A short circuit is a failure in the isolation of an electrical circuit that results in the flow of electrical current through an unintended path. A circuit can be shorted to a ground source, either frame ground or a ground circuit. A circuit can also be shorted to another circuit such as a battery circuit, power supply, or a signal circuit for a sensor.
A short circuit to a ground source will cause excessive current to flow through the circuit. Excess current will cause voltage in the circuit to decrease. In a circuit such as a lighting circuit, excess current will cause the fuse or the circuit breaker to open. In a circuit that is controlled by an ECM, the ECM will detect a change in the voltage. The ECM will activate a diagnostic code for the circuit with an FMI code. The FMI code will indicate whether the voltage is high or low.
A short circuit that occurs between two voltage sources can result in a voltage that should not be present. This type of short in ECM output circuit can create uncontrolled machine movement. This type of short on an ECM input circuit can cause machine functions not to operate when requested. The ECM will detect these conditions and can activate a diagnostic code for the circuit.
Short circuits can be caused by damaged wire insulation. The insulation may have been damaged as a result of repeated contact with the machine frame or with another wire. A short circuit can be caused by a damaged or contaminated machine harness connector. A failed component can also create a short.
The connection schematics that are provided in the back of this manual are a quick reference source to troubleshoot a machine. The complete machine electrical system schematic should be used to identify all of the machine wiring harness connectors that are in a specific circuit.
TEST FOR A SHORT CIRCUIT.
- Identify the connectors and the wire numbers of the suspect circuits. Use the Electrical System Schematic of the machine to identify the circuits.
- Turn the key start switch and the disconnect switch OFF.
- Disconnect the component and the ECM from the wiring harness.
- At the machine harness connector for the ECM, place one of the multimeter probes on the contact of the suspect circuit wire.
- Use the other probe to check the resistance between the suspect circuit wire and frame ground. Also, check all of the other contacts that are used in the connector for the ECM.
- While observing the multimeter measurement, wiggle the machine harness and the harness connectors that are involved in the circuit. Gently pull on the wires and move the wires around.
Expected Result: The resistance does not indicate a short.
OK - An obvious short is not evident. An intermittent short circuit could still be causing a problem.
NOT OK - One or more of the resistance measurements indicates a short. The short is between the suspected circuit wire and the lowest resistance measurement.
ISOLATING THE LOAVSION OF THE SHORT CIRCUIT
When a short circuit is present, the location of the short in the circuit must be found and repaired.
Perform the troubleshooting procedure for the specific diagnostic code that is activated. The procedure could help isolate the location of the short.
When the machine wiring harness is suspected, examine the circuit on the full Machine Electrical System Schematic. Starting at the ECM, identify, and locate each harness connector that is used in the circuit. Starting at the ECM, one at a time, disconnect each connector. Inspect the pins and the sockets on the male connector and on the female connector. Verify that all of the contacts are securely installed in the connector body. Ensure that the contacts are clean and the contacts are in good condition.
With each connector disconnected, use a multimeter to measure resistance between the suspect wire and frame ground or the wire with the low resistance. While observing the measurement, wiggle the connector and the machine harness in order to check for intermittent short circuits. Perform this test as you move outward from the ECM in order to isolate the section of the harness where the short circuit is occurring.
Reconnect the connector. Ensure that the locking clips on the connectors are engaged. Check all of the connectors that are in the suspect circuit in this manner. After the connectors have been examined, check to see if a short is still present in the circuit.
A "Short Finder" device is available from most tool distributors that will aid in locating the short. These devices detect the changes that a short creates in the magnetic field that is created by a current in the wire. Detailed instructions for the operation of the device are included with these devices.
When damaged insulation is present on a wire, use a good quality electrical tape or electrical heat shrink tubing to repair the section of insulation. Ensure that the section of wire does not contact any part of the machine frame. Do not use an "RTV" or silicone sealer to repair the insulation as these types of sealants are not compatible with the copper wire.