Charging System Output Test
The charging system output test measures system
voltage and current under maximum load. To check
output with a load tester, connect tester leads as
described by the manufacturer, as you may have either
an inductive (clip-on) amp pickup type or a non-
inductive type tester. Testing procedures for an
inductive type tester are as follows:
With the load tester controls set as prescribed by
the manufacturer, turn the ignition switch to the
RUN position. Note the ammeter reading.
Start the engine and adjust the idle speed to test
specifications (approximately 200 rpm).
Adjust the load control on the tester until the
ammeter reads specified current output. Do not
let voltage drop below specifications (about 12
volts). Note the ammeter reading.
Rotate the control knob to the OFF position.
Evaluate the readings.
To calculate charging system output, add the two
ammeter readings. This will give you total charging
system output in amps. Compare this figure to the
specifications within the manufacturers manual.
Current output specifications will depend on the
size (rating) of the alternator. A vehicle with few
electrical accessories may have an alternator rated at
35 amps, whereas a larger vehicle with more electrical
requirements could have an alternator rated from 40 to
80 amps. Always check the manufacturers service
manual for exact values.
If the charging system output current tested low,
perform a regulator voltage test and a regulator bypass
test to determine whether the alternator, regulator, or
circuit wiring is at fault.
Regulator Voltage Test
A regulator voltage test checks the calibration of
the voltage regulator and detects a low or high setting.
Most voltage regulators are designed to operate
between 13.5 to 14.5 volt range. This range is stated for
normal temperatures with the battery fully charged.
Regulator voltage test procedure is as follows:
Set the load tester selector to the correct position
using the manufacturers manual. With the load
control OFF, run the engine at 2,000 rpm or
specified test speed. Note the voltmeter reading
and compare it to the manufacturers
If the voltmeter reading is steady and within
manufacturers specifications, then the regulator
setting is okay. However, if the volt reading is steady
but too high or too low, then the regulator needs
adjustment or replacement. If the reading were not
steady, this would indicate a bad wiring connection, an
alternator problem, or a defective regulator, and
further testing is required.
Regulator Bypass Test
A regulator bypass test is an easy and quick way of
determining if the alternator, regulator, or circuit is
faulty. Procedures for the regulator bypass test is
similar to the charging system output test, except that
the regulator be taken out of the circuit. Direct battery
voltage (unregulated voltage) is used to excite the rotor
field. This should allow the alternator to produce
maximum voltage output.
Depending upon the system there are several ways
to bypass the voltage regulator. The most common
ways are as follows:
Sorting a test tab to ground on the rear of the
alternator (if equipped).
Placing a jumper wire across the battery and
field terminals of the alternator.
With a remote regulator, unplug the wire from
the regulator and place a jumper wire across the
battery and field terminals in the wires to the
Follow the manufacturers directions to
avoid damaging the circuit. You must NOT
short or connect voltage to the wrong wires or
the diodes or voltage regulator may be ruined.
When the regulator bypass test is being performed,
charging voltage and current INCREASE to normal
levels. This indicates a bad regulator. If the charging
voltage and current REMAINS THE SAME, then you
have a bad alternator.
Circuit Resistance Test
A circuit resistance test is used to locate faulty
wiring, loose connections, partially burnt wire,
corroded terminals, or other similar types of problems.