Electrical System for All Caterpillar Products Caterpillar

Circuit Voltage


D3400 01T

Circuit Faults

The following information will describe the circuit malfunctions of series, parallel, and series parallel circuits.

Circuit Malfunctions

There are several ways that a circuit can malfunction. Most electrical malfunctions are caused by the following conditions: opens, shorts, grounds, high resistance, or intermittents.


An open in any part of a circuit is an extremely high resistance that results in no current flow in the circuit. An open can be caused by any of the following components that have failed:

  • Switch

  • Fuse

  • Broken wire

  • Connector

  • Component (load)

The physical location of the open determines how the circuit will react. In a series circuit, any open connection will result in no current flow in the circuit. Illustration 1 shows an open in a series circuit. The switch acts as an open. Therefore, no current will flow through the two loads when the switch is open.

Illustration 1g01070634

Troubleshooting an open circuit is easily accomplished by using a multimeter and by measuring the source voltage. If source voltage is available at the connection ahead of the switch and not available on the load side of the switch, the switch contacts are open. If voltage is available on the load side it would be necessary to continue checking the circuit until the open is identified.

In a parallel circuit, identifying an open depends on where the open occurs. If the open occurs in the main line, none of the loads or components will work. All parallel branches will not operate. Additionally, an open in the return ground path would have the same effect as an open in the main line. An open in the return ground path is referred to as an open ground.

If the open occurs in any of the branches below the main line, only the load on that specific branch is affected. All other branch loads will operate normally. Illustration 2 shows an example of an open in the main line and in a parallel branch.

Illustration 2g01070636

When you are troubleshooting or diagnosing an open in a circuit, the result is normally a component that fails to operate or to function. Since most circuits are protected with some type of a fuse or circuit protection device, it is recommended that the fuse or device be checked visually. If a visual check does not reveal an open condition, remove the device and perform a continuity check in order to ensure that the device is functional. The next most probable place to check for an open is at the component. Using a multimeter and a electrical schematic, determine if the system voltage or source voltage is available. If voltage is not present at the component, the next step is to determine what other electrical devices, such as switches or connectors, are in the circuit path. Eliminate those devices, starting at the easiest location and working back toward the voltage source.


A short in a circuit is a direct electrical connection between two points. There is usually a very low resistance or opposition to current flow. A short in a circuit often describes an unwanted or an incorrect electrical connection. This may draw higher than expected current. In describing malfunctions that are caused by electrical shorts, the types of shorts are usually identified as a short to ground or a short to power.

A short to ground occurs when current flow is grounded before it was intended to be. This usually happens when wire insulation breaks and the conductor actually comes in contact with the machine ground. The effect of a short to ground depends on the design of the circuit and on the location in relationship to other circuit components (protection devices, switches, loads).

Illustration 3g01070638

Illustration 3 shows the short occurring after the protection device and the switch, but before the circuit load (lamp). In this example, a low resistance path to ground occurs whenever the switch is turned on and the source voltage is available. The result of this unwanted path will result in a blown fuse when the switch is turned on.

Illustration 4 shows the short to ground occurring before the switch. This condition is often referred to as a dead short. In this situation, the fuse will blow anytime that the circuit voltage is applied.

Illustration 4g01070640

A short to power occurs when one circuit is shorted to another circuit. The symptoms of a short to power depends on the location of the short. The result of this of condition generally causes one or both circuits to operate improperly. A component that is being energized when it is not supposed to be is an example. The root cause of this condition is caused by worn electrical wiring or frayed electrical wiring. Also, this condition seldom causes protection devices to open or cause damage to other components.

Illustration 5 shows the short to power occurring before the controlling devices (switches). This condition allows both switches to control the two loads.

Illustration 5g01070643

Illustration 6 shows the short to power that occurrs after the load in one branch, and before the load in the other. In this case, if the switch that controlls circuit no. 2 is turned on, the load does illuminate, but if the switch that controls circuit no. 1 is turned on, a direct short to ground occurs resulting in the fuse blowing.

Illustration 6g01070646


A grounded circuit usually results in a component that fails to operate. A grounded condition indicates that the circuit has an unwanted path to the machine frame. The effect on the circuit is determined by where the ground occurs.

High Resistance

Circuit malfunctions also occur when resistance levels become too high. The circuit effect usually results in the component failing to operate or the component does not operate according to specification. The cause of high resistance is a build up of corrosion on connections, or dirt on connections and on contacts.


An intermittent condition occurs when contacts or connections become loose or when internal component parts break. These problems usually results in lights flickering, or in components working intermittently. This problem usually appears as the result of vibrations or machines moving. These problems are not easily diagnosed because the condition corrects itself when the machine is stopped.

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