TEST 2: Making Sure The Alternator's Battery Circuit Has Continuity
All the current that your Ford alternator produces, to charge the battery, is transmitted to the battery across one wire.
Before this wire reaches the battery though, it's connected to an inline fusible link. This inline fusible link is in place to protect the circuit in case of an internal failure of the alternator or a short-circuit in the wire itself.
In certain cases, this inline fusible link will get blown. And once it does, the current that the alternator is producing will not reach the battery (even though the alternator is working correctly).
So, in this section we're gonna' check that this fusible link is OK by doing a simple multimeter continuity test. If the inline fusible link is blown, your multimeter will read an open circuit (no continuity).
IMPORTANT: Before you start this test, you need to disconnect the battery negative cable from the battery negative post. Failure to do this can damage your multimeter.
NOTE: All of the tests in this tutorial are done with the alternator in its place in your Ford's engine compartment. Do not remove the alternator to test it. In the illustration, the alternator is shown removed just to facilitate the explanation of the test connections.
Alright, let's get testing:
Disconnect the battery negative (-) terminal from the battery negative post. Leave the positive cable connected to the positive post.
IMPORTANT: Don't proceed to the next step without first disconnecting the battery from its negative cable.
Place your multimeter in Ohms mode.
Connect the black multimeter test lead to the center of the battery positive post.
Connect the red multimeter test lead to the stud on the rear of the alternator. The orange arrow, in the illustration above, points to this stud on the Ford Mustang (or Ford Thunderbird or Mercury Cougar) alternator.
You'll see one of two results: The multimeter will register continuity or it won't.
If the multimeter register's continuity, you'll usually see 0.5 Ohms. If it registers no continuity, you'll see the letters OL displayed.
Let's interpret your multimeter continuity test result:
CASE 1: Your multimeter registered continuity in the circuit. This is the correct test result.
Now there's one more test to do and this is to make sure that the alternator's voltage regulator is being fed 10 to 12 Volts DC. For this test go to: TEST 3: Making Sure The Voltage Regulator Is Getting 12 Volts.
CASE 2: Your multimeter indicates that the circuit does not have continuity. This test result usually means that the line fusible link that protects this wire is blown. If this inline fusible link is blown, the alternator's current will not reach the battery.
Your next step is to check that this 10 gauge inline fusible link okay. If the 10 gauge inline fusible link is blown, replace it with another and return to and repeat TEST 1.
TEST 3: Making Sure The Alternator's Voltage Regulator Is Getting 12 Volts
In this test section we're gonna' make sure that the voltage regulator (which is bolted onto the rear of the alternator) is getting 12 Volts DC.
In case you're wondering, these 12 Volts are what activate the alternator to start charging the battery. These 12 Volts are provided by an inline fusible link located in the engine compartment.
NOTE: Avoid probing the front of the voltage regulator's connector. To be able to read the voltage (inside the indicated terminal) you're gonna' have to use a back probe or a wire piercing probe on the wire. You can see an example of a wire piercing probe here Wire Piercing Probe.
OK, these are the test steps:
Unplug the alternator voltage regulator from its electrical connector. You'll find the voltage regulator bolted to the rear of the alternator (see illustration above).
Place your multimeter in Volts DC mode..
Verify that the terminal identified with the letter A has 10 to 12 Volts with the key on or the key off.
NOTE: Avoid probing the front of the terminal with your multimeter's test leads or you will damage it. You'll need to use a back-probe or a wire piercing probe to test for the presence of this voltage in the wire.
Let's interpret your multimeter voltage test result:
CASE 1: Your multimeter registered 10 to 12 Volts. This is the correct test result and lets you know that the voltage regulator is getting the power it needs to activate the alternator.
You can correctly conclude that the alternator is defective if you have:
- Confirmed that battery voltage is at 12 Volts with the engine running (TEST 1). And that this voltage decreases the longer the engine stays running.
- Confirmed that battery circuit wire that connects to the rear of the alternator has continuity (TEST 2).
- Confirmed that the voltage regulator is getting 10 to 12 Volts (TEST 3).
CASE 2: The multimeter did not register 10 to 12 Volts. Without 10 to 12 Volts, the voltage regulator will not activate the alternator to start charging the battery.
The most common cause of this lack of power (to the alternator's voltage regulator) is usually due to a blown fusible link. This 18 gauge fusible link connects to the under hood fuse box in the engine compartment.
Your next step is to make sure that this 18 gauge inline fusible link is not blown. If the 18 gauge fusible link is blown, replace it and repeat TEST 1.