Voltage drop circuit testing –Part One

By Des Davies AAE MIMI, Top Gear Motor Services

Testing electrical circuits and components can be difficult to comprehend but it's like everything we learn on the vehicle, understanding the basics of electrical circuits can only help us to test and locate problems within these systems. It’s not easy and can be time-consuming; most technicians I have worked with would rather repair mechanical problems than delve into electrical diagnostics. You cannot blame them, it can take hours or even days to locate a fault in the electrical system, whereas mechanical problems tend to be, but not always, easier to diagnose. Here, I’ve tried to explain the fundamentals as simply as possible. To test and diagnose electrical circuits, you will need:

• Access to the manufacturer’s schematic diagrams and technical information are essential

• When testing vehicle circuit faults and components you need a good digital multimeter to diagnose and test faulty circuits and components

• Do not use an old analogue voltmeter as these will draw too much current when working on modern cars, especially with systems using electronics and computers, they will fry them!

• The multimeter is your best friend when diagnosing electrical circuit faults, you do not need to purchase fancy or expensive lab scopes or meters, a good quality basic multimeter with 10MΩ of resistance is all that is required. If you need to test electric/hybrid vehicles as well, you need a higher spec Cat III 600V for safety.

When voltage testing you must always connect the black meter lead to the common port connection, which is the earth/ground lead on your multimeter, and the red lead to the Volts/Ohms ports. For these tests we are going to use the 20V range scale on the multimeter, because there is no 12V scale available on most meters.

The circuit must be on or loaded e.g., if you’re testing the horn circuit press the horn switch, if you’re testing the brake switch press the brake pedal to check the circuit. If possible, always connect the multimeter black lead to the battery earth/ground minus terminal for a good ground connection, for accuracy. Sometimes this will be difficult as the battery location could be in the boot area or another awkward location. If there is no positive connection under the bonnet area, make up some extension leads to connect to the battery if it is located away from the area you are testing.

Check your ground lead with a known good feed circuit e.g., battery positive, the OBD pin 16 connector when testing inside the vehicle, or fuse box live connector. When testing an electrical circuit with the engine running, remember that now the power source is the alternator/generator and not the battery voltage which is 12.6V, and this generated voltage can be between 13.6V to around 15V or more, depending on manufacturer specifications.

It's essential we understand the fundamentals and the relationship between Voltage, Current and Resistance:

Volts = Current multiplied by Resistance V = I * R

Current (I) = Volts divided by Resistance I = V / R

Resistance = Volts divided by Current R = V / I

We also need a good understanding of Ohm’s law (current in a circuit is equal to the potential difference divided by the resistance of the circuit).

Rules when using voltmeters to test circuits

Rule 1: A digital voltmeter displays the difference in voltage between where you place the black probe and to where you place the red probe.

Rule 2: The voltage after the last resistance in a circuit will always be zero (providing current can flow)

Rule 3: Volt drop will only occur across a resistance if current can flow.

Rule 4: The Volt drop across a resistance in a series circuit is in direct proportion to the comparative resistance values. Voltage drop across all the resistors in that circuit must add up to the source voltage. If you measure a 12V drop across the battery terminals, the 1 or more resistors in that circuit should all individually add up to the 12V system voltage. So, within a 3-resistor circuit, if they have all equal resistance then the voltage drop across the individual resistors will be 4V each, adding up to 12V system voltage. This is Ohm’s law.

* If the supply voltage is the same voltage to the component as the battery voltage, for example, both are displaying 12.6V on the multimeter, then you have an open circuit, no voltage drop and therefore no current flow.

* If current is flowing in the circuit then there must be a voltage drop across the resistors/components

* If the current is flowing in the circuit then it must be taking something out of the power source or battery, therefore the voltage must drop in the circuit. It cannot be the same voltage as the battery supply, if it is, no current is flowing in that circuit and it will not operate correctly

* If the voltage reading on the ground/earth connector of a wire is around 0V and fluctuating up and down you may have an open circuit, or you are not connected to that wire or part of the circuit you are testing. Shake the leads of the multimeter until you get a stable reading, this then indicates that you are now connected to a good source or point of that circuit, a good ground.

What does a voltmeter do? The voltmeter checks the reading at the negative point where you place the black meter lead in a circuit, and the positive point where you place the other second red meter lead in the circuit. The meter will display a voltage reading if there is a difference in that circuit between the red lead and the black leads. Think of the ground/ earth circuit as a low-pressure circuit and the live/power circuit as a high-pressure circuit, so when you check between these circuits with a meter/gauge you will get a difference in pressure between the low pressure and high pressure. If there is a difference in pressure it will display a reading on your meter/gauge.

So now back to the voltmeter readings, in between the two multimeter leads that we have placed is the part of the circuit we are testing for anomalies or faults. If I place my meter leads across the battery it will read a battery voltage of say 12V, because I am probing one side which is 0V (low pressure) ground/earth/negative post, and the other side of the battery which is 12V (high pressure) positive post/live, this is because there is a difference between 12V and 0V, hence the display of 12V on my multimeter.

Therefore, as an example, if I place the red lead on the battery positive terminal and the negative lead at the component live side or input, then I am checking for a fault between the battery live side and the component input. If there is no difference in the reading, to where we placed the leads in that circuit between the red and the black leads, then that part of the circuit must be good and the multimeter will display 0V.

If there is a problem in the circuit a difference in readings of where we have placed the two leads of the multimeter, it will display a voltage reading and this means that this part of the circuit has a problem (if the reading is above 0.5V on a 12V system).

If we test this circuit and have a reading on the multimeter, for example a 3V voltage drop, then we need to back track, towards the battery post/power source with our negative black lead and probe the cable back along the circuit until we get a reading of 0V (0.5V max), and the fault will lie between these two readings in that circuit (poor connection, corrosion or damaged cable, spread pins etc.)

In summary:

When we probe the circuit with the negative lead (black), to where we probe the positive lead (red) in the same circuit, in between these two probed leads is the part of the circuit that we are testing. If there is no difference in that part of the circuit, we should have 0V (max0.5V) on the multimeter, which indicates that that part of the circuit is good, if there is a reading on the multimeter e.g., 2V, then there is a difference, and that part of the circuit has the fault, and we need to investigate further.