Figure 17-2. Ammeter Operation

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Chapter 17. INSTRUMENTS, GAGES, AND ACCESSORIES

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TM-9-8000 Principles of Automotive Vehicles Manual

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Figure 17-4. Low Voltage Warning Lamp

TM 9-8000

directly from the alternator stator through the stator

terminal. When the ignition switch is closed, before the

engine is started, current flows through the resistor and

the indicator lamp to the alternator field, causing the

indicator lamp to light. After the engine is started, the

alternator begins to produce current, energizing the field

relay coil from the stator. The relay coil pulls the relay

points closed, shunting the alternator field directly to the

battery. This results in a zero potential across the

indicator lamp, causing it to go out.

17-3. Fuel Gages. Most fuel gages are operated

electrically and are composed of two units: the gage,

mounted on the instrument panel; and the sending unit,

mounted on the fuel tank. The ignition switch is included

Figure 17-2. Ammeter Operation

in the fuel gage circuit so that the electrical fuel gage

operates only when the ignition switch is on. Operation of

ignored because the average vehicle operator does not

the electrical gage depends on either coil action or

know how to Interpret its readings. The Indicator lamp

thermostatic action.

can be utilized in two different ways to indicate an

electrical system malfunction.

a. Thermostatic Fuel Gage: Self-Regulating (Fig. 17-

6). This gage configuration consists of an instrument

(1) Low-Voltage Warning (Fig. 17-4). The

panel gage and an electromechanical sending unit that is

indicator lamp can be set up to warn the operator

located inside of the fuel tank. The instrument panel

whenever the electrical system voltage has dropped

gage contains an electrically heated bimetallic strip that

below the normal operational range. The lamp is

is linked to a pointer. A bimetallic strip consists of two

operated by a calibrated relay that opens the circuit to it

dissimilar metals that, when heated, expand at different

whenever electrical system voltage is in the normal

rates, causing it to deflect or bend. In the case of the

range (13.2 to 14.5 volts for a 12-volt system). Whenever

instrument panel fuel gage, the deflection of the

the voltage falls below the normal range, the magnetic

bimetallic strip will result in the movement of the pointer,

field becomes insufficient to overcome the force of the

causing the gage to give

relay spring, which pulls the contact points closed. This

closes the circuit to the indicator lamp.

(2) No-Charge Indicator (Fig. 17-5). The

indicator lamp also can be set up to indicate whenever

the alternator is not producing current. The circuitry that

operates a no-charge indicator lamp usually is

incorporated in the voltage regulator. The voltage

regulator that is used on a vehicle equipped with a no-

charge indicator lamp contains a second element called

a field relay. The field relay has two contact points. One

contact point is connected to battery voltage through the

ignition switch. The other point also is connected the

same way, except for the inclusion of a series-parallel

arrangement of the no- charge indicator light and a

resistor. The resistor value Is matched with the

resistance of the indicator lamp so that their parallel

arrangement will produce a zero-voltage drop. When the

field relay Is open, alternator field current is supplied

through the resistor-indicator light combination. The

Figure 17-3. Voltmeter Operation

magnetic coil of the field relay is energized

17-2