Electrical Circuit Troubleshooting
CIRCUIT DIAGNOSISLOCATING OPENS
To locate an open circuit, a voltmeter or LED test light can be used. To check a portion of the circuit for continuity, an ohmmeter or self-powered test light can be used, but only after the circuit has been disconnected to remove the voltage. A jumper wire can also be used to bypass portions of the circuit in an attempt to make it operational. If it becomes operational with a jumper wire installed, that portion of the circuit jumped is open.
In a normally operating circuit, such as the simple parallel circuit shown, the left branch is receiving power. Current is flowing through lamps "One" and "Two". If tested with a voltmeter, the circuit would read 12 Volts on the "HOT" side and 0 Voltage on the "GROUND" side. Because the right branch of the parallel circuit is open at the switch, it is not powered.
If an open occurs in the ground side of the powered branch, as shown, the continuous path for electricity is broken. As a result, no current flows. Voltage will be at 12 Volts at all parts of the circuit between the power source and the open.
The lesson to be learned from this example is that in a simple series or a simple parallel circuit, no current will flow when it is open. All points that are connected to the power source will be at battery voltage.
In more complex circuits, however, opens often have different effects on voltage and current. The view, as shown, is a variation of the previously shown circuits. When this circuit is operating normally, lamps "One" and "Three" are powered when the switch is in the position shown.
However, when an open occurs in the ground side of the powered branch, an alternate path to ground exists through lamps "Two" and "Four" Lamps "One," "Two," and "Four" then are in series, and all are in parallel with lamp "Three" The higher resistance in the series branch of the circuit will cause lamps "One," "Two" and "Four" to glow dimly, while lamp "Three," having less resistance, will glow brightly. The voltage reading will be Twelve Volts at both lamp "One" and "Three." In diagnosing for opens, therefore, it is helpful to study the wiring diagram for common points in the circuit and to look for alternate current paths.
LOCATING SHORTS
Short circuits are most noticeable when they create an accidental current path which bypasses the current around the load or the control device. Short circuits happen whenever a "HOT" conductor accidentally makes contact with another conductor or with ground. Parts can also be shorted internally.
A short between the power source and the load will blow a fuse. If the circuit is unfused, the conductor will overheat. The insulation will melt, and the wire will probably burn and open.
In fact, when locating a short to ground that involves burnt wiring, keep in mind that the wire will burn or melt from the point of the short back toward the current supply (+) side of the circuit. Therefore, when tracing a burnt wiring harness, begin at the positive (+) side and work toward the load and/or ground side of the circuit. The point at which the short occurred is usually where you will find the end of the wire damage.
For example, if the burnt wire terminates at a load device, that component is probably shorted to ground internally. If the wire damage terminates at an inter-harness connector, the terminal is probably shorted to a ground terminal in the same connector.
When one wire gets hot enough to damage several other wires in the harness it can be more difficult to find the cause. However, the above information should still be very useful in locating the cause of the short.
In a short circuit to ground in which the wiring harness is not visibly or outwardly damaged, a LED test light can be substituted for the blown fuse to limit the current flowing in the circuit. If the light glows with all the load devices in the circuit disconnected, it indicates a short to ground. Connectors in the circuit can then be disconnected working back up the circuit from the load devices until the test light goes out. The short is located between the disconnected connector and the previously disconnected connector.
Another method used to find shorts to ground is with a Short Finder or Compass. A momentary circuit breaker is substituted for the fuse. This circuit breaker will open and close the circuit as it heats up and cools down. A Compass or Short Finder can be used to trace along the circuit. As the circuit breaker opens and closes it will cause a magnetic field to build up and collapse the conductor.
This will cause the compass or short finder needle to deflect back and forth as you move along the circuit. When you pass the point of the short the needle will stop deflecting as there is no more current flow in that portion of the circuit. The compass or short finder will show a deflection even through trim panels, carpeting, headliners, etc.
If there is a switch between the load and ground, as with engine warning lamps, another type of problem occurs. The load will be on constantly if the short is between the load and the switch.
SHORTS INTO OTHER CIRCUITS
Shorts into another circuit can be isolated by progressively disconnecting connectors in the circuit until the other circuit stops operating. If a fuse is blown, a test light can be substituted for the fuse to limit current flow in the circuit. The short is located between the connector which when disconnected turns off the other circuit and the connector which when disconnected makes no difference. Examine the wires and harness for damaged or bare wires touching. Repair or replace any damaged wires or wiring harness.
FEEDBACKS
Electrical feedbacks in automotive circuits can cause some unusual situations and be very difficult to diagnose. The most common cause of electrical feedback, in automotive circuits, is a blown fuse. Since some circuits are fed current from different sources, an open fuse can allow current to find another path to operate different load devices. Be sure to check all the fuses, both visually and with a tester. If the fuses are all right, consult the vehicle's wiring diagrams for the affected circuit. Check all the common circuit feed points.
Another cause of electrical feedback is poor or open grounds. As we saw in the example of a complex parallel circuit, an open ground in one branch can cause the current to seek ground through another branch. Again check the vehicle's wiring diagrams for common ground points.
After the electrical problem has been determined (open, short), and using the schematics the circuit has been isolated, the fastest way to find the actual fault is to split the circuit and trace the problem in the direction of current flow or against current flow. Where you choose to split the circuit should be determined by ease of access to fuses, components, or wires.
An example of this would be a dashboard circuit with an electrical problem. By using the schematic, you find that the fuse is somewhat in the middle of the circuit. The logical approach would be to check for voltage or amperage at the fuse before you dismantle the dash. If there is no measurable current at the fuse, your problem would be between the battery and the fuse. The problem would be in the engine compartment area, not in the dashboard. Your determination of this fact will have saved the time required to remove and replace or dismantle the dashboard.