Don't be caught short

Gary Lovett from Snap-on Diagnostics runs through some fault-finding basics

Published:  15 March, 2013

Finding intermittent problems can be difficult because the problem comes and goes. Wiggling and shaking wires or blowing them with a hot air gun may be necessary to simulate the conditions that cause the fault to occur. It is also possible to identify circuit problems on a scan tool with graph capability to narrow it down to a particular component and then move into test mode.

A voltmeter is the simplest and easiest tool but requires the user to understand the tests and the readings achieved. To find a short circuit in the 'old school' way, connect a 21W bulb in place of the blown fuse.... it will light up. Now wiggle test the wiring or disconnect components on that circuit waiting for the bulb to go out. A 12V buzzer can also be used for the same test.

Open circuits: A wiring diagram and patience is needed as you follow the circuit from source towards the component. Short/open circuit locators: This tool sends a frequency tone down the affected circuit. You can then follow the tone down the loom with a hand-held receiver listening for the tone to change.

Here's a working example:  A vehicle reads 'P0110 intake temp sensor voltage high' and 'P0100 mass airflow sensor voltage high'. When an open circuit occurs, the voltage being measured at the ECU is higher than expected. A wiggle test was done with the Snap-on Solus in graph mode (see visual). It confirmed that both outputs were affected so either both circuits were faulty or it was a supply or earth fault. The change in data occurred when the ECU connector was wiggled. Further checks with a multimeter using volt drop proved the ECU had failed internally. This was why simply replacing the airflow mass meter would not work.

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