ENGINE AR 8N3909 - Caterpillar

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. For example, a short across the wires in a circuit for a lamp produces too much current in the wires but no current is felt 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.
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 wire.
Use the other multimeter probe to check the resistance across all other contacts in the connector(s) of the ECM and frame ground. Expected Result: The resistance is greater than 5000 ohms for all the measurements.OK - The resistance is greater than 5000 ohms for all the measurements. The harness circuits are correct.Stop. NOT OK - The resistance is less than 5 ohms. 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 the machine harness or replace the machine harness.Stop.