How To Test The Alternator (1994-1997 2.2L Chevrolet S10, GMC Sonoma)

In this tutorial, I'll explain how to test the alternator on your 1994-1997 2.2L Chevrolet S10 and GMC Sonoma.

I think you'll be pleasantly surprised at how easy it is to figure out if the alternator is good or bad.

All of the test steps are explained in a step-by-step manner.

You can find this tutorial in Spanish here: Cómo Probar El Alternador (1994-1997 2.2L Chevrolet S10, GMC Sonoma) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 2.2L Chevrolet S10: 1994, 1995, 1996, 1997.
• 2.2L Chevrolet Sonoma: 1994, 1995, 1996, 1997.
• 2.2L Isuzu Hombre: 1996, 1997.

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, the alternator is the component in charge of charging the battery after you crank and start the engine.

Sooner or later, the alternator will fail, and when it does, it'll stop charging the battery.

An alternator failure will produce 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

When the alternator functions correctly, the battery will have a voltage between 13.5 to 14.5 Volts with the engine running.

So our first order of business is to start the engine and check the battery's voltage with a multimeter.

If the alternator has failed, you'll see a battery voltage of 12.5 Volts DC. These 12 Volts will decrease the longer the engine stays running.

These are the test steps:

1. 1

   Start the engine and let it idle.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

   Check the battery's voltage with your multimeter.

4. 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. 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. 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

I'm sure you've noticed that the alternator has two wires connected to it.

One wire is attached to the back of the alternator with a nut and is the one that delivers the alternator's current output to the battery.

This battery cable is protected by an inline fusible link known to get blown now and then.

In this test section, you'll check the integrity of this inline fusible link with a simple multimeter continuity test (on the wire).

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. 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. 2

   Set your multimeter to Ohms mode.

3. 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. 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. 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.

If your 2.2L S10 (Sonoma) is a 1994 model year, go to: TEST 3: Checking The Battery Voltage Sense Circuit Wire.

If your 2.2L S10 (Sonoma) is a 1995-1997 model year, your next step is to ensure that the alternator is receiving an activation signal. Go to: TEST 4: Checking The Alternator's Activation Signal

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.