Identify Types of Basic Circuit Faults and Wiring Harness Test

Basic Circuit Faults and Wiring Harness Test

Series Circuit

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Circuit Malfunctions
There are several ways that a circuit can malfunction.  Most electrical malfunctions are caused by opens, shorts, grounds, high resistance, or intermittent.

#Opens
An open in any part of a circuit is, in effect, an extremely high resistance that results in no current flow in the circuit.  An open can be caused by a failed component such as a switch or fuse, or a broken wire or connector.  The physical location of the open determines how the circuit will react.  In a series circuit, any open connection will result in no current flow in the circuit.  The illustration above shows an open in a series circuit.  The switch acts as an open and therefore no current will flow through the two loads when the switch is open.

Troubleshooting an open circuit is easily accomplished using a multimeter and measuring source voltage.  If source voltage is available at the connection ahead of the switch and not available on the load side of the switch, the switch contacts are open.  If voltage is available on the load side, it would be necessary to continue checking the circuit until the open is identified.

Parallel Circuit

In a parallel circuit, identifying an open depends on where the open occurs.  If the open occurs in the main line, none of the loads or components will work. In effect, all parallel branches will not operate. Additionally, an open in the return ground path would have the same effect as an open in the main line. An open in the return ground path is referred to as an "open ground".

If the open occurs in any of the branches past the main line, only the load on that specific branch is affected. All other branch loads will operate normally. The illustration above shows an example of an open in the main line and in a parallel branch.

When troubleshooting or diagnosing an open in a circuit, the result is normally a component that fails to operate or function. Since most  power circuits are protected with some type of a fuse or circuit protection device, it is recommended that the fuse or device be checked visually. If a visual check does not reveal an open condition, remove the device and perform a resistance or continuity check to ensure that the device is functional. The next most probable place to check for an open is at the component itself. Using a multimeter and an electrical schematic, determine if system or source voltage is available. If voltage is not present at the component, the next step is to determine what other electrical devices, such as switches or connectors, are in the circuit path. Eliminate those devices, starting at the easiest location and working back toward the voltage source.

#Short Circuits

A short in a circuit is a direct electrical connection between two points, usually a very low resistance or opposition to current flow. It most often describes an unwanted or incorrect electrical connection and may draw higher than expected current. In describing malfunctions caused by electrical shorts, the type of short is usually identified as a "short to ground" or a "short to power".

A short to ground occurs when current flow is grounded before it was intended to be. This usually happens when wire insulation breaks and the conductor actually comes in contact with the machine ground. The effect of a short to ground depends on the design of the circuit and on its location in relationship to other circuit components such as protection devices, switches, loads, etc.

The above illustration shows the short occurring after the protection device and switch, but before the circuit load (lamp). In this example, a low resistance path to ground occurs whenever the switch is turned on and source voltage is available. The result of this unwanted path will result in an opened fuse (or tripped breaker) when the switch is turned on.

The above illustration shows the short to ground occurring before the switch. This condition is often referred to as a "dead short". In this situation, the fuse will open any time circuit voltage is applied.

A short to power or supply occurs when one circuit is shorted to another circuit. The symptoms of a short to power again depends on the location of the short. The result of this type of condition generally causes one or both circuits to operate improperly, such as a component being energized when it is not supposed to be. The root cause of this condition is typically worn or frayed electrical wiring. Also, this condition rarely causes protection devices to open or damage to other components.

The above illustration shows the short to power occurring before the controlling devices (switches).  This condition allows either switch to control the two loads.

The above illustration shows the short to power occurring after the load in one branch and before the load in the other. In this case, if the switch controlling Circuit #2 is turned on, the load does illuminate, but if the switch controlling Circuit #1 is turned on, a direct short to ground occurs resulting in the fuse opening.


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Electronic Control Module (ECM)
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Wiring Harness (Open Circuit) - Test
TEST FOR AN OPEN CIRCUIT:
1. Identify the connectors and the wire numbers of the suspect circuits. Use the Electrical System Schematic of the machine to identify the circuits.
2. Turn the key start switch and the disconnect switch to the OFF position.
3. Disconnect the component and the ECM from the wiring harness.
4. At one of the disconnected harness connections, place a jumper wire from the contact of the suspect wire to frame ground.
5. At the other connector of the machine harness, use the multimeter probes to measure the resistance from the contact of the suspect wire to frame ground.

Expected Result:
OK - The resistance is less than 5 ohms (The harness circuit is correct).
Not OK - The resistance is greater than 5000 ohms. There is an open in the machine harness.


Wiring Harness (Short Circuit) - Test
TEST FOR A SHORT CIRCUIT:
1. Identify the connectors and the wire numbers of the suspect circuits. Use the Electrical System Schematic of the machine to identify the circuits.
2. Turn the key start switch and the disconnect switch to the OFF position.
3. Disconnect the component and the ECM from the wiring harness.
4. At the machine harness connector for the ECM, place one of the multimeter probes on the contact of the suspect wire.
5. Use the other multimeter probe to check the resistance across all other contacts in the connector (s) of the ECM and frame ground.

Expected Result:
OK - The resistance is greater than 5000 ohms for all the measurements (The harness circuits are correct).
Not OK - The resistance is less than 5 ohms. There is a short in the machine harness. The short is between the suspected wire and the wire with the lowest resistance measurement.







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