Autotechnician magazine October 24

Voltage drop circuit testing – Part Four

By Des Davies AAE MIMI, Top Gear Motor Services

This is the final article on voltage drop circuit testing, following on from the fundamentals of the circuit testing method, explained in Part One, Two and Three

My previous articles covered the basics and rules of these circuits and components. This example demonstrates a test to identify and locate a fault in the positive/live side of the circuit of a 2 resistor/bulb circuit.

Connect the multimeter across the 2 resistor/bulbs in the circuit, it should read battery voltage of 12.6V. The left side multimeter M1 indicates a 10V drop across the circuit, which should display a reading of 12.6V on this multimeter. We must have an unwanted resistance somewhere in the circuit, which could be in the positive/live side or the ground/earth side.

Place the red lead of your multimeter on the battery positive terminal and the black lead to the input terminal of the first resistor/bulb. If the multimeter M2 now reads 2.60V as in Figure 1, we must have a voltage drop somewhere in this part of the circuit.

Move your black lead backwards towards the battery at different points in this part of the circuit until your multimeter displays a maximum reading of 0.5V, in between where you previously had the reading of 2.60V and the reading of 0.5V max is the point where your high resistance or voltage drop is in the circuit.

Figure 2 shows how you check for the unwanted resistance in the circuit by placing the red multimeter lead on the battery positive terminal and leaving it connected to this terminal, whilst then placing the black lead at various points in the cable that goes from the battery positive to the input of the first resistor/bulb. Meter M1 now reads 2.60V, indicating a resistance before meter M1.

Move the black lead only and back track towards the battery positive terminal taking your multimeter readings. Meter M2 reads 2.60V, therefore the resistance is still in that part of the circuit, before this meter reading. Again, move the black lead only, and back track towards the battery positive terminal taking your next multimeter reading. Keep back tracking with the black lead until the meter now reads 0V (0.5V max) (M3).

Where the meter now displays 0V (0.5V max) (M3) and the previous meter reading of 2.60V (M2), in between these 2 readings is the unwanted resistance in that cable.

0V reading on positive side of the load indicates an open circuit in the live side of the circuit.

Note: A 0V reading on your meter can also indicate a short circuit or very high resistance in that circuit and these will not display a reading on the multimeter so be aware of this!

Again, back track with your red lead back to the positive battery terminal at different points until your multimeter reads 12.6V and in between these 2 readings of 0.00V and 12.6V, will be the open circuit in that location of the positive cable.

A reading of 12.60V on your multimeter indicates an open circuit in the ground circuit, no current flow and therefore it is not completing the circuit.

Remember wherever the voltage or current comes from, it must return to that source, to complete the circuit and function correctly.

Backtrack moving your red lead back towards the battery negative terminal until the reading on your multimeter displays a maximum of 0.5V, in between the readings of 12.60V and the 0.5V maximum is the fault in this circuit.

Three resistors in a series circuit

Let's now look at an electrical circuit with three resistors and test using a basic digital multimeter, Figure 7 overleaf. It does not matter how many resistors/bulbs you have in the circuit; they all must add up to battery/system voltage if the system is operating correctly. which is 12.6V.

This is a 3 bulb/resistor circuit, which is tested in the same way as the other series circuits. Remember they all share the total voltage in the circuit.

Don't forget the 4 important rules when using a multimeter to test any electrical circuit or components: Current is the same in all parts of the circuit; the applied voltage equals the sum of the volt drops around the circuit; the total resistance of the circuit (Rt) equals the sum of the individual resistance values (R1 + R2 + R3…); always check the vehicle battery first for system voltage, a fully charged battery will provide 12.6V.

All the individual resistors/bulbs must add up to battery/ system voltage, Figure 8, in this circuit each resistor/bulb has the same resistance and therefore has a voltage drop of 4.2V, adding the 3 together, 4.2V + 4.2V +4.2V = 12.6V, which is the battery/source voltage, this circuit is functioning correctly and to its full potential.

If they do not add up to battery/system voltage, then you must test the circuit using the multimeter to locate the unwanted voltage drop, which could be on the live side of the circuit or the earth/ground side.

With the black lead now connected to battery negative, earth/ground terminal and red lead connected around the circuit of this series circuit, Figure 9, we can test the individual inputs of these bulbs/resistors. The black lead must always be connected to the battery earth/ground terminal for these tests.

Meter M4 tests the input voltage of the first bulb/resistor in this circuit, 12.3V is a good reading. Meter M3 tests the input voltage of the second bulb/resistor in this circuit, 8.1V is a good reading. Meter M2 tests the input voltage of the third bulb/resistor, 4.2V is good. Meter M1 is testing the earth/ ground voltage of this circuit, 0.3V is a good reading.

Remember, if they all have the same resistance values, the voltage drops will be equal, 3 resistors X 4.2V volt drop =12.6V as they all share the system or battery voltage of 12.6V. If they did not have the same resistances, then they should all add up to battery or source voltage.

Having the correct knowledge of how the circuits operate correctly is essential, and having a good reliable source of technical information and the manufacturer’s data and circuit diagrams will help you to diagnose the electrical faults and fix the vehicle. This testing technique can be used on any circuit on the vehicle, but you may have to adapt the testing methods.

The easiest thing to do is fry an ECM or electronic component that costs a lot of money or starts a fire, the hardest thing is to tread carefully and only test what your knowledge and experiences have taught you. Think before you leap, it's a long way down!

Knowledge is power!

Because we cannot visually see the electrons move in a circuit it is more difficult to diagnose and understand. If you are not sure of your test result readings then you must have access to the manufacturer's data to proceed and work safely, test don't guess!

I used to practice by experimenting and testing known good circuits and components. I would do this as often as I could in my own time, to help me develop better skills by making lots of mistakes and learning from them.

I hope that I have helped to make it easier to understand the basic functionality of electrical circuits, and this helps you to better understand basic testing for voltage drops and faults in an electrical circuit, by using the correct equipment. Remember, practice makes perfect!

It is not that difficult, but it can be time consuming. That's life – nothing is easy, but it can also be rewarding!

Your multimeter is your best friend when testing and diagnosing circuit faults, buy a good make and look after it and this will last a very long time. You don't need expensive oscilloscopes to identify basic circuit or component faults.

Good luck with your diagnosing and testing!