used on cars is direct current (DC) and multi-meters can check current, voltage and resistance readings. they’ll also include other settings for measuring engine speed and dwell angle.

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 otherwise 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 see the dwell angle and engine speed. Always remember to attach the meter probe correctly. Check battery voltages by connecting the meter to the two terminal posts. Test resistance within the HT circuit by probing both ends of the lead.

Record alternator or dynamo current output using a meter connected across a shunt wire. Test voltage to the coil or any other circuit by connecting one side of the meter to the circuit and the other side to earth.

When using a multi-meter the primary thing is to form sure that you connect the meter leads the right way round. this relies on the polarity of your car. If your car uses a negative earth system you ought to attach the lead marked negative or (-) to the body. If your car uses a positive earth, the lead marked positive or (+) is connected to the car body. sign up your car handbook for the polarity of your car.

Make sure that the appropriate lead makes good contact which there’s no rust or paint within the contact area to upset the meter readings. Clean the connection if necessary with wet-or-dry paper. When working within the engine bay it’s best to connect the lead to the battery earth terminal.

Battery check

Before checking other circuits it’s a good idea to make a check on the battery itself to form sure it’s performing properly. Set the meter to the appropriate scale (0-20 volts ), then connect the meter leads across the battery terminals (not the lead connections). you ought to get a reading of between 11 volts (low charge ) 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 cells . 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’s a poor contact between the world lead and therefore the car body or battery terminal.

Repeat the method , this point connecting one meter lead to the earth terminal and therefore the other to the live lead connection at the starter solenoid . a low reading here indicates a poor connection between the live battery terminal and therefore the starter solenoid.

If any low readings are found on the battery leads, correct them now before checking other circuits on the car. pack 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’ll then use the reading as a reference for readings made on the other circuits.

Instrument tests

Many instruments are supplied with power that passes through a voltage stabilizer. If several instruments are showing erratic readings it might be due to a fault within the stabilizer. To see the voltage stabilizer, connect the meter to the stabilizer output terminal and turn on the ignition. The meter should read around 10 volts although it might pulsate slightly because of the regulator. Any lower or higher means the stabilizer needs replacing.

Sender unit check

The petrol tank sender unit uses a variable resistor , which you’ll 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 form 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’s likely that a number of the resistor tracks have shorted out, giving false readings on the gauge.

Ignition tests

When checking the low-tension circuit remember the contact breaker points have to be closed to finish the circuit. If the coil uses a ballast resistor the voltage at the input terminal will be lower (usually around 6 to 8 volts) thanks to the action of the resistor. to see the coil starting voltage, connect a lead between the coil points terminal and earth. Operate the starter briefly to bypass the ballast . the worth should be around 12 volts. Remove the lead.

If the value didn’t rise to this level there’s a fault within the low-tension circuit or the solenoid terminal connections. You can check the points by measuring any voltage drop 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 within 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 due to a fault with the distributor and you’ll check this by disconnecting the distributor lead. If the reading is still zero there’s a fault within the coil, but if it rises there’s a fault with the distributor.