DISC-INNER 4634965 - Caterpillar

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, or a circuit can 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 in the circuit to decrease. In a machine circuit that is not controlled by an ECM such as a lighting circuit, this will cause the fuse or the circuit breaker for the circuit to open. In a 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 be caused by damage 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.
Turn the key start switch and the 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 and frame ground and 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