Open Circuit Troubleshooting

Objective

This module will walk through the steps of efficient troubleshooting to find and fix an open circuit fault.

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

As a technician for a small gravel extraction company, you're pretty busy keeping the fleet of machinery running, and today is no exception. The operator of a front end loader has complained that the forward-facing headlamps on top of the loader cab are not illuminating when the light switch is toggled. All of the other headlights and marker lights come on, and the gauge cluster illuminates too.

The loader is not road-legal without the headlamps, and it is dangerous for the operator to work without sufficient light. It has been brought into your shop and you have locked out and tagged out the machine. It's troubleshooting time!

Step 1
Define the Problem

Electrical problems present in one of three ways.

No Go

E.g. The lamps do not light. At all.

Erratic Operation

E.g. The lamps work sometimes, but they are not reliable.

Incorrect Value

E.g. The lights work, but they are dimmer than they should be.

Our loader problem seems like a clear-cut case of a "No Go" problem.

Step 2
Consult the Schematic

Before visiting the loader, you stop by the bookshelf and grab the manual. A few page flips get you to the wiring diagram for the headlamps, and you take a moment to review the circuit.

Why It Helps

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 understanding a system; it can provide
  • a list of all of the components in the system, often including exact part numbers, codes and system-specific settings.
  • the exact location of test-points.
  • a diagram of the relationships between system components.
  • ways to isolate sub-circuits for testing purposes.

The manufacturer has provided a schematic with the manual, so you are ready to go.

Review The Schematic

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

Left and Right Front Work Lamps

These lamps will not light. What could the problem be?

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 (x4)

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

"Work Lights" Toggle 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.

There's nothing in the shop records about an issue with the lights. This is likely the first time this problem has come up.

Step 3
List "No Go" Suspect Components

In Step 1, you determined that this is most likely a "no go" problem. Now that you've consulted the schematic, make a list of ALL of the places where a failure could cause a "no go" fault.

Bonus!

This is a list that you will only have to create once. In fact, you can create this list, along with lists for Erratic Operation and Incorrect Value, before there is a malfunction. Once you have them, these lists become tools in your troubleshooting kit.

Don't worry about limiting it to just the places where it could cause this exact failure - think about ANY possible failure leading to "no go" at this point. The goal at this stage is to create a large list that does not overlook any possibilities.

Make a Master "No Go" Problem Suspect List

Touch the call-outs to examine each component and determine whether it should be on the list.

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?

Batteries

Store power for the circuit


What Can Go Wrong With It?

Could This Cause a "No Go" Fault?

Battery depleted

Yes

Short circuit across posts

Yes

Posts corroded

Yes

Disconnect

Connects/disconnects the power source and the circuit


What Can Go Wrong With It?

Could This Cause a "No Go" Fault?

Fails Open

Yes

Fails Closed

No

Fuse

Protects circuit components from excessive current


What Can Go Wrong With It?

Could This Cause a "No Go" Fault?

Fails Open

Yes

Fails to Open

No

Key Switch

Connects/disconnects the power source and the circuit


What Can Go Wrong With It?

Could This Cause a "No Go" Fault?

Fails Open

Yes

Fails Closed

No

Wires And Connectors

Connects components in the circuit by providing a path for electricity


What Can Go Wrong With It?

Could This Cause a "No Go" Fault?

Wiggle loose/breaks

Yes

Insulation worn away, shorting to frame, depleting battery

Yes

Temperature Switch

Closes when the sensor heats beyond 85°C/185°F


What Can Go Wrong With It?

Could This Cause a "No Go" Fault?

Fails Open

Yes

Fails Closed

No

Set Too High

Yes

Set Too Low

No

Lamp (x 10)

Illuminates


What Can Go Wrong With It?

Could This Cause a "No Go" Fault?

Burned-out filament

Yes

Hydraulic Valve Solenoid

Shifts the valve


What Can Go Wrong With It?

Could This Cause a "No Go" Fault?

Solenoid coil fails (does not shift valve)

Yes

Valve spool sticks open

No

Valve spool sticks closed

Yes

Valve spool travels incompletely

No

"Work Lights" Toggle Switch

Energizes the relays


What Can Go Wrong With It?

Could This Cause a "No Go" Fault?

Fails Open

Yes

Fails Closed

No

Relay (x2)

Connects lamps to the voltage source


What Can Go Wrong With It?

Could This Cause a "No Go" Fault?

Coil failure - will not operate

Yes

Contacts burned/oxidized

Yes

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We hope you enjoyed Open Circuit Troubleshooting

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

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