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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