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Question

1999 GMC SONOMA

Jul 13, 2017 12:13PM PDT

When I put the key in the volt meter reads about 9v and it's hard to start but once I get it started it jumps up to 14v I can put a volt meter on the battery and it reads about the same. As soon as I turn the car off the batterys volts slowly goes down. It's like something is on and drawing power. I have a new battery and had the battery and alternator checked. Both good. I have no idea what's going on.

Discussion is locked

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Answer
sounds to me like...
Jul 13, 2017 12:21PM PDT

...a dead or dying cell in the car battery. Of course you see better when vehicle is running because it's then measuring the charging power from the alternator, not your battery.

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Answer
Small world.
Jul 13, 2017 12:23PM PDT

My usually no drama car that I love is doing this too. It turns out something is pulling power when the car is off. So it drops from the usual 14V to 12.8/12.5 when shut off and dwindles down over time.

It's in the shop (warranty) and last word it was some ECU module but they have to let the maker OK the replacement.

No, I'm not telling you it's the ECU but sharing it's a fun thing to find where the draw is from.

-> Tip: Look to any non-stock devices that may be connected to the battery.

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Answer
Charging system
Jul 13, 2017 3:07PM PDT

Hi, Unforgiven; This is a very unscientific test, but turn off everything, doors closed, ect., disconnect the negative battery cable. Connect a test light to the negative post on the battery and touch the end of the test light to the disconnected cable. If the test light illuminates, it tells you that there is 'something' drawing power. With that being said, the PCM and stereo memories draw power all of the time. With the test light still connected, start pulling fuses, one at a time, until the test light goes out. You have found the system that is drawing the voltage. Once you've identified which system, get back to us.
Loren