How To Test The Alternator (2001-2003 4.3L V6 Chevrolet S10 Pickup, GMC Sonoma)

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

As mentioned at the beginning of the tutorial, the alternator is controlled by the fuel injection computer.

More specifically, the fuel injection computer sends the alternator an activation signal in the range of 1 to 5 Volts DC.

The activation signal will be in the 4 to 5 Volts range to get the alternator to produce more current to charge a low battery.

The alternator activation signal is delivered to the alternator by the red wire of the alternator's 1-wire connector.

In the illustration of the alternator connector above, the red wire connects to the terminal labeled with the letter B.

IMPORTANT: This test is done with the engine running. Please 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 tells you the alternator is receiving an activation signal to fully charge the battery.

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

• Confirmed that battery voltage 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).
• In this test section, you've confirmed that the fuel injection computer is sending the alternator an activation signal.

CASE 2: The multimeter DID NOT register 4 to 5 Volts DC. Without this activation signal, the alternator will not turn on 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).