Electrical Fault Finding

30-Mar-2013
Article video from the Australian Diesel Mechanic -March-
Electrical Fault Finding

During a recent training event we opened a discussion regarding the peculiar nature of electrical fault finding, It was pointed out that if you ask 100 mechanics how to change rear brake shoes, you will receive 100 answers, all of them pretty much identical. If you ask 100 mechanics how to diagnose a no start fault, you would probably only get 60 answers, and most of which would be a completely different. If you went a step further and compared different manufactures "diagnostic flow chats" you would again see different approaches for effectively, the same components. This is for us, in the shop and in the field, a confusing and intimidating state of affairs.

While the skill of electrical fault finding can not be systematised, the application of your knowledge and experience can be. Below are steps to empower your electrical fault finding processes.

The most important thing to acknowledge when fault finding is that the diagnostic time on electrical faults IS longer, and the repair time IS smaller, you would not say to a guy, 10 minutes into a clutch, why is that taking so long?

  • (1) APPROACH THE FAULT WITH AN OPEN MIND

Avoid arriving at a job with an "idea" of what could be wrong and trying to prove that fault, this more often than not, will only lead you astray.

  • (2) VERIFY THE FAULT

Listen to the customers complaint / reported fault. Are they the same- sometimes you need to read between the lines with the customers description of the fault, the reported fault and the vehicles history

  • Can you demonstrate the fault
  • Is it permanent or intermittent
  • (3) UNDERSTAND THE CIRCUIT

The wiring is on the vehicle, the diagnosis is in your understanding of the circuit

  • You don't diagnose wiring, your diagnose a circuit.
  • You FIX wires, You TEST circuits

Sure you can run over and start testing, but answer me this, what is a better use of your time (and the customers money).

  • Spending $20 online to download a wiring schematic and 30 minutes learning how the circuit functions and 10 minutes of testing and make a "rock solid diagnosis"
  • or
  • Spending 2 hours making tests, trying to figure it out yourself, to make a "pretty sure diagnosis"
  • CHALLENGE ACCEPTED

You can easily find wiring diagrams online for 90% of vehicles, Just remember to check international variants

  • "Holden Rodeo" limits your search to Australia
  • "Chevrolet D-Max" Will expand your search to American variants
  • "Isuzu D-Max" Will expand your search to Asian variants
  • (4) DETERMINE THE RELATED SYMPTOMS

Look at all symptoms holistically, are the faults related, are they independent of each other, think, how could they be related?

  • (5) FIND RIGHTS NOT WRONGS

Electrical fault finding is a process of elimination, It's not about finding what is wrong with a circuit, Its about finding whats right.

  • (6) KISS ASRE and be lazy

Keep It Simple Stupid (because it's) Always Something Really Easy. BE LAZY, Once you understand the circuit, figure out where you can eliminate the most sections, with the least amount of testing, in the easiest to access place.

  • What can you get to?
  • What will a test there eliminate?
  • (7) LEARN TO TRUST YOUR METER

Your meter can't lie & a circuit can't break the (ohms) law. Do the math on your readings, there are plenty of smart phone apps for calculating Ohms law so you can learn what means what. Learn by repetition.

Voltage and Resistance are the ONLY things in a circuit that can change.

  • Blowing a fuse?
  • You have too much current flow, Ohms law tells us we have either to much voltage or to little resistance?
  • (8) SAY IT OUT LOUD
  • Open
  • Short
  • High Resistance

These are the only faults you can have in a wire, A circuit is a called a circuit for a reason, because it needs to be a complete circle.
-A PLC not energising "Solenoid A" is an OPEN.
-The Symptom is the "Solenoid A" circuit is OPEN
-The fault is PLC input "Position Sensor D" is showing 0 Volts
-Say "Position Sensor D" circuit is SHORTED

  • (9) REPAIR THE FAULT

Repair and retest. DONE

There is nothing more satisfying the systematically working your way though a circuit to correctly identify a fault, conversely there is nothing more frustrating than being confused by a circuit and misdiagnosing a fault. Think about your own successes and failures in the past, how good did it feel to get it right.


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