How To Test The Alternator (1998-2000 4.3L Chevrolet S10 Pickup, GMC Sonoma)

TEST 3: Making Sure The Alternator Is Getting An Activation Signal

When the engine starts, the alternator receives an activation signal from the fuel injection computer.

This activation signal will range between 1 to 5 Volts depending on how much current output the fuel injection computer deems the alternator needs to produce to charge the battery.

The more current the alternator needs to produce, the higher the voltage of the alternator activation signal will be (close to or 5 Volts DC).

Since TEST 1 confirmed that the alternator is not charging the battery, the alternator activation signal should be at approximately 4 to 5 Volts DC.

The alternator receives its activation signal from the wire that connects to the terminal labeled with the letter B of the alternator's 1-wire connector (see the illustration above).

IMPORTANT: This test is done with the engine running. Be careful and take all necessary safety precautions.

These are the test steps:

1. 1

   With the engine off, disconnect the alternator from its 1-wire electrical connector.

2. 2

   Connect the red multimeter test lead to the red wire of the alternator's 1-wire connector.
   
   NOTE: The safest/easiest way to connect the red multimeter test lead to the wire is with a wire piercing probe. You can see an example of this tool here: Wire Piercing Probe.

3. 3

   Reconnect the alternator's 1-wire electrical connector to the alternator.
   
   CAUTION: Make sure the multimeter and test leads/wires are clear of the serpentine belt and any engine moving parts.

4. 4

   Connect the black multimeter test lead to the battery negative (-) terminal.
   
   IMPORTANT: The multimeter test lead must connect to a clean spot on the battery's negative (-) terminal.

5. 5

   Have your helper start the engine.

6. 6

   Your multimeter should register 4 to 5 Volts DC.

Let's analyze your test result:

CASE 1: The multimeter registered 4 to 5 Volts DC. This is the correct and expected test result and confirms that the fuel injection computer is activating the alternator.

You can conclude that the alternator is bad if you have:

• Confirmed that battery voltage is at 12.5 Volts DC and continues to fall as the engine runs (TEST 1).
• Confirmed that the inline fusible link that protects the alternator's output wire is not blown (TEST 2).
• This test section confirmed that the fuel injection computer is sending an activation signal to the alternator.

CASE 2: The multimeter DID NOT register 4 to 5 Volts DC. Without an activation signal, the alternator will not activate and charge the battery.

The most likely cause of this missing activation signal is:

• An open-circuit problem in the red wire between the connector and the fuel injection computer.
• A bad fuel injection computer (very rare).