How to Test a Ford F-350 Alternator

by Grace Mclain

The alternator on the Ford F-350 keeps the battery charged and the engine running. Once the alternator starts to go bad, the battery will become weaker and weaker until it can no longer crank the engine. Signs of a weak alternator are dim headlights and slow engine starting. The alternator regulates the entire electrical system of the truck. Testing the alternator on your Ford F-350 is a very easy process, and you can do it yourself in just a few minutes.

1

Crank the engine and increase the RPMs to 2000. This will put the alternator back to a full charge. Rev the engine at 2000 RPMs for about 20 seconds. Then let the engine come back to an idle.

2

Pop the hood and, while the engine is running, remove the negative battery cable with an adjustable wrench. The negative battery cable will have a (-) symbol stamped on the negative side of the battery. If the engine is still running after you have removed the negative battery cable, then the alternator output is good. Put the negative battery cable back onto the battery post.

3

Turn the headlights on from inside of the truck. Place the positive probe of the voltmeter on the positive battery post. Then put the negative probe of the voltmeter on the negative battery post.

4

Check the voltmeter while the engine is idling and the lights are on. A good alternator should have a volt output reading between 13 and 14.5 volts. If the voltage reading is below 13, then the alternator is weak and starting to go bad. If the voltage reading is above 14.5, then the voltage regulator inside of the alternator is overcharging the battery, and it is starting to go bad.

5

Remove the voltmeter probes from the battery and close the hood. Turn the lights off then shut the engine off.

Tips

  • check You can purchase a voltmeter at most auto parts stores.
  • check Make sure to tighten the negative battery cable onto the battery to get an accurate alternator output reading. Also make sure that the connectors on the back of the alternator are tight.

Warning

  • close Wear safety glasses when working around the battery.

Items you will need

About the Author

Grace Mclain has been writing professionally since 1998. Her articles have appeared on eHow.com, Answerbag.com and LIVESTRONG.COM, and she specializes in automotive and business topics. McIain has a professional writing certificate from JB Hunt in Little Rock, Ark.

Photo Credits

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