An open circuit is a failure of an electrical circuit that results in no flow of electrical current. An open circuit is can be caused by a break in a circuit wire. Most often, an open circuit is caused by a poor connection or a failed connection in one of the electrical connectors that is used in a circuit.
The following procedure is a way to check for an open or a poor connection in a circuit.
TEST FOR AN OPEN 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 one of the disconnected harness connections, place a jumper wire from the contact of the suspect wire to frame ground.
- At the other connector of the machine harness, use the multimeter probes to measure the resistance from the contact of the suspect wire to frame ground.
Expected Result: The resistance is less than 5 ohms.
OK - The resistance is less than 5 ohms. The harness circuit is correct.
Stop.
NOT OK - The resistance is greater than 5000 ohms. There is an open in the machine harness.
Repair: Repair the machine harness or replace the machine harness.
Stop.
When the resistance of a good circuit is measured, less than 5.0 ohms of resistance is expected. In a circuit that has an electrical wire or a connection that is completely open (broken or no connection), the flow of electrical current through the circuit is interrupted. A circuit that is completely open due to a broken wire or no connection in a connector will measure greater than 5000 ohms of resistance.
Many times a circuit is not completely open and will allow some current to pass through, however, not enough current will pass to allow the circuit to function properly. This can be caused by a wire that is partially broken but still is intact enough to allow a small amount of current to pass. Partial breaks in a wire are difficult to diagnose because the resistance measurement may check OK through the section of wire that is still intact. Partial breaks in a wire are not common. When a partial break is suspected in a wire, the current through the circuit should be checked under load in order to verify that the correct amount of current is present.
Most often, a resistance measurement of greater than 5.0 ohms and less than 5000 ohms is due to a poor connection in a circuit connector. The connector contacts may be corroded due to moisture that has entered the connector body. A connector pin or socket may not have been seated securely in the connector body and is not making good contact or a wire may be broken at the point where the pin or socket is connected. When checking for problems in a circuit, the connectors in the circuit should be examined closely.
When an open is suspected in a circuit, try to isolate the section of the machine harness where the open is occurring. The complete Electrical System Schematic for your machine must be used in order to identify and find the harness connectors that are in the suspect circuit.
Once the harness connectors for the circuit are identified, isolate the section of the circuit that is open. Start at either the component or the ECM end of the circuit. With the key start switch in the OFF position, disconnect the component or the ECM connector and use a jumper wire to connect two of the circuit wires. Go to the next harness connector in the circuit and disconnect the connector. Check the connector contacts for corrosion. Make sure the contacts are not damaged, are clean and are securely installed in the connector body. Use the electrical schematic to determine the contacts for the jumpered circuit wires. Check the resistance between the two wires. A good circuit will measure less than 5.0 ohms. Repeat the steps as you go to the next connector in the circuit until the faulty section of the circuit is identified.
Once the wire or connector that is causing the problem is found, replace the broken wire or repair the harness connector. Do not use a splice to repair a broken wire. A splice will not keep moisture out of the connection and will create another potential problem area in the circuit.
