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How to tell if alternator bad?

My apu is the only thing power my truck (lights). How do I tell if my batteries are fried or my alternator is bad? (I had a voltage surge today of 15.1 without any of my lights on. I turned my lights on and it went back down to 14.-13.9 then about 15 mins. later it dropped to 12.3 and my apu has been bringing it back up to 13.8 all day.)

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First inspect the alternator, for loose wires, connections, etc. Then start the truck up and make sure belts are tight and nothing is slipping. If you can safely get to the alternator while the truck is running you could test it with a multimeter set to 20v. Ideally it should be putting out 13.8 - 14.2 volts. If you can't safely get to the alternator you can test at the batteries, but it's not as accurate.

You also need to load test the batteries, the proper way to load test is to first fully charge the batteries, then remove the surface charge by turning on the lights (high beam) for several minutes. Then attach the load tester and run the test.

You can buy both the multi meter and the load tester at any auto parts store and they are fairly inexpensive.

You may even get the auto parts store to test them for you. Most of them will do it on a car for free.

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I did do all the above and found that it was my alt.. Mechanic at r&r said that they do sometimes surge. But he also said that it's like your coffee maker; one morning it works the next it don't ...

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Another quick and dirty check is to see if the alternator is putting out a magnetic field by placing a screw driver near the back of the alternator. (don't short out any of the terminals of-course). A strong magnetic field indicates the alternator is producing output.

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Alot of feedback thru the CB radio is always the first sign of alternator problems.

Voltage fluctuations as described maybe a resistor on the alternator going bad.

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I had to replace my alternator when it started surging the voltage up to 16 v, but it never went down below 14. At the shop, it tested fine twice, but the last time it was surging even at idle. The voltage regulator inside it was not functioning. After they replaced the alternator I have no problems with fluctuating voltage. This was a two year old alternator.

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