Short Circuit Troubleshooting Part 2

We're going to tackle a slightly more complex problem than you saw in the last Short Circuit Troubleshooting lesson. If you haven't completed that lesson yet, you might want to start there. This lesson will build on some of the ideas from that one.
Get ready for a deeper dive into Step 10: Root Causes!
Using the Ten Step Troubleshooting process as a guide, we'll concentrate on finding and fixing the fault as efficiently as possible, without unnecessary risk to equipment or safety.

The Ten Step Troubleshooting Process

This lesson focuses on a particular electrical failure that needs to be found and fixed. Rather than a typical Guess and Check, (sometimes known as Hit Or Miss) method, we will apply the Ten Step Troubleshooting Process, or TSTP.

The TSTP is a logical series of steps that can be adapted to nearly any troubleshooting problem, and followed for an efficient and effective equipment fix.

The Problem

You're a technician at a small gravel extraction company.

A loader with seemingly dead circuits has come in.


Earlier today, before the loader was brought in for repair...

The operator noticed that the hydraulic oil high-temperature warning light was on, and he stopped the machine and went for a coffee break. Afterward, when he restarted his machine, the light was off.

So, he resumed work.

The high temperature light soon came back on, and then went off almost immediately, along with the work lights and the lights in the operator's gauge cluster. The entire circuit seems to be dead.


Experience To The Rescue

This isn't your first day on the job, and you know exactly what these symptoms sound like - a blown fuse. Sure enough, when you check the fuse box, this circuit's fuse has obviously blown… violently.

It's easy to replace the fuse, but considering the scorch marks, the fuse almost certainly blew for a reason. It looks like there is a short somewhere in this circuit, and as long as it exists, replacement fuses will just blow as well. You're going to need to find and fix the short circuit - it's troubleshooting time!

Step 1
Define The Problem

Electrical problems present in one of three ways.

No Go

E.g. The circuit does not operate. At all.

Both open and short circuits will cause No Go conditions.

Erratic Operation

E.g. The circuit works sometimes, but is not reliable.

Incorrect Value

E.g. The circuit works, but results are wonky. Lights may be dimmer, motors turn faster or slower than they should, sensors display incorrect values.

Our loader problem is a textbook case of "No Go".

Specifically, it's almost certainly a short circuit. This will modify our troubleshooting process slightly.

Step 2
Consult Schematic & Gather Documentation

Before doing anything else, head to the bookshelf and grab the manual & schematics for this loader.

If you have a schematic that is up to date, and verified for accuracy, congratulations! You are miles ahead in troubleshooting this problem. An accurate and trustworthy schematic is the key to an electrical system; it can provide:

  • A list of all of the components in the system, often including exact part numbers, codes and system-specific settings.
  • A diagram of the relationships between system components.
  • The exact location of test-points.
  • Ways to isolate sub-circuits for testing purposes.

Review The Schematic

Touch each call-out to quickly review some circuit basics.

Battery

The electrical side of this machine is powered by a 24VDC battery bank.

Disconnect

The circuit can be locked out using this disconnect.

Fuse

A simple "blade" style automotive fuse provides circuit protection.

Ignition

When the ignition is off, key systems on the machine do not run.

Temperature Switch

This N.O. switch closes when the oil reaches 85C (185F) degrees.

High Temp Lamp

Illuminates when the temperature switch closes, indicating the oil is overheated.

Valve

When energized, this solenoid operated hydraulic valve shifts to connect the fan motor to a supply of hydraulic fluid flow.

Fan Motor

The fan motor requires flow of fluid in order to spin.

Forward & Rear-Facing Work Lamps

These lamps are mounted on top of the loader cab for nighttime work illumination.

"Work Lights" Switch

This N.O. toggle switch powers the coils in the lighting relays

Relay (x2)

These relays connect the high current work lamps to a power source.

Operator's Console Bulbs

These bulbs are backlights that illuminate various gauges on the operator's console.

Scan Existing Documentation & Records

Review the shop records. Has this loader or others like it been in for repairs recently? Look for evidence of a recurring problem, or a maintenance-induced problem.

It looks like something was up with the same electrical circuit a few months ago; the fuse in this circuit was replaced 4 times in just a few weeks. There's no sign of problems since, though.

Step 3
List Suspect Components

Think of the ways each component could fail. Could that cause a no-go fault?

In this step, try to list failures that could cause any no-go fault, regardless of how likely it may be.

Bonus!

This is a list that you will only have to create once.

In fact, you can create this list before there is a malfunction, along with similar master lists for erratic motion and incorrect value faults. Once you have it, these lists become extra tools in your troubleshooting kit.

As you consider each component, ask yourself these questions:

  • What is this component?
  • What does it do?
  • What can go wrong with it?
    • Can that cause a no-go fault?

Make a Master No-Go Fault Suspect List

Hey! You already created this list in Short Circuit TSTP 1! You can save time and just reuse it.
Hey! You already created this list in Short Circuit TSTP 1! You can save time and just reuse it.

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We hope you enjoyed Short Circuit Troubleshooting Part 2

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Vacuum Pressure
Atmospheric Pressure
Low Pressure
Medium Pressure
High Pressure
Ground/Common
Lowest Voltage
Medium Voltage
Highest Voltage
Magnetic Field
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