ALTERNATOR TEST 2: Checking Continuity of Battery Circuit
The alternator delivers its output to the battery across one single circuit (wire). This is the wire that attaches to the back of the alternator with a nut.
This wire has an Inline Fusible Link that protects the alternator in case of an electrical short. Every now and then, this Inline Fusible Link will get blown. When this happens, the alternator good be OK and creating a Charge, but the blown Inline Fusible Link keeps this charge from reaching the battery and the rest of the vehicle.
This Inline Fusible Link is usually located at the starter motor solenoid and with this test step, you'll be able to verify if this Inline Fusible LInk is blown or not by doing a simple resistance test with your multimeter. This is what you'll need to do:
The most important step, in this test, is to disconnect the battery's negative terminal only. Do not proceed to the next steps till you do this first.
Once the negative battery cable is disconnected from the battery, set your multimeter in Ohms mode.
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.
Place the RED multimeter lead on this stud.
Now, place the BLACK multimeter lead on the battery positive terminal (at the battery).
If the Inline Fusible Link is not blown, your multimeter will register 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 .5 Ohms: This result indicates that the Inline Fusible Link is OK and not blown.
So far, it's looking like the alternator is BAD. The last test to make sure it is is also a very simple test and it involves checking a fuse only. For this test, go to ALTERNATOR TEST 3.
CASE 2: Your multimeter DID NOT register .5 Ohms, it registered OL: This result tells you that the Inline Fusible Link is blown.
You'll need to replace the Inline Fusible Link and re-test the battery's voltage with engine running. This should solve your No Charge Condition.
Now, one thing to keep in mind, is that an Inline Fusible Link doesn't get blown for no apparent reason. You'll need to do some investigating to see why it did get blown.
ALTERNATOR TEST 3: Checking the Alternator Fuse
If you're driving an older 3.8L equipped GM vehicle, more than likely this test step does not apply to you. You may have to consult your vehicle's repair manual to see if the alternator has a dedicated plastic blade type fuse that supplies 12 Volts to the alternator's voltage regulator.
If your 3.8L GM is equipped with this alternator fuse it will be located in the Underhood Fuse Box. Why is this fuse important? Because if this fuse is blown, the alternator's voltage regulator will not get the 12 Volts it needs and this will result in the alternator not charging the battery.
One last thing, if this test step confuses you, in that you can't find this fuse... don't worry too much about it. It's rare for this fuse to get blown and if it's possible, the fuse should be checked. If not, that's OK too.
Reconnect the negative battery cable, that you disconnected in ALTERNATOR TEST 2.
Locate the alternator fuse, which will be located in the fuse box in the engine compartment.
To find the exact location of this fuse, you'll need to consult your owner's manual or repair manual, since it every 3.8L GM model may have it in a different location.
Once you have located the fuse, pull it out and verify that it is not blown.
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.