Not holding a charge

  • V6
  • RWD
I just got a new alternator for my truck. The people at o'reillys tested the battery and said it was fine but it won't hold a charge with a new alternator installed. I have no idea why. Could u please help?
Do you
have the same problem?
Tuesday, November 17th, 2015 AT 9:23 PM

1 Reply

Yup. By "won't hold a charge", do you mean the battery runs down overnight or it never charges while driving? The place to start to see if it's charging is to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If that is okay, the system is working and you likely have a drain that we have to find.

If the battery voltage is staying near 12.6 volts or less, the charging system is not working. We can figure out where to look next by taking three voltage readings at the three wires on the alternator.

The large bolted-on output wire must have full battery voltage all the time, regardless if the engine is running or not.

The engine has to be running when you take the readings on the two smaller wires. Typically you'll find a small black plastic block bolted to the rear with two metal tabs under two real small nuts. Those are the two nuts where you take the last two readings. If they're rusty, you'll have to scratch them with the voltmeter's probes to get a correct reading. One of them should have full battery voltage. The secret is what's on the second one. It should be less, but not 0 volts. Typically you'll find between 4 and 11 volts. Based on those readings, I'll tell you where to go next.
See the guide below
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Tuesday, November 17th, 2015 AT 10:02 PM

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