TEST 2: Checking Alternator's Battery Output Circuit
In this test section, we're gonna' test the battery output circuit of the alternator. The battery circuit cable is the thick cable that connects to the rear of the alternator (via a nut and stud). In the illustration above, the red multimeter test lead points to this location. All of the alternator's output (current and voltage) reaches the battery via this cable.
Before the alternator's output reaches the battery, via this cable, it has to go through a fusible link or mega fuse. Every now and then, the fusible link (or mega fuse) that protects this circuit gets blown. Because testing this battery output circuit is pretty easy and fast, in this section we're gonna' to focus on confirming that it's okay.
The fastest and easiest way to check to see if this circuit is OK is by checking its continuity. This continuity test is done with a multimeter in Ohms mode.
NOTE: The 1997-1998 Ford F150 uses a 175 amp mega fuse. The 1999-2002 Ford F150 uses a 12 gauge inline fusible link.
This is what you'll need to do:
- 1
Disconnect the battery negative and positive cables from the battery. Do not skip this step.
- 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 study to which the battery circuit cable attaches 2 with a nut. - 3
If continuity exist 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)
If you've reached this point you have verified that the alternator is not charging the battery and that its battery output circuit has continuity.
In this test section, we're now gonna' make sure that the alternator's in-line fuse is not blown. This fuse is identified with the letter B in the photo above. It's located next to the starter motor solenoid.
NOTE: This test section only applies to the 1997 through 1998 Ford F150.
This is what you need to do:
- 1
Locate the alternator's in-line fuse, which will be located next to the starter motor solenoid (see photo above).
- 2
Once you have located the 20 amp in-line fuse, remove it and check that it's not blown.
- 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 conclude that the alternator is fried and needs to be replaced only if the following are true:
- You confirmed that the alternator is not charging the battery (TEST 1).
- You confirmed that the alternator's output circuit has continuity (TEST 2).
- In this test, you have confirmed that the alternator's in line fuse is not blown.
Taken into account these three test results, you can now correctly conclude that the alternator is bad and needs to be replaced.
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.
