How To Test The Alternator (1994-1995 4.3L V6 Chevrolet S10 Pickup, GMC Sonoma)

TEST 3: Checking The Alternator's F Circuit (1994 Only)

NOTE: This test only applies to the 1994 4.3L V6 Chevrolet S10 pickup and GMC Sonoma with a 2-wire alternator pigtail connector.

The rotor (inside the alternator) needs 12 Volts to create a magnetic field so that the alternator can generate current. Terminal C of the alternator's electrical connector is tasked with supplying this voltage.

The 12 Volts that terminal C supplies to the alternator come from the GAUGES (20A) fuse located in the instrument panel fuse box.

We'll check that terminal C of the alternator's 2-wire pigtail connector is supplying 10 to 12 Volts with a multimeter voltage test.

Let's get testing:

1. 1

   Reconnect the negative (-) battery cable to the battery's negative (-) post.

2. 2

   Disconnect the 2-wire pigtail connector from the alternator.

3. 3

   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

4. 4

   Connect the red multimeter test lead to the female terminal labeled with the letter C of the alternator's 2-wire pigtail connector (see the illustration above).

5. 5

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

6. 6

   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.

Your next and last step is to go to: TEST 4: Checking The Alternator's L Circuit.

CASE 2: The multimeter DID NOT register 10 to 12 Volts DC. Check the alternator fuse to see if its blown.

If the fuse is blown, replace it with a fuse of the same rating and repeat TEST 1.

If the alternator still does not charge the battery, go to: TEST 4: Checking The Alternator's L Circuit.

TEST 4: Checking The Alternator's L Circuit

The alternator needs an activation signal to start charging the battery, and it comes from the circuit that illuminates the charge/battery lamp on your Chevy S10 pickup or Sonoma's instrument panel.

The terminal that delivers the 12 Volts activation signal (to the alternator) is the wire that connects to terminal B of your alternator's pigtail connector.

So we'll check that terminal B is supplying 10 to 12 Volts with a multimeter voltage test.

These are the test steps:

1. 1

   Disconnect the 1-wire or 2-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 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).
• Confirmed that the alternator's F circuit is receiving 10 to 12 Volts (TEST 3 -1994 only).
• 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).