7.6 Instruments, Gauges, and Accessories

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.

Battery 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, 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 (Figure 7-47). The coil is matched to the maximum current output of the charging unit, and this varies with different applications.

Figure 7-47 — Ammeter schematic.

The voltmeter provides a more accurate indication of the condition of the electrical system and is easier to interpret by the operator (Figure 7-48). During vehicle operation, the voltage indicated on the voltmeter is considered to be normal in a range of 13.2 to 14.5 volts for a 12-volt electrical system. As long as the system voltage remains in this range, the operator can assume that no problem exists. This contrasts with an ammeter, which gives the operator no indication of problems, such as an improperly calibrated voltage regulator, which could allow the battery to be drained by regulating system voltage to a level below normal.

Figure 7-48 — Voltmeter schematic.

The indicator lamp has gained popularity as an electrical system condition gauge over the years. Although it does not provide as detailed analysis of the electrical system condition as a gauge, it is considered more useful to the average vehicle operator. This is because it is highly visible when a malfunction occurs, whereas a gauge usually is ignored because the average vehicle operator does not know how to interpret its readings. The indicator lamp can be used in two different ways to indicate an electrical malfunction:

Figure 7-49 — Low voltage warning lamp schematic.

Figure 7-50 — No-charge indicator schematic.

Fuel Gauge

Most fuel gauges are operated electrically and are composed of two units—the gauge, mounted on the instrument panel; and the sending unit, mounted in the fuel tank. The ignition switch is included in the fuel gauge circuit, so the gauge operates only when the ignition switch is in the ON position. The basic fuel gauge circuit uses a variable resistor to operate either a bimetal or magnetic type indicator assembly (Figure 7- 51).

Figure 7-51 — Fuel gauge schematic.

Located in the trunk, the sending unit consists of a float and arm that operate a variable resistor. When the fuel tank is empty, the float is down so the variable resistance will be high. This allows only a little amount of current to flow through the fuel gauge. The bimetal arm stays cool and the needle shows that the tank is low.

When the tank is filled, the float rises to the top of the tank. This slides the wiper to the low resistance position on the variable resistor. More current then flows through the fuel gauge circuit. The bimetal arm heats up and warps to move the needle to the full side of the gauge.

Oil Pressure Gauge

A pressure gauge is used widely in automotive and construction applications to keep track of such things as oil pressure, fuel line pressure, air brake system pressure, and the pressure in the hydraulic systems. Depending on the equipment, a mechanical gauge, an electrical gauge, or an indicator lamp may be used.

The mechanical gauge uses a thin tube to carry an actual pressure sample directly to the gauge (Figure 7-52). The gauge basically consists of a hollow, flexible C-shaped tube called a bourbon tube. As air or fluid pressure is applied to the bourbon tube, it tends to straighten out. As it straightens, the attached pointer moves, giving a reading.

Figure 7-52 — Mechanical oil pressure gauge.

The electric gauge may be of the thermostatic or magnetic type as previous discussed (Figure 7-53). The sending unit that is used with each gauge type varies as follows:

Figure 7-53 — Electric oil pressure gauge.

The indicator lamp (warning light) is used in place of a gauge on many vehicles. The warning light, although not an accurate indicator, is valuable because of its high visibility in the event of a low-pressure condition. The warning light receives battery power through the ignition switch. The circuit to ground is completed through a sending unit. The sending unit consists of a pressure-sensitive diaphragm that operates a set of contact points that are calibrated to turn on the warning light whenever pressure drops below a set pressure.

Engine Temperature Gauge

The temperature gauge is a very important indicator in construction and automotive equipment. The most common uses are to indicate engine coolant, transmission fluid, differential oil, and hydraulic system temperatures. Depending on the type of equipment, the gauge may be mechanical, electric, or a warning light.

The electric gauge may be the thermostatic or magnetic type, as described previously. The sending unit that is used varies, depending upon application (Figure 7-54).

Figure 7-54 — Temperature sending unit.

The sending unit used with the thermostatic gauge consists of two bimetallic strips, each having a contact point. One bimetallic strip is heated electrically. The other strip bends to increase the tension of the contact points. The different positions of the bimetallic strip create the gauge readings.

The sending unit used with the magnetic gauge contains an electronic device called a thermistor whose resistance decreases proportionally with an increase in temperature.

The magnetic gauge contains a bourbon tube and operates by the same principles as the mechanical pressure gauge.

The indicator lamp (warning light) operates by the same principle as the indicator light previously discussed.

Transmission Temperature Gauge

A transmission temperature gauge operates on the same principles as the engine temperature gauge. The sending unit, a gauge and connection wire, may be mounted in the transmission oil pan, the cooling line between the radiator and the transmission, or in the valve body of the transmission. The importance of a transmission temperature gauge is that if the automatic transmission fluid gets too hot, it can actually start to boil. When this occurs, catastrophic transmission failure is eminent.

Speedometer and Tachometer

Mechanical Speedometers and Tachometers

Both the mechanical speedometer and the tachometer consist of a permanent magnet rotated by a flexible shaft. Surrounding the rotating magnet is a metal cup attached to the indicating needle. The revolving magnetic field exerts a pull on the cup that forces it to rotate. The rotation of the cup is countered by a calibrated hairspring. The influence of the hairspring and the rotating magnetic field on the cup produces accurate readings by the attached needle. The flexible shaft consists of a flexible outer casing made of either steel or plastic and an inner drive core made of wire-wound spring steel. Both ends of the core are molded square so they can fit into the driving member at one end and the driven member at the other end, and can transmit torque.

Gears on the transmission output shaft turn the flexible shaft that drives the speedometer. This shaft is referred to as the speedometer cable. A gear on the ignition distributor shaft turns the flexible shaft that drives the tachometer. This shaft is referred to as the tachometer cable.

The odometer of the mechanical speedometer is driven by a series of gears that originate at a spiral gear on the input shaft. The odometer consists of a series of drums with digits printed on the outer circumference that range from zero to nine. The drums are geared to each other so that each time the one farthest to the right makes one revolution, it will cause the one to its immediate left to advance one digit. The second to the right then will advance the drum to its immediate left one digit for every revolution it makes. This sequence continues to the left through the entire series of drums. The odometer usually contains six digits to record 99,999.9 miles or kilometers. However, models with trip odometers do not record tenths, therefore contain only five digits. When the odometer reaches its highest value, it will automatically reset to zero. Newer vehicles incorporate a small dye pad in the odometer to color the drum of its highest digit to indicate the total mileage is in excess of the capability of the odometer.

Electric Speedometers and Tachometers

The electric speedometer and tachometer use a mechanically driven permanent magnet generator to supply power to a small electric motor (Figure 7-55). The electric motor then is used to rotate the input shaft of the speedometer or tachometer. The voltage from the generator will increase proportionally with speed, and speed will likewise increase proportionally with voltage enabling the gauges to indicate speed.

Figure 7-55 — Electric speedometer and tachometer operation.

The signal generator for the speedometer is usually driven by the transmission output shaft through gears. The signal generator for the tachometer usually is driven by the distributor through a power takeoff on gasoline engines. When the tachometer is used with a diesel engine, a special power takeoff provision is made, usually on the camshaft drive.

Electronic Speedometers and Tachometers

Electronic speedometers and tachometers are self-contained units that use an electric signal from the engine or transmission. They differ from the electric unit in that they use a generated signal as the driving force. The gauge is transistorized and will supply information through either a magnetic analog (dial) or light-emitting diode (LED) digital gauge display. The gauge unit derives its input signal in the following ways:

An electronic tachometer obtains a pulse signal from the ignition distributor as it switches the coil on and off. The pulse speed at this point will change proportionally with engine speed. This is the most popular signal source for a tachometer that is used on a gasoline engine.

A tachometer that is used with a diesel engine uses the alternating current generated by the stator terminal of the alternator as a signal. The frequency of the AC current will change proportionally with engine speed.

An electronic speedometer derives its signal from a magnetic pickup coil that has its field interrupted by a rotating pole piece. The pickup coil is located strategically in the transmission case to interact with the reluctor teeth on the input shaft.


The horn currently used on automotive vehicles is the electric vibrating type. The electric vibrating horn system typically consists of a fuse, horn button switch, relay, horn assembly, and related wiring. When the operator presses the horn button, it closes the horn switch and activates the horn relay. This completes the circuit, and current is allowed through the relay circuit and to the horn.

Most horns have a diaphragm that vibrates by means of an electromagnetic. When the horn is energized, the electromagnet pulls on the horn diaphragm. This movement opens a set of contact points inside the horn. This action allows the diaphragm to flex back towards its normal position. This cycle is repeated rapidly. The vibrations of the diaphragm within the air column produce the note of the horn.

Tone and volume adjustments are made by loosening the adjusting locknut and turning the adjusting nut. This very sensitive adjustment controls the current consumed by the horn. Increasing the current increases the volume. However, too much current will make the horn sputter and may lock the diaphragm.

When an electric horn will not produce sound, check the fuse, the connections, and test for voltage at the horn terminal. If the horn sounds continuously, a faulty horn switch is the most probable cause. A faulty horn relay is another cause of horn problems. The contacts inside the relay may be burned or stuck together.

Windshield Wipers

The windshield wiper system is one of the most important safety factors on any piece of equipment. A typical electric windshield wiper system consists of a switch, motor assembly, wiper linkage and arms, and wiper blades. The descriptions of the components are as follows:

The windshield wiper switch is a multi-position switch, which may contain a rheostat. Each switch position provides for different wiping speeds. The rheostat, if provided, operates the delay mode for a slow wiping action. This permits the operator to select a delayed wipe from every 3 to 20 seconds. A relay is frequently used to complete the circuit between the battery voltage and the wiper motor.

The wiper motor assembly operates on one, two, or three speeds (Figure 7-56). The motor has a worm gear on the armature shaft that drives one or two gears, and in turn operates the linkage to the wiper arms. The motor is a small shunt-wound DC motor. Resistors are placed in the control circuit from the switch to reduce the current and provide different operating speeds.

Figure 7-56 — Wiper motor assembly.

The wiper linkage and arms transfer motion from the wiper motor transmission to the wiper blades. The rubber wiper blades fit on the wiper arms.

The wiper blade is a flexible rubber squeegee-type device. It may be steel or plastic backed and is designed to maintain total contact with the windshield throughout the stroke. Wiper blades should be inspected periodically. If they are hardened, cut, or split, they should be replaced.

When electrical problems occur in the windshield wiper system, use the service manual and its wiring diagram of the circuit. First check the fuses, electrical connections, and all grounds. Then proceed with checking the components.

Test your Knowledge

14. Which type of battery condition gauge provides the most accurate indication of the condition of the electrical system?

A. Ammeter
B. Voltmeter
C. Ohmmeter
D. Inductive ammeter

15. The signal generator for an electric tachometer used on a gasoline engine is driven by what component?

A. Transmission
B. Distributor
C. Coil

16. What type of oil pressure gauge has a bourbon tube?

A. Mechanical
B. Electrical
C. Magnetic
D. Electronic