CIRCUIT BREAKERS AND FUSES

Fuses are safety devices placed in electrical circuits to protect wires and electrical units from a heavy flow of current. Each circuit, or at least each individual electrical system, is provided with a fuse that has an ampere rating for the maximum current required to operate the units. The fuse element is made from metal with a low-melting point and forms the weakest point of the electrical circuit. In case of a short circuit or other trouble, the fuse will be burned out first and open the circuit just as a switch would do. Examination of a burnt-out fuse usually gives an indication of the problem. A discolored sight glass indicates the circuit has a short either in the wiring or in one of its components. If the glass is clear, the problem is an overloaded circuit. Be sure when replacing a fuse that it has a rating equal to the one burned out. Ensure that the trouble of the failure has been found and repaired.

A circuit breaker performs the same function as a fuse. It disconnects the power source from the circuit when current becomes too high. The circuit breaker will remain open until the trouble is corrected. Once the trouble is corrected, a circuit breaker will automatically reset itself when current returns to normal levels. The fuses and circuit breakers can usually be found behind the instrument panel on a fuse block (fig. 2-72).

REVIEW 4 QUESTIONS

Q1. By what percentage is light output increased when using halogen headlights?

Q2. What component of the headlight switch allows for adjusting the brightness of the instrument panel lights?

Figure 2-72. - Fuse block.

Q3. When using a headlight-aiming screen, you place the screen at what distance in front of the vehicle?

Q4. On a 20 percent downgrade, blackout taillights should be invisible from the air at what distance?

Q5. On most automotive vehicles, the brake light switch is mounted at what location?

INSTRUMENTS, GAUGES, AND ACCESSORIES

Learning Objective: Identify instrument, gauges, and accessories, their functions, and maintenance procedures.

The instrument panel is placed so that the instruments and gauges can easily be read by the operator. They inform the operator of the vehicle speed, engine temperature, oil pressure, rate of charge or discharge of the battery, amount of fuel in the fuel tank, and distance traveled. Vehicle accessories, such as windshield wipers and horns, provide the operator with much needed safety devices.

BATTERY CONDITION GAUGE

The battery condition gauge is one of the most important gauges on the vehicle. If the gauge is interpreted properly, it can be used to troubleshoot or prevent breakdowns. The following are the three basic configurations of battery condition gauges - ammeter, voltmeter, and indicator lamp.

The AMMETER is used to indicate the amount of current flowing to and from the battery. It does NOT give an indication of total charging output because of other units in the electrical system. If the ammeter shows a 10-ampere discharge, it indicates that a 100 ampere-hour battery would be discharged in 10 hours, as long as the discharge rate remained the same. Current flowing from the battery to the starting motor is never sent through the ammeter, because the great quantities of amperes used (200 to 600 amperes) cannot be measured due to its limited capacity. In a typical ammeter (fig. 2-73), all the current flowing to and from the battery, except for starting, actually is sent through a coil to produce a magnetic field that deflects the ammeter needle in proportion to the amount of current. The coil is matched to the maximum current output of the charging unit, and this varies with different applications.

The VOLTMETER (fig. 2-74) provides a more accurate indication of the condition of the electrical 2-51