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Hi everyone thanks for your input I have a 1994 Isuzu Rodeo and the alternator goes negative when I rev the engine also when staring the engine it goes to negative and I have to wait until it builds up to go positive Reeving the engine does not work I have to wait while it idles With lights on and making a turn down a hill alternator goes to negative and will not go positive if I accelerate I have to wait at idle Alternator has been rebuilt 4 times by 2 different companies Alternator belt is good

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    Welcome to Motor Vehicle Maintenance & Repair! What do you mean by "it goes to negative"? And how are you detecting it does this? Apr 22, 2022 at 0:29
  • On the gauge in the car it drops below 12 volts Apr 22, 2022 at 0:51
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    Have you verified the readings with a digital multimeter (DMM)? If the gauge is bad or misreading, you could be chasing your tail. Apr 22, 2022 at 1:20

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If your gauge in the car is reading correctly, it sounds like it could be a bad regulator or a broken wire.

  • The first thing to check is the alternator. Most auto parts shops have an alternator test bench and will test it for free. Once you know the alternator is indeed doing what it needs, then you have to chase down the real issue.
  • Next check the wires connected to the alternator. Look for chafing, exposed copper, corrosion and broken wires. When you say making a turn down a hill stops charging, that makes me think you will find your issue here. Some wire is seemingly hanging on and when the right conditions move it, it stops charging. Alternators have a small wire called the "sense wire" that tells the alternator the battery voltage so it knows how much to charge. If this wire is not making good connection, a good alternator will not charge.
  • Ensure the big positive cable connected to the alternator is tight and clean on both ends. It should run from the alternator to the battery.
  • If your car has a PCM controlling the alternator, that could be failing and not telling the alternator to charge.
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  • Thank you Mr. Anderson I will check this next week and let you know the results Apr 22, 2022 at 15:15
  • Replaced the bad wires new belt and fan clutch all is good thanks again Mr. Anderson! May 3, 2022 at 19:46

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