If you suspect that the alternator has failed and isn't charging the battery, this tutorial will show you how to test it.
The three tests described in this tutorial will help you quickly and easily determine if your alternator is good or bad.
By the way, all three tests are done with a multimeter, so you don't need expensive diagnostic test equipment.
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 (1998-2000 4.3L V6 Chevrolet S10 Pickup, GMC Sonoma) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 4.3L Chevrolet S10 Pickup: 1998, 1999, 2000.
- 4.3L Chevrolet Sonoma: 1998, 1999, 2000.
- 4.3L Isuzu Hombre: 1998, 1999, 2000.
Important Testing Tips
TIP 1: To properly diagnose the alternator as good or bad, the battery must be fully charged.
If the battery is not fully charged, charge it before starting the tests described in this tutorial.
TIP 2: You can use a digital multimeter or an analog multimeter.
TIP 3: Some of the alternator tests are done with the engine running. Take all necessary safety precautions and remain alert.
Symptoms Of A Bad Alternator
The alternator has two important functions:
- To charge the battery after the engine has started.
- To supply the current your vehicle's accessories (radio, wipers, blower motor, etc.) need to run.
When the alternator fails, 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.
- The headlights glow dim when turned on while driving at night.
- The engine 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.
TEST 1: Checking Alternator Voltage Output With A Multimeter
The first thing we'll do to start diagnosing the alternator is check the battery voltage with the engine running.
If the alternator has failed and isn't charging the battery, your multimeter will report a voltage of approximately 12.5 Volts DC. This voltage will decrease the longer the engine runs.
If the alternator is functioning correctly, your multimeter should report a voltage of 13.5 to 14.5 Volts DC regardless of how long the engine runs.
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.
These are the test steps:
- 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: Battery voltage remained at 13.5 to 14.5 Volts throughout the 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: Battery voltage DID NOT stay at 13.5 to 14.5 Volts throughout the test. This test result confirms that the alternator is struggling to charge 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.
CASE 3: Battery voltage is 12.5 Volts and dropped 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
The current produced by the alternator is fed to the battery via a single cable.
This cable attaches to a stud on the rear of the alternator. In the photo above, the stud is labeled with the orange arrow with a '+' symbol.
An in-line fusible link protects the cable and it is not uncommon for this fusible link to blow.
We'll check the continuity of the cable with a multimeter to determine if the inline fusible link is blown.
NOTE: The photo above shows the alternator removed from the vehicle to better illustrate 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 output cable is OK.
Your next step is to see if the alternator is receiving an activation signal. Go to: TEST 3: Making Sure The Alternator Is Getting An Activation Signal.
CASE 2: Your multimeter DID NOT register continuity, it registered OL. This test result confirms the inline fusible link protecting the cable is blown.
Your next step is to replace the inline fusible link and retest.
