Charging system output test-measures current and voltage output of the charging system.

Regulator voltage test- measures charging system voltage under low output, low load conditions.

Regulator bypass test- connects full battery voltage to the alternator field, leaving the regulator out of the circuit.

Circuit resistance tests- measures resistance in insulted and grounded circuits of the charging system.

Charging system tests are performed in two ways- by using a load tester or by using a volt-ohm-millimeter (VOM/ multimeter). The load tester provides the accurate method for testing a charging system by measuring both system current and voltage.

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 manufacturer's 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 manufacturer's 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 manufacturer's 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 manufacturer's specifications.

If the voltmeter reading is steady and within manufacturer's 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 alternator.

CAUTION

Follow the manufacturer's 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.