Alternator Output Current Check

This check determines whether the alternator produces current equal to its rated output.

Preparation for Check

PROCEDURE

1. Before testing, check the following items:
– condition of the battery installed in the vehicle;
– alternator drive belt tension.

The battery used for the alternator output current check must be partially discharged. If the battery is fully charged, the check cannot be performed because the alternator load is insufficient.

2. Turn the ignition switch ON.
3. Disconnect the negative battery cable.
4. Disconnect the wire from alternator terminal "B".
5. Connect a DC ammeter with a 0–100 A range in series between terminal "B" and the wire disconnected from the alternator. Make sure the negative ammeter lead is connected to the wire disconnected from the alternator.

Connect the wires and ammeter leads very securely, because a large current will flow through the circuit.

6. Connect a voltmeter (0–20 V) between alternator terminal "B" and ground. Make sure the positive voltmeter lead is connected to alternator terminal "B".
7. Connect the tachometer and the wire to the negative battery terminal.

8. Leave the hood open.

Inspection

PROCEDURE

1. Make sure the voltmeter scale is clearly visible for reading the voltage. If the voltmeter reads 0 V, the circuit between alternator terminal "B" and the negative battery terminal is open, the fuse has blown, or there is a poor ground connection.

2. Start the engine and turn on the headlights.
3. Turn on the high beam headlights and set the heater fan to maximum airflow. Briefly increase the engine speed to 2,500 rpm and read the gauges; the ammeter will show the current output of the alternator.

After the engine starts, the battery charging current decreases very quickly, so this check must be performed very quickly.

Check result

PROCEDURE

1. The ammeter reading must exceed the specified limit value. If the current is lower than the limit value and the wire connected to the alternator output is in good condition, remove the alternator from the vehicle and inspect it.
Limit current value:
52.5 A for 75 A alternator
63.0 A for 90 A alternator

The limit current value is specified on the label attached to the alternator housing.

The current output of the alternator depends on temperature and load. Therefore, to obtain the limit current value, partially discharge the battery by turning on the headlights on the vehicle with the engine not running.

The limit current output of the alternator cannot be obtained if the alternator or ambient air temperature is too high.

2. After completing the alternator limit current test, return the engine to idle and turn the ignition off.
3. Disconnect the wire from the negative battery terminal.
4. Remove the tachometer, voltmeter and ammeter.
5. Connect the wire to the "B" terminal of the alternator.
6. Connect the wire to the negative battery terminal.