Lesson 4/Learning Event 3
Voltmeters are gaining popularity as an instrument panel battery condition indicator. This is
because the electrical system voltage is a more accurate indication of the condition of the electrical
system than the amperage and is easier to interpret by the operator. 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 that is below normal.
Indicator Lamp
Over the years, the indicator lamp has gained popularity as an electrical system condition gage.
Although it does not provide as detailed an analysis as a gage, it usually is considered more useful
to the average vehicle operator. This is because it is highly visible when a malfunction occurs,
whereas a gage 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
system malfunction.
- Low-voltage warning indicator. The indicator lamp can be set up to warn the
operator whenever the electrical system voltage has dropped below the normal
operational range. The lamp is operated by a calibrated relay that opens the circuit to
it whenever electrical system voltage is in the normal range (13.2 to 14.5 volts for a
12-volt system). Whenever the voltage falls below the normal range, the magnetic
field becomes insufficient to overcome the force of the relay spring, which pulls the
contact points closed. This closes the circuit to the indicator lamp.
- No-charge indicator. 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 is connected to
battery voltage through the ignition switch; the other 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
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