How To Test The Alternator (1997-2002 4.2L Ford F150)

TEST 2: Checking Alternator's Battery Output Circuit

Checking Alternator's Battery Output Circuit. How To Test The Alternator (1997-2002 4.2L Ford F150)

As you're probably already aware, the charge that the alternator creates reaches the battery across a cable. This cable is connected to a stud on the back of the alternator via a nut. In the illustration above the red multimeter test lead points to this stud.

We need to make sure that the fuse that protects this circuit isn't blown. So what we're going to do in this test section, is make sure that this circuit has continuity.

If the fuse is blown, your multimeter will show no continuity (usually by showing you the letters OL). If the fuse is OK you'll see continuity.

This is what you'll need to do:

  1. 1

    Disconnect the battery negative and positive cables from the battery. Do not skip this step.

  2. 2

    Place your multimeter in ohms mode. With the red multimeter test lead, probe the battery positive cable (at the battery).

    With the black multimeter test lead, touch the stud on the rear of the alternator. This is the stud to which the battery circuit cable attaches to with a nut.

  3. 3

    If continuity exists in this cable, your multimeter should register a value of 0.5 Ohms or less. If continuity does not exist, your multimeter will read a value in thousands of Ohms or simply display the letters OL (which stand for over the limit).

Let's take a look at what your test results mean:

CASE 1: Your multimeter confirms that the circuit has continuity. This test result tells you that the mega fuse or inline fusible link is OK. In other words, if the alternator were working and charging, all of its charge would actually reach the battery.

If your F150 is a 1997-1998 year model, your next step is to check the alternator inline fuse. For this test go to: TEST 3: Checking The Alternator Inline Fuse (1997-1998 F150).

If your F150 is a 1999-2002 year model, your next step is to check the alternator fuse located in the under-hood fuse and relay box. For this test go to: TEST 4: Checking The Alternator Mini-Fuse (1999-2002 F150).

CASE 2: Your multimeter confirms that the circuit DOES NOT HAVE continuity. This test results let you know that the alternator's charge is not reaching its destination -the battery. Your next step is to replace the 175 amp mega fuse (1997-1998 F150) or the 12 gauge in-line fusible link (1999-2002 F150) to resolve the alternator's no charge problem.

If your F150 is a 1997-1998 year model, you can see the location of the 175 Amp mega-fuse here: Location Of The Alternator 175 Amp Mega-Fuse (1997-1998 F150).

If your F150 is a 1999-2002 year model, alternator's battery output fuse is located in the under-hood fuse and relay box. You can see it's location here: Location Of The Alternator Battery Output Fuse (1999-2002 F150).

TEST 3: Checking The Alternator Inline-Fuse (1997-1998 F150)

Checking The Alternator's 20 Amp Inline Fuse (1997-1998 4.6L Ford F150)

In this test, we're now gonna' check that the inline 20 Amp in-line fuse is not blown. In the photo above, the arrow with the letter B identifies this 20 Amp in-line fuse.

This 20 amp inline fuse is located right next to the 175A mega fuse. Both of these fuses are located next to the starter motor relay.

NOTE: This test section only applies to the 1997 through 1998 Ford F150.

This is what you need to do:

  1. 1

    Locate the alternator's in-line fuse, which will be located next to the starter motor solenoid (see photo above).

  2. 2

    Once you have located the 20 amp in-line fuse, remove it and check that it's not blown.

  3. 3

    If it is blown, replace it with a new one and repeat TEST 1.

Let's take a look at what your test results mean:

CASE 1: The alternator's in-line fuse IS NOT blown. This is the correct and expected test result.

You can now correctly conclude that the alternator is defective only if you have confirmed the following:

  1. Battery voltage is at 12.5 Volts DC or less with the engine running (TEST 1).
  2. The alternator's battery output circuit has continuity (TEST 2).
  3. In this test, you have confirmed that the alternator's in-line fuse is not blown.

CASE 2: The alternator's in-line fuse is blown. Replace the fuse and repeat TEST 1. If the alternator now charges the battery (after replacing this fuse), then this has resolved the no charge problem.

Ford Vehicles:

  • Ford F150
    • 1997,
      1998,
      1999,
      2000,
      2001,
      2002