770G and 772G Off-Highway Truck/Tractors Caterpillar

A short circuit is a failure of an electrical circuit that results in undesired electrical current. Usually, a short circuit is a bypass of the circuit across a load. A short across the wires in a circuit for a lamp produces too much current in the wires but no current at the lamp. The lamp is shorted out. The resistance in a normal circuit can vary, but the resistance between a particular circuit and other unrelated circuits is always greater than 5000 ohms. The following procedure explains the test for a short circuit:

Reference: For a complete electrical schematic, refer to Electrical System Schematic for the machine that is being serviced.

Test For A Short Circuit

Table 1

Troubleshooting Test Steps	Values	Results
1. Identify Short Circuit. A. Identify the connectors and the wire numbers of the suspect circuits. Use the Electrical System Schematic of the machine to identify the circuits. B. Turn the key start switch and the disconnect switch to OFF. C. Disconnect the component and the ECM from the wiring harness. D. At the machine harness connector for the ECM, place one of the multimeter probes on the contact of the suspect wire. E. Use the other multimeter probe to check the resistance across all other contacts in the connectors of the ECM and frame ground.	The resistance is greater than 5 kΩ for all the readings.	OK - The harness circuits are correct. STOP NOT OK - The resistance is less than 5 Ω. There is a short in the machine harness. The short is between the suspected wire and the wire with the lowest resistance measurement. Repair: Repair or replace the machine harness. Note: A resistance that is greater than 5 Ω but less than 5kΩ would indicate a loose connection or a corroded connection in the circuit. A resistance measurement that is greater than 5 kΩ would indicate an open in the circuit. STOP