Right job, wrong tool?

Even though modern vehicles are advanced, basic principles still apply - and a power probe wrongly used can be very dangerous, as Tony Kitchen explains

Published:  27 February, 2012


Basic principles

So what tools do we need? Long gone are the days when a mechanic could get by using a test lamp. As for power probes, they are always a dangerous tool in the hands of so many who only think they know what they're doing. A good quality digital multimeter and the knowledge to use its various functions effectively, is an absolutely essential starting point. However, these days a multimeter is not enough and other tools are required to assist with diagnosing vehicle electrical systems.

The diagnostic scan tool now plays a vital role in just about every aspect of vehicle servicing and diagnostics work. As already mentioned, the functions of electrical systems are distributed between several control modules. It is often physically impossible to access electrical connectors for testing purposes. Use of a scan tool is the only way to verify the correct functionality of some switches, sensors and actuators through live data parameters and the initiation of actuator tests.

Classic example

Operating principle

So, when it comes to evaluating live data parameters using a scan tool, we can rely upon scan tool live data parameters. Unless the signal is correct, the data will not be plausible. Therefore, we can use the scan tool to quickly eliminate components from the diagnostic process. This saves us time and money by not having to dismantle body panels and other parts of the vehicle to gain access to test electrical components, unless a scan tool live data test result proves absolutely necessary.

Where a live data parameter or failed actuator test identifies a problem, the next step (so often not carried out properly or at all) is to test the component and its associated wiring. Now, with the evidence of the scan tool's tests, we know we are justified in having to dismantle parts of the vehicle to gain access to carry out further testing, using a multimeter or oscilloscope. This is where it is important to have good background knowledge of electrical system operating principles and circuit testing techniques.

Wrong tools

Non-disturbance voltage drop testing is by far the most effective way of testing any electrical circuit and saves time, effort and money if done properly. No piercing of wiring insulation or disconnection of components is necessary either. Understanding the principle of voltage drop, both on the positive and earth return side of a circuit, enables us to differentiate between open circuit and resistive faults as well as test for short circuits. If you know what to look for then you can also use voltage measurements to check for continuity, identify a good reference earth for testing purposes and even establish whether or not current is actually flowing through the circuit. Understanding the difference between good and bad test readings is the key to successful diagnosis.

One other very important factor is that many components receive either a pulse width modulated (PWM) or frequency based voltage supply. This can lead to some spurious and confusing multimeter test results for the unsuspecting technician. Sometimes, an oscilloscope is also necessary for general electrical testing. Examples of circuits controlled in this way include tail lamps, electric window lifters, central locking micro switches and instrumentation illumination control. The principle of operation using frequency and PWM signals is often also combined with a so called voltage divider circuit. This is where resistors of differing Ohmic values are used to enable multi-function switches (window lifters for example) to create several separate signals using just one signal wire.

To successfully diagnose and repair a faulty circuit we need to know how it should work. Determining the correct functionality of a system, particularly on an unfamiliar vehicle, is not always easy. This is where reference to a wiring diagram is essential because this will tell you how the circuit should work, but not directly of course. The technique is to be able to read and understand the diagram. By identifying some basic factors such as live and earth current paths, it's possible to establish the direction of current flow on the page. From here, you can then start to relate what you see on the page to the actual vehicle, thereby turning the information into something you can use to test the circuit effectively. The study of a wiring diagram and available technical data in advance of tackling the problem saves a great deal of time and money later on. There are many factors which influence how we go about diagnosing faults.

The developments of modern vehicle electrical systems place further demands upon the knowledge and expertise of today's vehicle technicians It requires them to be familiar with a wider range of diagnostic tools and fault finding techniques than ever before.

Despite these challenges however, things need not always be as complicated as they seem. One of the most important points when diagnosing an electrical problem is to 'think basic first'. Basic principles still apply and by keeping a cool head and having the confidence to tackle problems in a methodical way, we can get to the bottom of most electrical problems as quickly as possible.

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