If you suspect that the alternator has failed on your 2.2L Chevrolet S10 or GMC Sonoma, this tutorial is a simple and straightforward guide to testing the alternator with a multimeter.
You'll quickly find out if the alternator is good or bad in three easy tests.
NOTE: This is an on-car alternator test. No need to remove it to test it.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar El Alternador (1998-2003 2.2L Chevrolet S10, GMC Sonoma) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 2.2L Chevrolet S10: 1998, 1999, 2000, 2001, 2002, 2003.
- 2.2L Chevrolet Sonoma: 1998, 1999, 2000, 2001, 2002, 2003.
- 2.2L Isuzu Hombre: 1998, 1999, 2000.
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.
Symptoms Of A Bad Alternator
Cranking and starting the engine drains the battery and as you've guessed it, it's the alternator that's tasked with charging it back up.
Sooner or later, the alternator will fail, and 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 car cranks and starts is if you charge the battery.
TEST 1: Checking Alternator Voltage Output With A Multimeter
The battery in your Chevy S10 (GMC Sonoma) will have a voltage between 13.5 and 14.5 Volts (with the engine running) when its alternator is functioning correctly.
So the first thing I'll ask you to do is to start the engine and check the battery's voltage with a multimeter.
If the alternator has failed, your Chevy S10 or GMC Sonoma's battery voltage will be around 12.5 Volts DC. These 12 Volts will decrease the longer the engine stays running.
If the alternator is OK, then you'll see a battery voltage around 13.5 to 14.5 Volts DC.
NOTE: If you don't have a multimeter or need to upgrade yours, take a look at my recommendatin here: Buying A Digital Multimeter For Automotive Diagnostic Testing.
Let's get going:
- 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 maintained a 13.5 to 14.5 Volts value thru' out the whole test. This is the correct test result and it tells you the alternator is functioning correctly.
Since the alternator is charging the battery, no further testing is required.
CASE 2: The multimeter DID NOT maintain 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 alternator delivers its output (amperage) to the battery through the cable that connects to the stud located at the rear of the alternator.
In the image above, I've labeled the stud (the cable connects to) with the orange arrow with the '+' symbol.
This cable is protected by an inline fusible link and in this test section, you'll check the integrity of this inline fusible link with a simple multimeter continuity test.
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.
You can conclude that the alternator is bad and needs replacement 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 (this test section).
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.
