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How to Test 12V Batteries on a Diesel Truck

by Gregory Crews

Testing the batteries in your diesel truck does not have to be a complicated process. The batteries on a diesel truck are wired to the truck differently than in a standard automobile. In the diesel truck three to four batteries are required to crank the truck's engine. If one of the batteries is dead, then the truck will not crank. The load test is the most efficient way to test the battery as it actually puts a load on the battery then gauges it to determine if the battery is weak.


Rent a battery load test tool from your local automotive parts store. This is a device that tests the current on the battery and tells you how much charge the battery has.


Locate and remove the battery cover on your truck. Bigger trucks will have a battery box by the passenger door. The battery cover has two straps holding it in place.


Disconnect each battery with a socket wrench by taking off all the positive cables and the negative terminals. It does not matter in which order you remove the battery cables.


Attach the load tester to the first battery by placing the positive cable onto the positive terminal. The positive terminal is indicated by the "Plus" (+) symbol. Attach the negative cable to the negative terminal. The negative side is indicated by a "Negative" (-) symbol.


Observe where the needle on the gauge stops. This will tell you how much charge the battery has in it. The areas will be color-coded. Green will mean there is a good charge on the battery. If the needle falls in the red zone, that means the battery is weak.


Flip the toggle switch on the bottom of the tester. This will place a strain on the battery. Observe where the needle sits. This will be color-coded as well, with red indicating a low charge and green a good one. If the battery is bad, the needle will automatically land in the area designated "Bad."


Repeat these steps on the remaining batteries. Replace each bad battery.


  • Ensure all the batteries are completely isolated from one another by removing all battery cables.


  • Use caution, when working around electrical systems, as failure to do so may result in electrical shock.

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About the Author

Gregory Crews has been in the film industry for three years and has appeared in more than 38 major motion pictures and 16 television shows. He also writes detailed automotive tutorials. His expertise in the automotive industry has given him the skills to write detailed technical instructional articles.

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