How to Test a Ford F-350 Alternator

by Grace Mclain

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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.

Step 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.

Step 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.

Step 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.

Step 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.