Short, Open, Loads, FInding Short Circuits

SHORT CIRCUITS

There are two kinds of short circuits, a short to power and a short to ground. A short to power can occur within a component or between circuits. If the short is within a component, the component will not operate properly or may not operate at all. Check the resistance of the component with an ohmmeter; if there is a short, the resistance will be less than specified.

A short to power between circuits usually results in components operating unintentionally. This can be due to worn, abused or burned wiring insulation. A short to ground will usually cause a fuse to blow as soon as it is installed. However, if the short is on the ground side of the load but before a ground-side switch, the fuse will not blow but the component will not turn off. A voltmeter or a circuit probe will be useful for diagnosing electrical faults. 

A short to ground also can be detected using a compass-like device called a “short finder.” The short finder is connected between the fuse terminals (in place of the blown fuse) and will create a pulsing magnetic field in the circuit. After installing the short finder, turn on the load device. Move the short finder meter along the circuit, beginning at the fuse box. The meter needle will deflect with each magnetic current pulse. When the meter is moved past the short, the needle will stop deflecting. This area should be checked for the cause of the short.

FINDING SHORT CIRCUITS

Occasionally you will come across a burned fuse with no evidence of a short. If you plug in a new fuse, the fuse holds, and all seems to work well. This may be due to a transient or intermittent short. The current flow may be high enough to pop the fuse or breaker, but only during a transient period. This condition could be due to faulty insulation where the wiring harness rubs the body or some other object, or where moisture may enter the wire through the worn spot. The circuit will temporarily ground and the fuse will blow, but not until the next time it rains, or slush is thrown up by the wheel to wet the circuit. This condition occurs often in bumper-integral light circuits.

Testing for a short to ground with a short finder. (Courtesy: Hyundai Motor America)

A mysterious short may be caused by a momentary overload. This may be due to the high load of a defective motor unit as it starts up. Usually it takes a lot more current to get an electric motor moving than to keep it moving. 

A jammed unit may cause a fuse to blow. If a motor or its mechanism is jammed, the protection device will perform its job - motor protection! A conversation with the driver may tip you off to the real cause of this kind of transient condition. More than one wiper motor or door window motor frozen in place with ice has popped its protective device when the customer turns the switch on. In the case of wiper blades, the circuit breaker will likely automatically reset. 

In the event a normal blade fuse blows, or a fusible link burns out, something is faulty and needs to be corrected.

Lastly, you may have a transient condition that occurs in a circuit which has more than one load; one in parallel with another. For example, many cars have a dome light and a cigarette lighter on the same fuse. Replace the fuse, and the interior lights work great. Try as you might, you can’t reproduce the short until you check the diagram and realize the dual nature of the circuit. Check the schematic for such possibilities when troubleshooting short circuits.

OPEN CIRCUITS

The current flow through a circuit is interrupted when the circuit is opened by a break in the conductor. The open may be created by a defective switch, relay or wire. In order to restore the circuit, we must find and repair the open. Check the wiring diagrams carefully and trace the normal current flow. Then trace the circuit using a voltmeter. 

CIRCUIT LOADS

The load device in a circuit is the unit that does the work, such as the lights, horn, wiper motor, etc. Once adequate voltage gets to the load, it should operate - that is, if the circuit isn’t shorted, open internally, or improperly grounded.

You can use a voltmeter to check for power and proper voltage drops across load devices. Source voltage should be dropped across any load in a simple or parallel circuit (not in a series circuit). 

Remember too that all circuits need a ground. Some loads are internally grounded to the frame of the unit; others are externally wired to ground. With the move to the greater use of plastic and fiberglass panels in vehicles, you may find a larger number of units using external grounding methods. When replacing a part, especially where wiper motors are concerned, be sure to reconnect the ground wire before you apply energy.