A short circuit is a failure 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 in the machine wiring harness such as a system voltage (+battery) circuit, a sensor power supply circuit, or a sensor signal circuit.
A short circuit to a ground source will cause excessive current to flow through the circuit and will cause the voltage to decrease. In a machine circuit that is not controlled by an ECM such as a lighting circuit, will cause the fuse or the circuit breaker to open. In an ECM controlled circuit, the ECM will detect these conditions and can activate a diagnostic code for the circuit with an FMI 04 (voltage below normal) or an FMI 06 (current above normal) condition code.
A short circuit to another voltage source circuit can result in voltage being present in the circuit when no voltage should be present. When this occurs in ECM controlled output circuits (solenoids), uncontrolled machine movement can occur. When this occurs in ECM input circuits (sensors), machine functions may not operate when requested due to signal interference. The ECM will detect these conditions and can activate a diagnostic code for the circuit with an FMI 03 (voltage above normal) or possibly an FMI 08 (abnormal frequency, pulse width or period) condition code.
Short circuits can occur by damages to the insulation of a wire as a result of repeated contact with the machine frame or with another wire. A short circuit can also be caused by a damaged or contaminated machine harness connector. A short circuit can also be caused by a failed circuit component.
When troubleshooting a circuit for a short, the connection schematics that are provided in the back of this manual are a good quick reference source, however, the complete Machine Electrical System Schematic should be used in order to identify all of the machine wiring harness connectors that are in a specific circuit.
ReferenceFor a complete electrical schematic, refer to Electrical System Schematic for the machine that is being serviced.
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.
- Disconnect the negative battery cable and turn the key start switch to the OFF position.
- 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 multimeter 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 ECM connectors.
- As each measurement is made, observe the multimeter resistance measurement and wiggle the machine harness connectors for the ECM. Gently pull on the wires and move the wires around.
- While observing the multimeter resistance 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 measurement is greater than 5000 ohms for each of the measurements.
OK - The resistance is greater than 5000 ohms for all the measurements. 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 is less than 5 ohms. There is a short in the machine harness. The short is between the suspected circuit wire and the circuit wire with the lowest resistance measurement or the short is between the suspected circuit wire and the machine frame ground.
ISOLATING THE LOCATION OF THE SHORT CIRCUIT
Once it has been determined that a short circuit is present, the location of the short in the circuit must be found and repaired.
Use the troubleshooting procedure for the specific diagnostic code that is activated in order to isolate whether the problem is caused by a system component or is present in the machine wiring harness.
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 machine harness connector that is involved 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.
While each of the connectors is disconnected, use a digital multimeter to measure the resistance between either frame ground or measure the resistance between the suspect circuit wire and the circuit wire with the low resistance measurement. While observing the measurement, wiggle the connector and the machine harness in order to check for intermittent short circuits. Do this 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. In order to locate 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 on 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 or harness is not in contact with 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.