Back to the Basics: States of a Wire

Modern electrical systems can be complex.  Understanding different failures that can happen to the wires that connect components can reduce troubleshooting time and ensure correct repairs.  Using the proper wiring diagram will help you isolate the specific wires used in the circuit so you can quickly perform the required tests to locate the failure.


A perfectly good wire.  The wire will not affect the normal operation of a circuit.  A wire should show very little voltage drop when current is flowing through it.  A maximum of 0.2 VDC is an acceptable reading.


An open wire is physically broken and will not allow current to flow through it.  The circuit will not function.  Since no current is flowing through the circuit, no voltage drop will be seen across the circuit's load.  Supply voltage will be available from the voltage source to the open.  No voltage will be available after the open.  Use a voltmeter to follow the positive side of the circuit.  The voltage will stop where the open is.


A shorted wire has path to ground before the circuit's load.  This is also referred to as a dead short.  In this case the load in the circuit has been bypassed.  The power source will now push as much current through the circuit as possible, several hundred amps in the case of most batteries.  This will blow the fuse for that circuit or cause the wire to melt until it becomes an open circuit.

To locate the short in the circuit that has blown its fuse, replace the fuse with a small light bulb (an old bow light works well).  While the short is present, the bulb will remain lt.  When the short is located and repaired, the bulb will go out.  Replace the fuse and verify proper circuit operation.

If the short has some resistance to the ground, it may not blow the fuse, but can cause excessive amperage to flow through the circuit.  This can cause connectors or the insulation on the wire to get too hot and start to melt.  Remove the fuse again and use the bulb in its place.  This will once again allow you to find the short.


Some circuits are controlled on the ground side of the load.  These circuits may use relays as the switch to control current flow through the circuit.  This will allow the ECM to control the circuit.  The high pressure fuel pump inside the vapor separator circuit is a good example.  A grounded circuit will have a path to the ground after the load but before the controlling switch, in this case the ECM.  This will cause the circuit to be always active, the ECM will not be able to turn the circuit off, anytime the main switch is on the pump will be running.

Series Resistance

In a normal circuit, the load should be the only resistance in the circuit.  Series resistance is an unwanted additional load in the circuit, such as corrosion at the connector.  This unwanted resistance will consume some of the available voltage.  When this occurs, circuits may not operate correctly and begin to operate in very unpredictable ways.  This is particularly true when the resistance affects the sensors used on modern fuel-injected units.

Performing a voltage drop test on portions of the circuit will quickly locate the unwanted resistance.  To quickly isolate which side of the circuit the failure is in, perform a voltage drop on the entire positive side.  If the reading is acceptable, repeat the test on the ground side of the load.

Now that the failure has been determined to be between the relay and the ECM, a few more quick tests will isolate the failure down to the affected wire or connector.

Understanding how different wire failures affect a circuit allows you to quickly decide what the most effective testing procedure is.  This will cut down on diagnostic time and helps ensure that the failure is correctly identified and repaired.