How To Test The Alternator (1987-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma)

The alternator can be easily tested with a simple multimeter without having to remove it from the vehicle.

In this tutorial, you'll find step-by-step instructions on how to test the alternator and find out if it's good or bad.

You can find this tutorial in Spanish here: Cómo Probar El Alternador (1987-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 2.5L Chevrolet S10 Pickup: 1987, 1988, 1989, 1990, 1991, 1992, 1993.
• 2.5L GMC S15 Pickup: 1987, 1988, 1989, 1990.
• 2.5L GMC Sonoma: 1991, 1992, 1993.

Important Testing Tips

TIP 1: The battery must be fully charged before starting the indicated tests in this tutorial.

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 probably already know, the alternator has two main jobs. These are:

• Charge the battery.
• Provide the electrical current needed to keep the engine and vehicle accessories running once the engine is started.

In the event the alternator fails, you usually cannot crank and start the engine because the battery is dead.

Besides a dead 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 has three wires connected to it. One wire (cable) is attached to a stud on the back of the alternator with a nut.

The other two wires go to the alternator pigtail connector. Here is 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

A properly working alternator will keep the battery voltage between 13.5 and 14.5 Volts while the engine is running.

With this in mind, let's start the engine and check the the battery's voltage. If your multimeter shows a voltage of 13.5 to 14.5 Volts, you can conclude that the alternator is charging the battery.

If the alternator is not charging the battery, your multimeter will show a battery voltage of about 12 Volts. These 12 Volts will decrease the longer the engine runs.

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: Battery voltage is and stayed between 13.5 to 14.5 Volts even with everything you on. This is the correct test result and it confirms your 2.5L Chevy S10 pickup or GMC S15 pickup's alternator is functioning correctly.

Since the alternator is charging the battery, no further testing is required.

CASE 2: Battery voltage is 12.5 Volts and decreased as you turned everything on. 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 cable that connects to the stud on the rear of the alternator is the one that delivers the alternator's output to the battery.

This cable is protected by an inline fusible link and it's not uncommon for it to get blown.

In this test section, we'll check the continuity of the cable to determine if the inline fusible link is good or blown.

If the inline fusible link is blown, your multimeter will report an open-circuit problem in the cable.

OK, let's begin:

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 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. 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. 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). So far so good since this test result confirms the inline fusible link protecting the cable 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.