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 unit.

The fuse element is made from a metal that has a low-melting point and is the weakest point in the electrical circuit. In case of a short circuit or other trouble, the fuse burns out first and this opens the circuit just as a switch would do. Visual examination of a burned-out fuse usually provides a quick indication of the problem. A discolored sight glass indicates the circuit has a short either in the wiring or one of its components. If the glass is clear, the problem may be an overload in the circuit.

When replacing a fuse, you should ensure that it has a rating equal to the one burned out. Also, ensure that the malfunction that caused the failure has been determined and repaired.

GAUGES

Just because everything checks out okay during the prestart operation does not mean it will stay that way throughout the workday. You must continually monitor certain conditions, such as water temperature, oil pressure, and so forth, to ensure the equipment is running correctly. You monitor them by watching the indicator (warning lights or gauges) on your equipment. On equipment, you may see an analog type of gauge (fig. 4-7) or color-coded indicators, as shown in figure 4-8.

Figure 4-7.-Analog gauges.

Figure 4-8.-Color-coded gauges.

Water Temperature Gauge

When operating a piece of equipment, you must monitor the water or coolant temperature gauge. If your machine has an analog type of gauge (fig. 4-7), you must know the correct operating temperature. Refer to the operator's manual to determine the operating temperature. Anytime the temperature reading on the gauge starts to rise, stop and determine the reason. Do not wait until the gauge is in the red zone to investigate.

WARNING

Use extreme caution when removing a hot radiator cap. Steam coming from the radiator will cause severe burns.

CAUTION

Do not add cold water to a hot engine when it is not running. Rapid cooling of an overheated engine will cause severe damage to the cylinder head and block. To cool an overheated engine down, leave the engine running and add water slowly. Watch for the steam that may be produced when adding water. Should a radiator hose burst, secure the engine before the temperature gets too high.

Oil Pressure Gauge

The oil pressure indicator is one of, if not, the most important gauges to watch. Every vehicle or equipment has an oil pressure warning light or a gauge. Should you start to lose oil pressure and the warning light comes on, secure the engine immediately. Operating equipment without proper oil pressure causes severe damage to the engine.

Check the oil level in the engine. If the oil level is low, add oil until it is at the proper level. Start the engine; if oil pressure does not register on the gauge or the light stays on for 30 seconds, secure the engine. Then document and report the problem.

Should the oil level be correct and no pressure registers on the gauge or the warning light stays on, secure the engine. Then document and report the problem.

NOTE: A rule of thumb is that after starting an engine, the oil pressure gauge should show 30 pounds of oil pressure after the engine runs for 30 seconds. Should the oil level be correct but no pressure registers on the gauge, secure the engine. Then document and report the problem.

Air Pressure Gauge

There is an air pressure gauge on each vehicle equipped with an air-brake system. The air pressure must be maintained within a range of 100 pounds per square inch (psi) to 120 psi. A warning light or buzzer should come on if the air pressure drops below 60 psi. If there is a rapid loss of air pressure, an air unit may have burst. A slow leak may sometimes be repaired by tightening a fitting.

WARNING

Do not operate any equipment with air brakes if there is an air leak.

With the engine at operating rpm, the air pressure system should build from 85 to 100 psi within 45 seconds in dual-air systems. In single-air systems (pre-1975), the pressure should build up from 50 to 90 psi within 3 minutes.