2005 Chevrolet Silverado Voltage fluctuates

Tiny
JHWGRAY
  • MEMBER
  • 2005 CHEVROLET SILVERADO
  • 4.3L
  • V6
  • 2WD
  • AUTOMATIC
  • 130,000 MILES
While driving and sometimes idle the headlights and interior lights while dim and then go bright and will repeat. Sometimes for a little time or for like 20 min drive. The battery meter dips from about 15 or 16 down to below 14 and then back up to normal. The battery in the truck is a optima yellow top and is about 1 to 2 years old and the alternator is about 8 to 10 months old. When I replaced the old alternator and tension pulley. With the one thats in now it fixed the problem for a couple months and then started back up again. I do have a aftermarket radio in the truck but the problem did not occur when first installed or parts were replaced. Have tried stock radio and that did not fix the problem. As you can imagine this issue is very annoying.
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Wednesday, April 15th, 2015 AT 10:23 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
First of all, dash gauges are notoriously inaccurate. All they're good for is for you to notice when something is not normal. The place to start is by measuring battery voltage when the engine is running. It must be between 13.75 and 14.75 volts. (In particular, I need to know if it goes outside that range when the lights go bright or dim.

Second, there's no way to sugar-coat this. Your generator is the world's worst design. GM had probably the world's second best generator from the early '70s through 1986. For the '87 model year they redesigned them and have had a huge problem. Without going into lots of electrical theory, they develop large voltage spikes, just like an ignition coil does, and those spikes can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. It is common to go through four to six replacement generators in the life of the vehicle, but to reduce the number of repeat failures, you must replace the battery at the same time unless it is less than about two years old. The battery is the key component in damping and absorbing those voltage spikes, but as they age and the lead flakes off the plates, they lose their ability to do that.

Given that you have an uncommon battery, even though it's not that old, it sounds like you're running into this same problem. I suspect there is a characteristic of your battery that is not in the best interest of helping the generator survive. The lead, for example, could be packed onto the plates very densely to provide a longer reserve capacity, but that could reduce its ability to absorb rapidly-occurring voltage spikes. It sounds like you need another replacement generator, but if you have problems again, replace the battery with what is supposed to be in your vehicle. Don't fall for any gimmicks or advertising hype.

Measuring battery voltage with the engine running is only the first half of the test. For the next part you need a professional load tester. Under full load, you need to measure maximum current output and "ripple" voltage. Under a brief full load, you will get either the rated current capacity of the generator or very near exactly one third of that value. If one of the six diodes fails, which is real common on these generators, you will only be able to get one third of the rated current. 30 amps from the common 90 amp generator is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.

Diodes are rarely intermittent, so it's more likely your voltage regulator is failing. That will typically turn the generator on like it is supposed to be, then off, and that will cause lights to be bright, then dim.
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Wednesday, April 15th, 2015 AT 11:30 PM
Tiny
JHWGRAY
  • MEMBER
So basically your telling me I need to get a new alternator and battery. And this was happening with a stock battery when I had gotten this truck. Also the generator in 87' year is the same in the 2005 year. If I get a higher amp alternator then a oem replacement would that help the issue?
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Wednesday, April 15th, 2015 AT 11:44 PM
Tiny
CARADIODOC
  • EXPERT
A generator with a higher capacity wont do anything. It will only develop exactly the amount of current needed by the electrical system, and no more. Just because it has a higher capacity doesn't mean it will develop more current.

If the first generator failed while there was an old battery in the truck, that would explain that failure. If the replacement generator was in there for a time when the old battery was still in there, that could explain the second failure. If the replacement generator was only in there with the new battery, I suspect there is some characteristic of that battery that is not conducive to damping those voltage spikes. That's the main thing that causes repeat generator failures.

The generators are quite different among years and engine sizes. That's another one of the drawbacks that GM has designed in. With their older design, any generator could easily be adapted to work on any engine by just spinning the rear half of the case to one of three orientations. With the new design, there's so many different mounting ears, offsets, pulleys, and connector locations that you have to get the right one for your truck. Some of them still interchange, but not like in years past.
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Thursday, April 16th, 2015 AT 12:40 AM
Tiny
JHWGRAY
  • MEMBER
If I go back to the old battery would I need to replace the current generator
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Thursday, April 16th, 2015 AT 12:48 AM
Tiny
CARADIODOC
  • EXPERT
If testing part one with a voltmeter shows insufficient voltage or too much voltage, the generator must be replaced. If testing part two with a load tester shows insufficient current and / or high ripple voltage, the generator must be replaced. While these generators can fail on their own, by far the largest percentage of failures are due to the aging battery. Replacing the battery reduces the likelihood of another generator failure, but replacing the battery doesn't fix a generator that has failed already.

I know I'm not explaining it very well. Consider Elmer Fudd going wabbit hunting in a row boat. There's a good chance he's going to accidentally fire his shot gun and put a hole in the bottom of the boat. Taking the gun away from him won't fix the boat. It has to be replaced, but taking the gun away reduces the likelihood of the same failure happening again.
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Thursday, April 16th, 2015 AT 1:15 AM

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