How To Test The Alternator (1996-1997 4.3L V6 Chevrolet S10 Pickup, GMC Sonoma)

TEST 3: Checking The Alternator's L Circuit

For our final test, we'll check that the alternator is receiving a 12 Volts activation signal.

This 12 Volts activation signal comes from the circuit that illuminates the charge/battery lamp on your Chevy S10 pickup or GMC Sonoma's instrument panel.

The 12 Volts activation signal is delivered to the alternator via the brown (BRN) wire of the alternator's pigtail connector.

In the illustration of the alternator's pigtail connector above, the terminal labeled with the letter B connects to the BRN wire.

Let's get started:

1. 1

   Disconnect the 1-wire pigtail connector from the alternator.

2. 2

   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

3. 3

   Connect the red multimeter test lead to the female terminal labeled with the letter B of the alternator's pigtail connector (see the illustration above).

4. 4

   Have your helper turn the key to the ON position (RUN position) but ask him NOT to crank the engine.

5. 5

   Your multimeter should register 10 to 12 Volts DC.

Let's analyze your test result:

CASE 1: The multimeter registered 10 to 12 Volts DC. This is the correct and expected test result and confirms that the alternator's L circuit is OK.

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).
• In this test section, confirmed that the alternator's L circuit is receiving 10 to 12 Volts.

CASE 2: The multimeter DID NOT register 10 to 12 Volts DC. Without this voltage signal, the alternator will not turn activate and charge the battery.

The most likely cause of this missing activation signal is:

• The fuse that supplies 12 Volts to the battery (charge) lamp is blown.
• An open-circuit problem has occurred in the L circuit wire between the connector and the instrument panel or fuse box.
• The battery lamp is blown (in the instrument cluster).