Previous Next

Alternator

TESTING

General Information

There are many possible ways in which the charging system can malfunction. Often the source of a problem is difficult to diagnose, requiring special equipment and a good deal of experience. This is usually not the case, however, where the charging system fails completely and causes the dash board warning light to come on or the battery to discharge. To troubleshoot a complete system failure, only two pieces of equipment are needed: a test light, to determine that current is reaching a certain point and a current indicator (ammeter), to determine the direction of the current flow and its measurement in amps. This test works under three assumptions:

  1. The battery is known to be good and fully charged.
  2. The alternator belt is in good condition and adjusted to the proper tension.
  3. All connections in the system are clean and tight.

    NOTE: In order for the current indicator to give a valid reading, the vehicle must be equipped with battery cables which are of the same gauge size and quality as original equipment battery cables.

    Before commencing with the following tests, turn off all electrical components on the vehicle. Make sure the doors of the vehicle are closed. If the vehicle is equipped with a clock, disconnect the clock by removing the lead wire from the rear of the clock.

Battery No-Load Test
  1. Ensure that the ignition switch is turned OFF.
  2. Connect a tachometer to the engine by following the manufacturer's instructions.
  3. Using a Digital Volt Ohmmeter (DVOM) measure the voltage across the positive (+) and negative (-) battery terminals. Note the voltage reading for future reference.

    Ensure that all electrical components on the vehicle are turned off. Be sure the doors of the vehicle are closed. If the vehicle is equipped with a clock, disconnect the clock by removing the lead wire from the rear of the clock.

  4. Start the engine and have an assistant run it at 1500 rpm.
  5. Read the voltage across the battery terminals again. The voltage should now be between 14.1–14.7 volts.
    1. If the voltage increase is less than 2.5 volts over the base voltage measured in Step 3, perform the Battery Load test.
    2. If there was no voltage increase, or the voltage increase was greater than 2.5 volts, perform the Alternator Load and No-Load tests.
Battery Load Test
  1. With the engine running, turn the air conditioner ON (if equipped) or the blower motor on high speed and the headlights on high beam.
  2. Have your assistant increase the engine speed to approximately 2000 rpm.
  3. Read the voltage across the battery terminals again.
    1. If the voltage increase is 0.5 volts over the base voltage measured in Battery No-Load test Step 3, the charging system is working properly. If your problem continues, there may be a problem with the battery.
    2. If the voltage does not increase as indicated, perform the Alternator Load and No-Load tests.
Alternator Load Test

WARNING
Do NOT use a normal Digital Volt Ohmmeter (DVOM) for this test; your DVOM will be destroyed by the large amounts of amperage from the car's battery. Use a tester designed for charging system analysis, such an Alternator, Regulator, Battery and Starter Motor Tester unit.

  1. Switch the tester to the ammeter setting.
  2. Attach the positive (+) and negative (-) leads of the tester to the battery terminals.
  3. Connect the current probe to the B+ terminal on the alternator.
  4. Start the engine and have an assistant run the engine at 2000 rpm. Adjust the tester load bank to determine the output of the alternator. Alternator output should be within ten percent of the alternator's output rating; if so, continue with the Alternator No-Load test. If the output is not within ten percent of the alternator's output rating, there is a problem in the charging system. Have the system further tested by a Ford qualified automotive technician.
Alternator No-Load Test
  1. Using the same tester as in the Alternator Load Test, switch the tester to the voltmeter function.
  2. Connect the voltmeter positive (+) lead to the alternator B+ terminal and the negative (-) lead to a good engine ground.
  3. Turn all of the electrical accessories off and shut the doors.
  4. While an assistant operates the engine at 2000 rpm, check the alternator output voltage. The voltage should be between 13.0–15.0 volts. If the alternator does not produce voltage within this range there is a problem in the charging system. Have the system further tested by a Ford qualified automotive technician.

Previous Next