Question about 1999 Chevrolet Silverado 1500

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Alternator may be overcharging?

My Service Engine Soon light is on, and my Voltmeter reads 18 while the vehicle is running. I was thinking if this could be the alternator overcharging. I have a 1996 Chevrolet pickup truck 1500.

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Yes It sounds like the voltage regulator inside the alternator has gone wide open .It can also ruin your battery and explode the battery.I would have it hooked up to a alternator tester and just to make sure it's not just your gauge.hope this helps

Posted on Jan 24, 2009

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Well, 18 volts is an issue. And can cook your battery. Proper voltage range is 13.6 to 14.2 volts
The lower the voltage, the higher the amperage so 18 volts is doing nothing as far as charging but is doing alot towards ruining the battery. Have both checked. The battery could have wasted the alternator. good luck. I bet your battery needs lots of water. If so, it may already be trash.

Posted on May 12, 2009


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If it is overcharging ( how do you know if the voltmeter is not working) then the problem will be either in the regulator of the alternator or the rectifier of the alternator. I would be looking at a blown diode in the rectifier allowing the alternator to produce ac current that will not work an voltmeter that is dc. The ac current is affecting the ability of the regulator to control the output of the alternator.

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The alternator is vital part of your car or truck's electrical system. When the vehicle is running, the alternator provides a constant charge to the battery, as well as to other accessories. Without the alternator, the battery will eventually discharge. However, if the alternator isn't working correctly, it may send too great of a charge to the battery, which is known as overcharging. This condition is dangerous to your car's battery and electrical system. You can test to see if the alternator is overcharging, using a simple voltmeter. Start your vehicle and open the hood. Be aware of the moving parts within the engine compartment, as you do not want to get your hands or tools anywhere near them.

  • 2 Locate the vehicle's battery. It may be obscured by protective shields, the air filtration or intake system or a fuse box. Whatever the case, you need to remove everything that covers the battery. You must have access to both the negative and positive terminals of the battery.
  • 3 Turn the digital voltmeter on and adjust it to the proper settings, if necessary. The voltmeter must be set to "DC" and 12 volts.
  • 4 Connect the clamps or leads of the voltmeter to the battery. You must connect positive to positive and negative to negative. The positive lead is usually red or yellow, while the positive terminal on the battery will be marked with a plus sign. The negative lead on your voltmeter is black, while the negative battery terminal is marked with a minus sign.
  • 5 Examine the voltmeter. If you've hooked it up properly, you should see a reading somewhere between 13.6 volts and 14.3 volts. A reading higher than 14.4 volts warrants further testing by a professional. If your alternator is found to be overcharging, you will need to have it replaced.
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    The charge rate increases as the load and engine revs go up but only supposed to be a little, thats what the voltage regulator on the alternator is for.. when the regulator fails and alternator is overcharging eventually enough power is sent through the ground to light everything up and the computer cannot run properly.
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