The alternator on your 4.3L V6 Chevy S10 pickup, GMC S15 pickup or GMC Sonoma can be tested with a multimeter, and in this tutorial, I'll show you how.
With your test results, you'll quickly determine if the alternator is good or bad, all without expensive diagnostic equipment.
NOTE: This tutorial applies to both the TBI and CPI fuel injected 4.3L V6 engines.
Contents of this tutorial:
- Important Testing Tips.
- Symptoms Of A Bad Alternator.
- Alternator Connector Circuit Descriptions.
- TEST 1: Checking Alternator Voltage Output With A Multimeter.
- TEST 2: Testing The Continuity Of The Alternator's Output Wire.
- TEST 3: Checking The Alternator's F Circuit.
- TEST 4: Checking The Alternator's L Circuit.
You can find this tutorial in Spanish here: Cómo Probar El Alternador (1988-1993 4.3L V6 Chevrolet S10 pickup, GMC S15 pickup, GMC Sonoma) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 4.3L Chevrolet S10 Pickup: 1988, 1989, 1990, 1991, 1992, 1993.
- 4.3L GMC S15 Pickup: 1988, 1989, 1990.
- 4.3L Chevrolet Sonoma: 1991, 1992, 1993.
NOTE: You can find the 1994-1997 4.3L V6 Chevy S10 pickup and GMC Sonoma's alternator test here:
- How To Test The Alternator (1994-1995 4.3L V6 Chevrolet S10 Pickup, GMC Sonoma).
- How To Test The Alternator (1996-1997 4.3L V6 Chevrolet S10 Pickup, GMC Sonoma).
Important Testing Tips
TIP 1: Before you start the alternator tests, make sure the battery is fully charged (since you'll have to crank and start the engine to test the alternator).
TIP 2: You can use a digital multimeter or an analog multimeter.
TIP 3: Take all necessary safety precautions. Be alert and think safety all of the time since you'll be working around a running engine.
Symptoms Of A Bad Alternator
As you're already aware, every time you crank and start the engine, the battery loses some of its charge.
It's the alternator's job to recharge the battery as soon as the engine starts and receives its activation signal.
When the alternator fails, it stops charging the battery. If the alternator isn't charging the battery, you'll see one or more of the following symptoms:
- The charge light (also known as the battery light) will be shining nice and bright on your vehicle's instrument cluster.
- Whenever you turn on the headlights (night driving), they glow very dim.
- The car won't crank. It will only crank and start if you jump start your vehicle.
- The only way the car cranks and starts is if you charge the battery.
Alternator Connector Circuit Descriptions
The alternator on your 4.3L V6 Chevrolet S10 pickup, GMC S15 pickup or GMC Sonoma has three wires connecting to it.
One wire (cable) connects to the stud on the rear of the alternator. The other two wires belong to the alternator's pigtail connector.
Here's a brief description of each:
| Wire | Color | Description |
|---|---|---|
| B+ | Red (RED) | 12 Volts (Battery). Stud On Rear Of Alternator |
| F | Pink with black stripe (PNK/BLK) | 12 Volts (Field Coil) |
| L | Brown (BRN) | 12 Volts (Charge Lamp Circuit) |
TEST 1: Checking Alternator Voltage Output With A Multimeter
The very first you need to do to diagnose the alternator as good or bad is to see what the battery's voltage is with the engine running.
This involves checking the battery's voltage with a multimeter. If the alternator charges the battery, you should see a voltage between 13.5 to 14.5 Volts DC.
If the alternator isn't charging the battery, you'll see a battery voltage of around 12 Volts, decreasing the longer the engine runs. Easy peasy, right?
Let's get started:
- 1
Start the engine and let it idle.
- 2
Place your multimeter in Volts DC mode.
- 3
Check the battery's voltage with your multimeter.
- 4
The multimeter should register 13.5 to 14.5 Volts.
If it doesn't, don't worry about this just yet, continue to the next step. - 5
Turn on every accessory possible while observing the multimeter. Like the headlights, the A/C or heater (high blower speed), the windshield wipers, the radio, the rear window defroster, etc.
As each accessory comes on, they'll place a load on the charging system (alternator). - 6
As each accessory comes on, your multimeter will do one of two things:
1.) The multimeter's voltage reading will decrease slightly and then stabilize around 13.5 to 14.5 Volts DC (when something comes on).
2.) The DC voltage reading will decrease to 10 Volts DC.
Let's analyze your multimeter test results:
CASE 1: The multimeter read 13.5 to 14.5 Volts value thru' out the whole test. This is the correct test result and it confirms your Chevy S10 pickup, GMC S15 pickup or GMC Sonoma's alternator is functioning correctly.
Since the alternator is charging the battery, no further testing is required.
CASE 2: The multimeter DID NOT read a 13.5 to 14.5 Volts value. This test result confirms that the alternator is not charging the battery.
The next step is to test the continuity of the wire that connects the alternator to the battery. For this test go to: TEST 2: Testing The Continuity Of The Alternator's Output Wire.
TEST 2: Testing The Continuity Of The Alternator's Output Wire
The battery receives the alternator's amperage and voltage output via a single wire (a cable to be more exact).
This output cable connects to the stud in the rear of the alternator. This stud is labeled with the orange arrow with the '+' symbol in the illustration above.
The engineers at GM, in all their wisdom, placed an inline fusible link to protect the cable (that connects to this stud), and it's not uncommon for it (the fusible link) to get blown.
When the fusible link gets blown, none of the alternator's output will reach the battery.
The focus of this test section is for you to check the cable's continuity and see if it has suffered an open-circuit problem (caused by a blown inline fusible link).
OK, these are the test steps:
- 1
Disconnect the battery negative (-) cable from the battery but leave the positive (+) cable connected to the positive (+) post.
IMPORTANT: Do not proceed to the next steps until you do this first. - 2
Set your multimeter to Ohms mode.
- 3
Connect the red multimeter test lead to the stud shown in the photo above.
The alternator's output wire connects to the stud the arrow points to (in the photo above). - 4
Connect the black multimeter test lead on the battery positive (+) terminal (at the battery).
The battery negative (-) wire must remain disconnected from the battery. - 5
Your multimeter will register one of two values:
1.) Continuity (usually an Ohms value of about 0.5 Ohms).
2.) No continuity (an infinite Ohms reading (OL)).
OK, let's interpret your test results:
CASE 1: Your multimeter registered continuity (usually 0.5 Ohms). This is the correct and expected test result and it tells you that the inline fusible link protecting the alternator's output wire is OK.
So far, it's looking like the alternator is bad. There's still one more test to do, go to: TEST 3: Checking The Alternator's F Circuit.
CASE 2: Your multimeter DID NOT register continuity, it registered OL. This test result confirms the inline fusible link protecting this wire is blown.
Your next step is to replace the inline fusible link and retest.
