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.

Here are the contents of this article at a quick glance:

  1. ALTERNATOR TEST 1: Checking Battery Voltage With Engine Running.
  2. ALTERNATOR TEST 2: Checking Continuity Of The Battery (+) Circuit.
  3. ALTERNATOR TEST 3: Checking The Alternator Fuse.
  4. Symptoms Of A BAD Alternator.
  5. Related Test Articles.

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 you multimeter in Volts DC mode.

  2. 2

    With the RED multimeter lead, touch the battery positive terminal. Now, using the BLACK multimeter 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.

  1. 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...