The alternator on the 1996-1997 4.3L Chevy S10 pickup and GMC Sonoma can be easily tested with a multimeter and in this tutorial, I'll show you how.
With the results of the three tests described in this tutorial, you'll easily and quickly determine if the alternator is good or bad.
By the way, the alternator test in this tutorial is an on-car test of the alternator. It does not have to be removed from the vehicle to test it.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar El Alternador (1996, 1997 4.3L V6 Chevrolet S10 Pickup, GMC Sonoma) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 4.3L V6 Chevrolet S10 Pickup: 1996, 1997.
- 4.3L V6 GMC Sonoma: 1996, 1997.
NOTE: You can find the alternator tests for the 1988-1995 4.3L V6 Chevy S10 pickup, GMC S15 pickup, and GMC Sonoma in the following tutorials:
- How To Test The Alternator (1988-1993 4.3L V6 Chevrolet S10 Pickup, GMC, S15 Pickup, GMC Sonoma).
- How To Test The Alternator (1994-1995 4.3L V6 Chevrolet S10 Pickup, GMC Sonoma).
Important Testing Tips
TIP 1: The battery must be fully charged to get the most accurate result from the alternator test described in this tutorial (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.
TIP 4: This is an on-car alternator test. You don't need to remove it to test it. The photos I'm using may show it off the vehicle, but this is just to better explain the test connections.
Symptoms Of A Bad Alternator
The starter motor requires quite a bit of battery power to crank and start the engine. This process of cranking the engine causes the battery to lose some of its charge.
Once the engine has started, the alternator charges the battery in preparation for the next time you need to crank and start the engine.
In addition to charging the battery, the alternator provides all the electrical power that your Chevrolet S10 pickup or GMC Sonoma's accessories (radio, wipers, headlights, etc.) need to run.
Sooner or later, the alternator will wear out and fail. When it does, 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 engine cranks and starts is if you charge the battery.
Alternator Connector Circuit Descriptions
The alternator delivers the current it produces to the battery through the cable connected to the stud on the back of the alternator. This circuit is known as a B+ circuit.
Only one wire comes out of the alternator's pigtail connector and it provides the alternator activation signal.
| Wire | Color | Description |
|---|---|---|
| B+ | Red (RED) | 12 Volts (Battery). Stud On Rear Of Alternator |
| B | Brown (BRN) | 12 Volts. Charge/Battery Lamp Circuit And Voltage Regulator Activation Signal |
TEST 1: Checking Alternator Voltage Output With A Multimeter
To get this show on the road, we'll check the battery's voltage with the engine running.
This battery voltage check will tell us immediately whether the alternator is charging the battery or not.
Specifically, if the battery voltage is between 13.5 Volts and 14.5 Volts with the engine running, you can conclude that the alternator is working properly and charging the battery.
If the battery voltage is 12.5 Volts with the engine running, you can conclude that the alternator is not charging the battery.
NOTE: If you don't have a multimeter or need to upgrade yours, take a look at my recommendation here: Buying A Digital Multimeter For Automotive Diagnostic Testing.
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 see what your test results mean:
CASE 1: Battery voltage stayed at 13.5 to 14.5 Volts during the entire test. This is the correct test result and it tells you the alternator is charging the battery.
Since the alternator is charging the battery, no further testing is required.
CASE 2: Battery voltage DID NOT stay at 13.5 to 14.5 Volts during the entire test. This test result generally tells you the alternator is failing under load.
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.
CASE 3: Battery voltage was at 12.5 Volts and decreased during the test. 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
In this test section, we'll check the continuity of the cable connecting the alternator to the battery.
This cable is protected by an inline fusible link and we need to check if it's blown.
If the inline fusible link is blown, no output from the alternator will reach the battery.
The quickest and easiest way to check the inline fusible link is to run a multimeter continuity test on the cable.
If the inline fusible link is blown, your multimeter will register the letters OL, or an Ohms reading the in the K-Ohms range.
If the inline fusible link is good, your multimeter will show continuity in the circuit.
NOTE: The photo above shows the alternator off of the vehicle to better explain the test connections. Do not remove the alternator from the vehicle to perform this test.
OK, let's start:
- 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 black 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 red 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.
Your next step is to go to: TEST 3: Checking The Alternator's L 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.
