It's logical Captain!

Component testing hints and tips from James Dillon

Published:  27 January, 2014

By James Dillon

When troubleshooting systems it is useful to remember that there are only three faults that affect wiring:

Much of the troubleshooting information made available to technicians provides resistance values for various system components. I question whether these are given because it's easy to do so, rather than this being the right thing to do. Now for a rather sweeping statement: If you are mainly measuring resistance values during your component checks then you are going to misdiagnose a lot of components. I can say this confidently because of the following:

Resistance checks require you to break the circuit to see if it is broken. In short, resistance tests can only be carried out when the component or wiring is not passing current. Have you ever known a car to break whilst it's not working?

Input, logic, output - all computer-based control systems operate using this methodology. Data is received from the sensors by the computer, it carries out logical evaluation (its program), it calculates an output based on the logic and it commands an actuator to do something - simple! Therefore, if we prove the input is correct, yet the system doesn't function correctly, we can suspect either the logic (the computer) or the output (the actuator). By accurately testing inputs and outputs we can diagnose the root cause of the system failure correctly. However, some faults are difficult to diagnose, even for the more skilled diagnostic technician but the fact remains that if there is a fault, there has to be a cause. If you can't find the cause, it's not because it doesn't exist, it is due to the fact that you're not looking in the right way (with the right tool), in the right area or under the right conditions.

The best advice I heard concerning system testing and misdiagnosis is this: So you 'think' that you have diagnosed the fault correctly but before you push the button and order a new component, think about what will happen if the new component doesn't fix the fault. What is the first test you'll do after fitting the component that doesn't cure the fault? Simply, perform this test before you order the new component. This will help prevent misdiagnosis.

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