How To Test The Alternator (1990-1994 3.1L V6 Chevrolet And Pontiac Vehicles)

The alternator is one of the easier components to test on your 3.1L V6-equipped Chevrolet or Pontiac vehicle.

In this tutorial, I'll explain how to test the alternator step by step. Your test results will help you determine if the alternator is good or bad.

You can find this tutorial in Spanish here: Cómo Probar El Alternador (1990-1994 3.1L Chevrolet y Pontiac) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 3.1L Chevrolet Beretta: 1990, 1991, 1992, 1993.
• 3.1L Chevrolet Cavalier: 1990, 1991, 1992, 1993, 1994.
• 3.1L Chevrolet Celebrity: 1990.
• 3.1L Chevrolet Corsica: 1990, 1991, 1992, 1993.
• 3.1L Chevrolet Lumina: 1990, 1991, 1992, 1993, 1994.
• 3.1L Pontiac 6000: 1990, 1991.
• 3.1L Pontiac Grand Prix: 1989, 1990, 1991, 1992, 1993.
• 3.1L Pontiac Sunbird: 1991, 1992, 1993, 1994.

Important Testing Tips

TIP 1: The battery must be fully charged before starting TEST 1 of 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're already aware, the battery discharges every time the engine is started.

It's the alternator's job to recharge the battery as soon as the engine starts.

The alternator also provides all the electrical current (amps) and voltage required by the accessories (radio, headlights, wipers, etc) when the engine is running.

Sooner or later the alternator fails, and when it does, it simply stops charging the battery.

When the alternator fails, you will 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 starting point of our alternator diagnosis is checking the battery voltage with the engine running.

If everything is OK with the alternator, you'll see a battery voltage between 13.5 and 14.5 Volts DC.

If the alternator is not charging the battery, you'll see a voltage of 12.5 Volts DC, decreasing the longer the engine runs.

IMPORTANT: The battery must be fully charged to get the most accurate test result.

Let's get started:

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 the correct and expected test result and confirms the alternator is charging the battery.

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 the alternator is not charging the battery.

Let's go to the next test which is checking the continuity of the wire connecting 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 and voltage output from the alternator is fed to the battery via the cable attached to the stud on the back of the alternator.

I've labeled this stud with the orange arrow with the "+" symbol in the image above.

An inline fusible link protects this output cable, and we'll verify that it hasn't blown by doing a simple multimeter continuity test (on the cable).

If there is continuity in the wire between the positive (+) battery terminal and the alternator, the inline fusible link is good, and we'll proceed to TEST 3.

NOTE: Do not disconnect the cable from the BAT stud. In the image above, the alternator is shown off of the vehicle to help explain the multimeter test connections.

Let's get the ball rolling:

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 have done 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 output wire connects to the stud indicated by the arrow (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). This is the correct and expected test result and confirms the inline fusible link (protecting the alternator's output cable) is OK.

So far, it's looking like the alternator is bad. Let's move on to the next test: TEST 3: Checking The Battery Voltage Sense Circuit Wire.

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, which is located on the starter motor solenoid, and retest.