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Q: Voltage drops from 12.5 to 3.0 when key is turnned to acc

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I just bought this 2001 lesaber from an older lady . Super nice car , though it won't crank over. She had gone to start it herd a loud pop from under the hood like a fire cracker. Stater would engage but it wouldn't crank over. Engine turns fine by hand . So we check the voltage at the jump point under the hood , it showed 12.5 . When I went to start it it dropped to 3.0 volts . So I'm thinking bad battery maybe, took the one from my truck and put it in the Buick . Now nothing works at all !! No accessories what so ever. Checked all the grounds and cleaned them , no evidence of bad ground cables or anything. Voltage still drops when I turn the key on , starter will no longer engage! Any advice would be appreciated. She had a starter put on it before I looked at it too, no difference, by the way

My car has 117000 miles.
My car has an automatic transmission.

Hello, thank you for writing in. Considering the pop sound seemed to end the working condition of the vehicle, you will want to turn your attention there. If there was an electrical surge of any kind, the battery is likely damaged. The first thing to do is check all of the fuses and relays in the main fuse box. Also make sure that the starter is capable of turning over, and have it bench tested. The pop may have also been something mechanical, which would mean there is something more seriously wrong with the vehicle. Have you tried scanning the vehicle for diagnostic codes? This may give some insight if the issue is monitored with a sensor. Only further testing will tell. For more help, contact our service department to schedule an appointment.

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