Voltage gauge fluctuates high then low why?

Hi SAdams,

As for what is happening, these vehicles the alternator should be putting out a constant voltage that will fluctuate about 1 volt depending on RPM. However, the BCM is responsible for taking the state of charge from the PCM and sending it to the cluster which displays what you are seeing.

I attached the detail operation of this system. Please view page 4 the highlighted portion. You spread of 3 volts from 11.5 to 14.5 is the same spread that is called out here but it is a little low.

The reason, I suggest you get it documented is for two reasons other than you are clearly under warranty. First, it could just be a drained battery from sitting on the dealer's lot. Unfortunately, some dealers are not the best at maintaining the batteries in their inventory. Second, if the battery is okay, this could be a battery sensor issue or a software issue. If it is a software issue and the correction is not available yet, you can request they alert you when it is.

Either way, just getting it documented is not a bad thing even if they tell you it is normal based on the info I provided. Clearly, GM says it should not go below 12.5V and yours is, so I would put my money on a weak battery. This guide can help

https://www.2carpros.com/articles/car-battery-load-test

Let me know what you find because this info can help others if they experience something like this in their new truck. Thanks

It may be the gauge itself as they are digital and will do that. you can check fuse 46 underhold as that powers the dash but it may be something like a CAN serial data problem or body module but to be sure lets do a CAN scan which is the future of automotive repair you can get a CAN scanner from Amazon for about $50.00 here is a video to show you how.

https://youtu.be/u-4syLc-ifQ

let us know.

The is normal the system senses the battery state of charge and will "back off" when the battery is charged.

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.

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.

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.