You can easily test the alternator with a multimeter to find out if it's bad or not. Not to mention that you don't need any expensive testing equipment to do it!
In this tutorial, I'll explain how to do it in a step-by-step manner. You'll easily and quickly find out if the alternator is causing a no-charge condition on your 4.0L Ford Explorer or 4.0L Ford Aerostar.
NOTE: You can find the 4.0L Ford Ranger, Mazda B4000, and Mercury Mountaineer alternator test tutorials here:
- How To Test The Alternator (1998-2001 4.0L V6 Mercury Mountaineer).
- How To Test The Alternator (1996-2001 4.0L Ford Ranger And Mazda B4000) (at: easyautodiagnostics.com).
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 4.0L V6 Ford Aerostar: 1992, 1993, 1994, 1995, 1996, 1997.
- 4.0L V6 Ford Explorer: 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000.
- Alternator Circuit Diagram (1996-1997 4.0L Ford Explorer).
- Alternator Circuit Diagram (1998-2001 4.0L Ford Explorer, Mercury Mountaineer).
Important Testing Tips
The following tips will help you accomplish the tests in this tutorial:
TIP 1: Before starting the alternator tests, you must fully charge the battery.
TIP 2: You can use a digital multimeter or an analog multimeter.
TIP 3: Take all necessary safety precautions. Be alert and think safety all of the time since you'll be working around a running engine.
Symptoms Of A Bad Alternator
When the alternator fails and doesn't charge the battery anymore, the first thing you'll notice is the battery malfunction indicator illuminated on your instrument panel.
Besides the battery light shining nice and bright on the instrument panel, you'll also see one or more of the following symptoms:
- The engine only start with a jump start. After a few minutes of run time, it dies.
- The engine starts fine if you charge the battery. After your initial trip, the battery needs a jump-start to crank and start the engine.
- Dim headlight glow dim.
TEST 1: Checking Alternator Voltage Output With A Multimeter
For your first diagnostic test is to test the battery's voltage with the engine running.
If the alternator is charging the battery, you'll see a voltage between 13.5 to 14.5 Volts DC on your multimeter.
If the alternator has failed and isn't charging the battery, you'll see a battery voltage around 12.5 Volts. These 12.5 Volts will decrease the longer the engine stays running.
NOTE: If you don't have a multimeter or need to upgrade yours, check out my recommendation here: Buying A Digital Multimeter For Automotive Diagnostic Testing (at: easyautodiagnostics.com).
IMPORTANT: You must perform this test with a fully charged battery.
These are the steps:
Place your multimeter in Volts DC mode.
Start the engine and let it idle.
Probe the battery terminals with your multimeter test leads.
The red multimeter test lead to battery positive (+) post. The black multimeter test lead to the battery negative (-) post.
You should see 13.5 to 14.5 Volts on your multimeter. If you don't, don't worry about this just yet, continue to the next step.
Place a load on the battery. You can do this by turning on the headlights, the A/C or heater on high, the windshield wipers, the radio, the rear window defroster, etc.
You'll see one of two things:
1.) The voltage will decrease slightly and then stabilize around 13.5 to 14.5 Volts DC when something comes on.
2.) The voltage reading will start to decrease till it reaches 10 Volts DC and your vehicle may stall.
Let's analyze your test results:
CASE 1: The voltage stayed at 13.5 to 14.5 Volts thru' out the whole test. This is the correct and expected test result and it confirms that the alternator is charging the battery.
No further testing is required.
CASE 2: The voltage is at 12.5 Volts or less and decreased as you turned on all accessories. This test result tells you the alternator is not charging the battery.
Around 90% of the time, replacing the alternator will solve the no-charge problem. But to be sure the alternator is bad, I suggest doing two more very simple tests. For the first one, go to: TEST 2: Testing The Continuity Of The Alternator's Battery Circuit.
CASE 3: The voltage started at 13.5 Volts to 14.5 Volts, but decreased to 12.5 Volts or less as you turned on all accessories. Generally, this test result indicates the alternator is bad.
Before you replace the alternator, I suggest doing two more tests. For the first one, go to: TEST 2: Testing The Continuity Of The Alternator's Battery Circuit.