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

TEST 4: Checking The Alternator Mini-Fuse (1999-2002 F150)

In this last test we're gonna' make sure that the alternator's 20 Amp mini-fuse is not blown.

This 20 amp mini-fuse is located in the under hood fuse and relay box. In the illustration above, this 20 Amp mini-fuse is identified with the arrow with the number 11.

NOTE: This test section only applies to the 1999 through 2002 Ford F150.

This is what you need to do:

1. 1

   Locate the alternator's mini-fuse. This mini-fuse is located in the under-hood fuse and relay box and is identified by the number 11 in the illustration above.

2. 2

   Once you have located the 20 amp mini-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 20 Amp mini-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 mini-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.

Location Of The Alternator 175 Amp Mega-Fuse (1997-1998 F150)

The alternator's 175 Amp mega fuse is located behind a plastic dust cover. It's on the passenger side of the firewall besides the starter motor solenoid (the starter motor solenoid is located next to the battery).

The arrow with the letter A points to the 175 Amp mega fuse that protects the alternators battery output circuit. The arrow with the letter B identifies the alternator fuse.

Location Of The Alternator Battery Output Fuse (1999-2002 F150)

The fuse that protects the alternator battery circuit on the 1999 through 2007 Ford F150 is a cartridge type fuse. I's located in the under-hood fuse and relay box. In the illustration above, this cartridge type of use is identified with the number 111.

On the 1999 Ford F150, this fuse will be a 50 amp fuse. On the 2000 through 2002 Ford F150 this fuse will be a 40 amp fuse.

More 4.2L Ford Diagnostic Tutorials

If this tutorial save the day, then you might be interested in a few others that I have written up. You can check out the rest of the 4.2L V6 Ford tutorials in this index:

• Ford 4.2L Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

• How To Test The Fuel Pump (4.2L Ford E150, E250, F150).
• How To Test The Fuel Injectors (4.2L Ford).
• How To Test The Throttle Position Sensor (4.2L Ford F150, E150, E250).
• How To Test The Ford EGR Valve EGR Vacuum Solenoid, DPFE Sensor.
• How To Test The Coil Pack (Ford 3.0L, 3.8L, 4.0L, 4.2L).