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Voltage fluctuating after jumping another car


jrhone

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So on thanksgiving i jump started my mother in laws car. Before that the voltage readings in the 02 were almost always 13.5 steady. Maybe it would drop to 13 at idle but it was always above 13 volts. After I jumped the other car I notice it drop to as low as 10 volts. Never goes to 13.5 like before the highest is about 12. Did I fry something?

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Update. So I did more checking. With engine running. No lights nothing electrical on the voltage is 13.8. When I turn on the lights and electric fan it drops down to about 11.8. Definitely wasn't doing this before I jump started the other car. Alternator is an upgraded 320 80amp I believe and it's been on there at least 5 years.

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Either the battery is going bad or you need a new regulator/brush assembly. Very first thing: battery load tester. You can pop into Sears if you don't have one. This will tell you the real condition of the battery. I would say that's the problem from what you describe. Cables? What kind of shape are your grounds and cable ends in? If they are somewhat corroded they will cause problems too. You can do a visual on the brushes by taking the two screws off to pull it out and take a look at how much they are worn down but It's probably the battery as when you have no loads on it you are getting the right voltage out of the alternator. When you put a load on the battery it shows its weakness. Jumping the other car was probably a heavy load on it and that may have pushed it over the brink. Most of the time it's simpler then you think.

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Test the battery with the engine off.  You should get 12.6 vols. With the engine running, you should get 13.6 V.  When you turn an accessory on or turn on the lights, the voltage should drop momentarily but move right back to 13.6 V.  Your symptoms seem to indicate a defective voltage regulator.

When you jumped the car, was your car running?  While our cars don't have any electronics to fry, it is recommended that you keep the "donor" car off so that to protect its electrical circuit.  The alternator may boost the voltage but when you really need is amps.

If your car is running and then you jump another car, when they hit their starter and current flows, your voltage regulator goes bizerk trying to compensate.

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