How To Test The Alternator (1988-1993 4.3L V6 Chevrolet S10 Pickup, GMC, S15 Pickup, GMC Sonoma)

TEST 3: Checking The Alternator's F Circuit

In this test section, we'll check that terminal F of the alternator's electrical connector is supplying 10 to 12 Volts to the alternator's field coil circuit.

The 12 Volts that terminal F supplies to the alternator come from the IGN-GAU (20A) fuse located in the instrument panel fuse box.

We'll check that terminal F is supplying 10 to 12 Volts with a multimeter voltage test.

NOTE: The IGN-GAU Fuse (20A) is the fuse that supplies 12 Volts to the F and L terminals on all 1988-1993 4.3L V6 Chevrolet S10 pickup, GMC S15 pickup and GMC Sonoma.

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 F 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

For our final test, we'll check that terminal L has 10 to 12 Volts with the key in the ON position.

This circuit does two very important things:

• Illuminates the battery light on your instrument cluster when the alternator is not charging the battery.
• Activates (excites) the alternator's voltage regulator to begin charging the battery.

These are the test steps:

1. 1

   Disconnect the alternator's 2-wire connector if it isn't already.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

   Connect the black multimeter test lead to the negative (-) battery terminal.

4. 4

   Turn the key to the ON position but don't crank or start the engine.

5. 5

   With the red multimeter test lead, probe the female terminal labeled with the letter L of your alternator's connector.

6. 6

   Your multimeter should register 10 to 12 Volts DC.

Let's interpret your test result:

CASE 1: The multimeter registered 10 to 12 Volts DC. This is the correct and expected test result.

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 protects the alternator's output wire is not blown (TEST 2).
• Confirmed that the IGN-GAU fuse is supplying 10 to 12 Volts to the F terminal (TEST 3).
• Confirmed in this last test section that the L terminal has 10 to 12 Volts DC.

CASE 2: The multimeter DID NOT register 10 to 12 Volts DC. This result tells you that one of three things have gone wrong:

• 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).

Although its beyond the scope of this tutorial to diagnose this issue, your next step is to find out why the L terminal does not have 10 to 12 Volts and resolve the problem.