How To Test The Alternator (1998-2003 2.2L Chevrolet S10, GMC Sonoma)

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

The alternator gets an activation signal to start charging from your Chevy S10 or GMC Sonoma's fuel injection computer (PCM = Powertrain Control Module) when you crank and start the engine.

This activation signal is in the form of 5 Volts and are 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. 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 4.5 to 5 Volts DC.

Let's analyze your test result:

CASE 1: The multimeter registered 4.5 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 below 12.5 Volts DC and continues to fall as the engine runs (TEST 1).
• Confirmed that the inline fusible link that protect 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 4.5 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).