How To Test The Alternator (GM 3.1L, 3.4L)

This article will walk you, step by step, through the testing of the alternator on your 3.1L or 3.4L GM car. No expensive testing equipment is needed, all you need is a multimeter to accomplish this test.

This is an on-car test of the alternator that's accomplished in three easy test steps.

To see if this alternator test article covers your specific 3.1L or 3.4L GM vehicle and alternator, you can take a look at the list of applications on the box labeled ‘Applies To:’ on the column on the right and scroll with the prev and next links.

Symptoms Of A Bad Alternator

The most common symptom of a bad alternator are:

• The most obvious will be that the battery light will be On in your vehicle's instrument cluster.
• When you turn on the headlights, the glow very dim.
• Your vehicle starts easily with a jump-start, but only runs a few minutes until it stalls.
• Every time you re-charge the battery, the vehicle starts fine, but after the initial trip, it needs to be jump-started.

ALTERNATOR TEST 1: Checking Battery Voltage With Engine Running

You'll need to crank and start the car, so the battery has to be charged enough so that the vehicle will stay running for at least 15 minutes. If possible, charge the battery for at least 45 minutes before attempting the alternator test.

Also, whether you use a digital or an analog multimeter, it doesn't matter. Both will do the job. Alright, the very first thing you need to do is find out how much voltage the alternator is producing.

This is what you'll need to do:

1. 1

   Have your helper start the vehicle. Place your multimeter in Volts DC mode.

2. 2

   With the red multimeter test lead, touch the battery positive (+) terminal. Now, using the black multimeter test lead, touch the negative battery terminal.

3. 3

   Observe the DC voltage your multimeter is registering. You'll need to know what this voltage is for the next step.
   
   Now, if the alternator is charging, you'll notice that the reading, on your multimeter, will be above 13.5 Volts. If it's not charging, this reading will be around 12 Volts DC.

4. 4

   Now, have your helper turn On the headlights, the A/C (or Heater) on high, windshield wipers, radio, etc. Have him or her turn everything inside that can be possibly be turned on
   
   The idea here is to place an electrical load on the charging system to see if it can respond and meet the electrical demand.

5. 5

   Now, eyeball your multimeter display, after you have turned all those things On.

6. 6

   After something is turned On, inside the vehicle, and if the alternator is working properly, the multimeter's voltage reading will drop slightly and then stabilize around 13.5 to 14.5 Volts.
   
   Or the multimeter will drop in voltage and continue to drop further the more things get turned On and the longer the vehicle runs. Usually dropping down to 9 Volts or less.
   
   OK, let's interpret your multimeter test results in the next page.

Take a look at the following test interpretations to find out which one best fits your multimeter test results:

CASE 1: Your multimeter registered 13.5 to 14.5 Volts from the beginning of the test to the end of the test: This test result lets you know that the alternator is working and charging.

Here's why: One indicator, that the alternator is producing enough electrical power to meet the demand of the vehicle is by producing at least 2 Volts over battery voltage (12.5 Volts). Since your multimeter test has confirmed this, the alternator is good.

CASE 2: If your multimeter DID NOT register 13.5 to 14.5 Volts from beginning to end and the voltage dropped to 9 Volts DC: This test result lets you know that the alternator is probably bad.

Here's why: An alternator that is not working (producing a charge) will not be able to produce any voltage. So, then the dropping voltage reading you saw on the multimeter is just the battery's voltage potential slowly being drained.

You could stop here and replace the alternator. Around 90% of the time, you would hit the nail right on the head and solve the no-charging condition. But to be absolutely sure, I suggest doing two more very simple tests. For this first one, Go to ALTERNATOR TEST 2.