Diagnostic Strategies
Fault tracing, generalFaults in electrical systems may often be due to defective wires and connections. These faults may often be caused by interference from other electrical systems or by mechanical and chemical factors.
In the case of intermittent faults, the cause is often in the wires and connections. Gently shake and pull connections to find this type of fault.
Procedure
1. Check connections visually.
2. Open circuit.
3. Short to ground.
4. Short to other circuits.
5. Resistance at contacts or connections.
Check connections visually
For all readings and checks where connectors are separated, check the connections visually.
Look for signs of oxidation that may affect the connectors.
Check that connector pins are intact and that the copper wires are properly connected to the pins.
Open circuit
Open circuits are revealed by functions not working. They may be caused by chafing of wires or by connections working loose.
Disconnect connectors at both ends of the wire. Use an ohmmeter to measure across both ends of the wire. If the wire is sound, the ohmmeter should read about 0 ohms
Short to ground
A short circuit between a live wire and ground is usually revealed by a fuse blowing when power is applied to the wire.
Activate all switches and sensors in the circuit and see whether the fuse blows.
While taking readings, gently shake the wires and pull the connectors to reveal intermittent faults as well.
Unplug connectors to components in the circuit so that they do not affect the readings.
Use an ohmmeter to measure between wires and ground to locate any shorts to ground.
The ohmmeter should indicate infinite resistance if no connectors are plugged in.
Short between wires
A short between wires does not always cause a fuse to blow. Usually it is revealed by something not working.
While taking readings, gently shake the wires and pull the connectors to reveal intermittent faults as well.
1. Using a voltmeter, take readings at different points in the circuit while operating switches and sensors. The required voltmeter reading depends on the circuit involved and the position of switches and sensors. Use the circuit diagram to find the correct voltage for a given circuit.
2. Using an ohmmeter, take readings across any wires suspected of having a short between them. The ohmmeter should indicate infinite resistance between wires that are not connected in the circuit.
Resistance at contacts and connections
The resistance at contacts and connections should actually be zero, but there may be a slight resistance. If this resistance becomes too high, operation of the equipment will be affected. How much resistance is tolerable depends on the load on the circuit. Normally it should not be more than a few ohms.
While taking readings, gently shake the wires and pull the connectors to reveal intermittent faults as well.
Check with a separate male pin that the female pin contact is good and that the male pin remains connected under light tension.
Using an ohmmeter, take readings between the ends of the wire. Take readings with switches and sensors in circuit as well, to locate excessive contact resistance in those components. The ohmmeter should not read more than a few ohms in a circuit in which the resistance at contacts and connections is acceptable.