Checking The Alternator On A 4.6L F150

So, you are the proud owner of a Ford F-150, but the alternator just failed on you. At least you think it is the alternator. The only way to know for sure is to test it. Don’t worry, since this won’t require any specialized equipment or a visit to the garage. This whole testing phase will require a simple voltmeter that can test DC voltage and a little know how. Below, you will learn more information about the alternator and how to test it.

Understanding The Alternator 

Before you just go and start trying to test the alternator you really need to understand what the alternator is and what functions it performs. An alternator has two main jobs. The first job is to charge the battery so that your vehicle will start every time you crank it. You might not know this, but starting your vehicle puts a huge load on the battery and drains it. The alternator will recharge the battery, so that the vehicle can start again when needed.
The other main job of the alternator is to feed DC voltage to all the accessories in your vehicle that need to be running. However, this could include accessories that are optional as well. For instance, the air conditioner, or radio would be optional accessories.

Signs That The Alternator Might Be Defective

There are several waning signs that you might see when the alternator fails or starts to fail. These signs and symptoms will be listed below.
• You might notice the battery light appears on your instrument panel
• The vehicle will only start if you jump it with jumper cables
• Your rpm’s will race up when you are just sitting in idle
• The headlights will be very dim as if they are not getting enough power

Voltage Test With Vehicle Running

So, now you know a little bit about the alternator. It is now time to start testing. In order to perform this test, the vehicle will need to be running. This might even require that you jump-start the vehicle. Allow the vehicle to run for 5 to 10 minutes before performing this test. This will ensure that the battery has enough time to properly charge.

Now, take your voltmeter, set it on DC volts, put the black lead on the negative battery terminal, and place the red lead on the positive battery terminal. After you have the voltmeter leads attached to the battery, hold them there for a minute or two and take notice of the readings. You are going to get two possible results.
The first result could be anywhere from a steady 13.5 volts to 14.5 volts DC. The second possible reading could be 12.5 volts, but with this reading, the volts will drop off the longer the engine is running.
The first reading of a steady 13.5 or 14.5 volts DC is what you should see if your alternator is working properly. If the reading drops off the longer the engine is running, you are looking at a defective alternator.

Voltage Testing With Accessories Running

Unfortunately, just because you got a good reading by performing the test above, it does not mean that you are done testing. You now need to turn all the accessories on inside the vehicle. Turn on the wipers, the heater, the lights, and the radio. Now perform the same test again. If the alternator is working like it should, the test result will read a steady 13.5 to 14.5 volts DC. If the voltage drops off the alternator is not charging the battery properly.

I recommend for aftermarket alternators.


*Henry Avera 3/30/2016

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