CTS 2003-2005

Circuit Protection


The purpose of circuit protection is to protect the wiring assembly during normal and overload conditions. An overload is defined as a current requirement that is higher than normal. This overload could be caused by a short circuit or system malfunction. The short circuit could be the result of a pinched or cut wire or an internal device short circuit, such as an electronic module failure.

The circuit protection device is only applied to protect the wiring assembly, and not the electrical load at the end of the assembly. For example, if an electronic component short circuits, the circuit protection device will assure a minimal amount of damage to the wiring assembly. However, it will not necessarily prevent damage to the component.

The vehicle's power distribution system consists of fuses, fusible links, circuit breakers and the ignition switch. Fusible links are short pieces of wire several sizes smaller than the circuit wire to which they supply power. They are covered with special high temperature insulation. When conducting an improperly high current flow, they will melt and stop current flow. They are designed to protect the vehicle's electrical system from electrical shorts in circuits not protected by circuit breakers or fuses.

There are three basic types of circuit protection device: Fuses, Fusible links and Circuit breakers.

The most common method of automotive wiring circuit protection is the fuse. A fuse is a device that, by the melting of its element, opens an electrical circuit when the current exceeds a given level for a sufficient time. The action is non-reversible and the fuse must be replaced each time a circuit is overloaded or after a malfunction is repaired.

Fuses are color-coded with standardized color identification and ratings. For service replacement, non-color fuses of the same respective rating can be used.

Examine a suspect fuse for a break in the element. If the element is broken or melted, replace the fuse with one of equal current rating.

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Fig. Typical fuses used on GM W-Body vehicles

There are additional specific circuits with in-line fuses. These fuses are located within the individual wiring harness and will appear to be an open circuit if blown.

In addition to circuit breakers and fuses, some circuits use fusible links to protect the wiring. Like fuses, fusible links are "one-time" protection devices that will melt and create an open circuit.

Not all fusible link open circuits can be detected by observation. A blown link often, but not always, has "bubbly" appearing insulation making troubleshooting easier. Always inspect that there is battery voltage past the fusible link to verify continuity.

Fusible links are used instead of a fuse in wiring circuits that are not normally fused, such as the ignition circuit. For AWG sizes (wire gauges), each fusible link is four wire gauge sizes smaller than the wire it is designed to protect. For example: to protect a 10 gauge wire, use a 14 gauge link (or for metric, to protect a 5mm wire, use a 2mm link). Links are marked on the insulation with wire-gauge size because the heavy insulation makes the link appear to be a heavier gauge than it actually is. The same wire size fusible link must be used when replacing a blown fusible link.

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Fig. Good and damaged fusible links

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Fig. Double wire feed fusible link repair

Choose the shortest length that is available. A fusible link should NEVER be longer than nine inches. Fusible links longer than this will not provide sufficient overload protection.

To replace a damaged fusible link, cut it off beyond the splice. Replace with a repair link. When connecting the repair link, strip the wire and use staking-type pliers to crimp the splice securely in two places. To replace a damaged fusible link which feeds two harness wires, cut them both off beyond the splice. Use two repair links, one spliced to each wire harness.

A circuit breaker is a protective device designed to open a circuit when a current load is in excess of rated breaker capacity. If there is a short or other type of overload condition in the circuit, the excessive current will open the circuit between the circuit breaker terminals. There are two types of circuit breakers used in GM vehicles: cycling and non-cycling.

The cycling circuit breaker will open due to heat generated when excessive current passes through it for a period of time. Once the circuit breaker cools, it will close again after a few seconds. If the cause of the high current is still present, it will open again. It will continue to cycle open and closed until the condition causing the high current is removed.

There are two types of non-cycling breakers. One type is mechanical and is nearly the same as a cycling breaker. The difference is a small heater wire within the non-cycling circuit breaker. This wire provides enough heat to keep the bimetallic open until the current source is removed.

The other type is solid state, called an Electronic Circuit Breaker (ECB). This device has a Positive Temperature Coefficient. It increases its resistance greatly when excessive current passes through it. The excessive current heats the ECB. As it heats, its resistance increases, therefore having a Positive Temperature Coefficient. Eventually the resistance gets so high that the circuit is opened, removing voltage from its terminals. Once voltage is removed, the circuit breaker will re-close within a second or two.

The primary application of a circuit breaker is the headlights. If the headlamps were protected by a fuse and fault developed driving at night, the headlamps would go out, an unacceptable situation. So a cycling-type circuit breaker is built into the headlamp switch. If a fault develops, the headlamps will go our momentarily, then the breaker quickly resets and the headlamps will come on again. If the fault is still there, the headlamps will go out, the breaker will reset again, and the headlamp will come on again. If the cycle keeps repeating, the result is the headlamps will seem to continuously flash on and off. This at least allows the driver to safely stop the vehicle.

The windshield wiper motor is also protected by a circuit breaker. If the wiper motor overheats (wipers frozen to the windshield or stuck in heavy snow, stalling the motor, for example) the circuit breaker will trip, remaining off until the motor cools or the overload is removed.

There may also be some circuit breakers in the fuse box for the power door locks, power seats and power windows.