A circuit can also be shorted to another circuit in the machine wiring harness. A circuit can be shorted to 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. a short to ground will cause the voltage in the circuit to decrease. In a machine circuit that is not controlled by an ECM such as a lighting circuit, a short to ground will cause the fuse or the circuit breaker for the circuit 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) condition code. An FMI 06 (current above normal) condition code can also be activated.
A short circuit to a voltage source circuit can result in voltage being present in the circuit when no voltage should be present. When a short circuit to a voltage source occurs in ECM controlled output circuits (solenoids), uncontrolled machine movement can occur.
When a short circuit to a voltage source occurs in ECM input circuits (sensors), machine functions may not operate properly due to signal interference. The ECM will detect these conditions. The ECM 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 be the result of damage to the insulation of a wire. The short can be caused by 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 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 for the machine to identify the circuits.
- Turn the key start switch and the battery disconnect 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, frame ground and all of the other contacts that are used in the ECM connectors.
- As each measurement is made, observe the multimeter resistance.
- 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. The short can also be between the suspected circuit wire and the machine frame ground.
ISOLATING THE LOCATION OF THE SHORT CIRCUIT
When 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 if the problem 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, the contacts are dry, and the contacts are in good condition.
While each of the connectors is disconnected, use a digital multimeter to measure the resistance between the involved circuit and frame ground. also 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. Check the harness as you move outward from the ECM 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. 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. Instructions for the operation of the device are included with these devices.
When damaged wire insulation is present, 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. A high-quality non-corrosive RTV sealant that is rated for use with electrical components or insulation can be used to repair light damage to electrical circuits.