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

ALTERNATOR TEST 2 Checking Continuity Of The Battery (+) Circuit

The Power that the alternator produces, on your 3.1L or 3.4L V6, is delivered to the battery across a sigle wire (circuit). This circuit is protected by an inline fusible link, that in the older vehicles is located on the starter motor solenoid and in the newer vehicles, it's located on the Underhood Fuse Box.

Every now and then, this fuse or inline fusible link blows and the alternator's Power output never reaches the battery. This test step will help you to verify is this fusible link is blown or not.

This test is accomplished by doing a simple Ohms (resistance) test of the wire. This is what you'll do:

1. 1

   Disconnect the negative cable from the battery. This is important! The battery positive cable must remain connected to the battery.

2. 2

   After disconnecting the battery negative (-) cable, set your multimeter in Ohms mode.

3. 3

   Now, locate the one and only stud behind the alternator (see the photos in the image viewer). This is where the big wire, that supplies the alternator's output to the battery, is attached to with a nut.
   
   Touch this stud with the red multimeter test lead.

4. 4

   Now, with the black multimeter test lead, touch the battery positive (+) terminal at the battery.

5. 5

   If all is OK in this circuit and the inline fusible link is not blown, your multimeter will show an Ohms value of about 0.5 Ohms.
   
   If the inline fusible link is blown, your multimeter will register an infinite Ohms reading, which if you're using a digital multimeter, it'll display the letters OL.

OK, now that the testing part is done, let's take a look at what your results mean:

CASE 1: Your multimeter registered 0.5 Ohms. This result indicates that the inline fusible link is OK and not blown.

Now, there's just one more thing to check and you're done! The next test is to make sure that the alternator fuse in the underhood fuse box is not blown. For this test, go to: ALTERNATOR TEST 3.

CASE 2: Your multimeter DID NOT register 0.5 Ohms, it registered OL. This result tells you that the inline fusible link is blown.

Replace the fusible link and retest. More than likely your problem will be solved and the alternator will begin charging again. The exact location of this inline fusible link varies from one GM vehicle to another. You'll need to check a repair manual to be sure.

ALTERNATOR TEST 3: Checking The Alternator Fuse

OK, this last test verifies that the alternator fuse in the underhood fuse box is OK. This fuse provides the voltage regulator inside of the alternator with 12 Volts. Without these 12 Volts, the alternator will not produce a charge. This fuse is normally labeled GEN BATT and is a 10 Amp fuse

In this test, you're gonna' verify that there is indeed power (12 Volts) available in this circuit. This is what you'll need to do:

1. 1

   Reconnect the battery to it's negative battery cable, that you disconnected in the previous test.

2. 2

   Locate the alternator fuse, which will be located in the fuse box in the engine compartment.
   
   The exact location of this alternator fuse (GEN BATT) will vary from one vehicle to another and may be labeled differently. You'll need to consult your owner's manual or repair manual to find its exact location.

3. 3

   Once you have located the ALT fuse, pull it out and verify that it is not blown.

Let's analyze your test results:

CASE 1: The fuse is NOT blown. This means that the alternator's voltage regulator is getting power.

Also, this confirms that the alternator on your GM 3.1L or 3.4L equipped vehicle is bad and needs to be replaced.

CASE 2: The fuse IS blown. This tells you that the voltage regulator is not getting power.

Replace the fuse and start the vehicle up and check to see if the alternator is charging the battery.

Related Test Articles

I have written quite a few articles on the GM 3.1L and 3.4L equipped cars to help you solve the most common types of problems with specific troubleshooting tests. You can find the ones I've written for this web page, by checking out this link: GM 3.1L, 3.4L Index Of Articles.

At easyautodiagnostics.com, you'll find the following articles:

• How To Test The Ignition Coil Packs (GM 3.1L, 3.4L) (at: easyautodiagnostics.com).
• Testing The Ignition Module And Crank Sensor (GM 3.1L, 3.4L) (at: easyautodiagnostics.com).
• How To Test The MAF Sensor (at: easyautodiagnostics.com).
• How To Clean The MAF Sensor (at: easyautodiagnostics.com).
• How To Test The Throttle Position Sensor (GM 3.1L, 3.4L) (at: easyautodiagnostics.com).