TEST 3: Checking The Alternator's F Circuit
The pigtail connector of the alternator of your 2.5L Chevrolet S10 pickup (GMC S15 pickup, GMC Sonoma) has two wires coming out of it.
One wire is the F (Field) wire and the other is the L (Lamp) wire.
In this test section, we'll check the F terminal of the connector to see if its delivering 10 to 12 Volts to the alternator.
The 12 Volts that terminal F supplies to the alternator come from the IGN-GAU (20A) fuse located in the instrument panel fuse box.
NOTE: The IGN-GAU Fuse (20A) is the fuse that supplies 12 Volts to the F and L terminals on all 1987-1993 2.5L Chevrolet S10 pickup, GMC S15 pickup and GMC Sonoma.
Let's get testing:
- 1
Reconnect the negative (-) battery cable to the battery's negative (-) post.
- 2
Disconnect the 2-wire pigtail connector from the alternator.
- 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
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
Have your helper turn the key to the ON position (RUN position) but ask him NOT to crank the engine.
- 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
In this final test section, we'll check that terminal L has 10 to 12 Volts with the key in the ON position.
This circuit does two 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
Disconnect the alternator's 2-wire connector if it isn't already.
- 2
Place your multimeter in Volts DC mode.
- 3
Connect the black multimeter test lead to the negative (-) battery terminal.
- 4
Turn the key to the ON position but don't crank or start the engine.
- 5
With the red multimeter test lead, probe the female terminal labeled with the letter L of your alternator's connector.
- 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 12.5 Volts DC and drops 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.
More 2.5L Chevy S10 Pickup, GMC S15 Pickup, And GMC Sonoma Tutorials
You can find a complete list of 2.5L Chevy S10 pickup, GMC S15 pickup, And GMC Sonoma tutorials here:
Here's a sample of the tutorials you'll find in the index:
- How To Test Engine Compression (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
- How To Test For A Blown Head Gasket (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
- How To Test The TPS (1987-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
- How To Test The MAP Sensor (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
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