After replacing the alternator the gauge is erratic

Try a new battery before you go looking for the more elusive and less common stuff.

Due to the design, since the 1987 model year, GM has had a huge problem with repeat generator failures. They develop huge voltage spikes that can destroy the internal diodes and voltage regulator, and interfere with computer sensor signals. The battery is the key component in absorbing and damping those spikes, but as they age, the lead flakes off the plates and they lose their ability to do that. That battery will still work fine in an '86 or older model. No battery tester can test for the spike-damping ability.

Any time the generator needs to be replaced, the battery should be replaced at the same time, unless it is less than about two years old. Failure to do so has been a common cause of going through four to six replacement generators in the life of the vehicle.

Why did you replace it? Was it in an attempt to solve the erratic gauge, or some other problem?

I should have also mentioned to check the various ground wires for rust or looseness, especially the smaller negative battery wire that runs from the cable clamp to the body sheet metal.

The symptoms you described can be caused by a perfectly good, but aged battery, which I already described. In addition, when you have an electromechanical instrument cluster, the generator's internal voltage regulator monitors system voltage right at its output terminal, but when you have a digital dash, its brightness is very susceptible to tiny system voltage fluctuations, so they use an additional wire in the generator's small plug to monitor system voltage right at the dash. That is done to maintain the supply voltage perfectly steady there. Any minor fluctuations won't be noticed in other vehicle systems such as the power seats and radio.

Voltage fluctuations can also be caused by a failed diode inside the generator. The voltage regulator has to fight to hold output voltage steady and usually doesn't do a good job of that. Most charging system testers, other than the simple hand-held style, include a diode test as part of the load test. A few models actually print it out as a voltage, but most just use a relative bar chart to show "ripple voltage" between "low" and "high". All generators put out a little ripple voltage, but that goes a real lot higher when one of the six diodes has failed. The voltage regulator looses control of system voltage when it tries to keep up with this varying output voltage.

The additional clue to a failed diode is the generator will develop exactly one-third of its maximum rated current under the full-load test. The standard generator for your vehicle is a 120-amp unit. With one failed diode, the most it will develop is 40 amps. Forty amps is what some older generators were capable of, so some testers will interpret 40 amps as "good" under the full-load test, but it's 120 amps you really want to see.

If your testing did get close to 120 amps, we can rule out a failed diode. That would again point to an aged battery or rusty ground wire connections.

Another test is to unplug the generator's small plug while the engine is running, then observe if the erratic gauge becomes steady. If it does, we know this is related to the generator and battery. If there is still unstable voltage fluctuations, look in a different direction, mainly the ground wires, but also the 12-volt feed circuits. Loose or corroded fuse terminals and mating connector terminals become good suspects for that.

Keep in mind too someone could have previously rerouted a wiring harness, especially on the engine, and that could place some vulnerable wires in pulsing or varying magnetic fields from other wires they aren't normally supposed to be near. This has been especially common when those generator's voltage spikes interfere with computer sensor circuits.

Keep me updated on what you find, then we'll figure out where to go next.

If you're measuring voltage across the battery terminals, we want to see between 13.75 and 14.75 volts, but on later models, 15.1 volts is not something to get excited about. As far back as the late '70s, GM came out with their "Freedom" batteries that had calcium in the grids for the plates. That resisted over-charging due to high system voltage, and 15 volts was not uncommon. (As a side note, the electrolyte was different too, and they didn't want it to become contaminated with electrolyte from conventional batteries, so they sealed the vent caps so you couldn't stick a hydrometer in there that had acid on it from a different battery).

If you're getting that 15.1 volts from the dash gauge, that is not accurate and shouldn't be used for diagnostics. Dash gauges are notoriously inaccurate. Their main purpose is for you to see when something is out of the ordinary or has changed from what you normally see.

By 2005, GM had a lot of vehicles with digital dashes. Their display brightness is very susceptible to flickering due to voltage fluctuations. To avoid that, they used an additional circuit for the generator's voltage regulator to monitor system voltage right at the dash instead of at the generator's output terminal. Rather than holding charging voltage perfectly steady at the generator, and allow voltage to fluctuate harmlessly at other places around the vehicle, they want to hold system voltage perfectly steady at the instrument cluster, and let it fluctuate harmlessly at other places.

At the same time, on many models, they stopped taking system voltage readings at the instrument cluster to send to the "Volts" gauge. Instead, system voltage is monitored by the Engine Computer, then it gets sent out onto the "data buss" along with all kinds of other information. The instrument cluster and all the other computers look at that information and take what they need, and work with that. In this case the instrument cluster, which is another computer, reads that data, then decides where to place the "volts" gauge. It's a round-about way of doing the same thing, but while the voltage regulator is holding system voltage steady at the instrument cluster, the gauge is reading the slightly-fluctuating voltage at the Engine Computer.

Regardless how it's done, don't panic if charging voltage is a little high. If the voltage regulator were to short and cause the generator to charge wide-open, you'd see as much as 18 volts, or even more, at the battery. Also be aware some voltage regulators are incapable of bringing charging current down to near 0 amps, which means they're always producing some output. There may be times when the load on the electrical system is real low, such as when head lights, heater fan, and most other systems are turned off. That will cause battery voltage, and therefore system voltage, to rise a little. That is not indicative of a defect, and in most cases drivers wouldn't even notice.

It's flickering lights or dash gauge that is more concerning to me. Did you do something to stop that from occurring?

I have a suspicion there currently isn't a problem.

As a side note, I just noticed on my truck there are no numbers on the gauges. They likely did that to avoid this scenario.

It's common today and considered normal for all engines to use a little oil between oil changes. To address the numerous complaints, dip sticks are no longer labeled with "Add" and "Full". Since at least as far back as the mid '90s, they're labeled with "Min" and "Max". As long as the level remains between those two points, there's no cause for concern.

Likewise, there's no cause for concern as long as the gauges stay within their "normal" range. I'm not worried about 15.1 volts or that it appears to vary over time. I don't want to hear that lights are flickering. That would be something we'd want to address.

You might consider unplugging the small connector at the rear / side of the generator. If the flickering still occurs, it's not charging system related. It would have to be due to something cycling on and off that draws considerable current.

I've been driving my truck in snow for the fist time since I got it four years ago. I've noticed the lights flicker brighter and dimmer when the ABS or stability control activates. It's not enough to get excited about, but it is noticeable. I don't see any change on the "Volts" gauge.

As a point of interest, Chrysler had temperature compensation built into their older firewall-mounted voltage regulators up through some '89 models. That's because charging a battery is a chemical reaction, and that becomes harder to do when it gets cold. They bump up the charging voltage in cold weather. Today they build the regulator into the Engine Computer so it can take advantage of knowing all the data the computer knows. Now it will vary charging voltage for a number of variables, including cutting the alternator off at wide-open-throttle to make that five horsepower available for passing freight trains up a steep hill, or to reduce load if the engine is running hot, as some examples.

When other manufacturers want to adjust charging voltage for various conditions, they run an additional circuit to the regulator. As I mentioned with GM's digital dashes, those are the most important to prevent brightness flickering, so they're more concerned with the voltage at the instrument cluster than at the battery or other systems. Even on vehicles that don't have a digital dash, they may still use the same sensing circuit there rather than design and build an unnecessary second circuit for those vehicles. Then it becomes an issue of where the dash gauge is getting its information from. That is very likely not right at the instrument cluster. It's more likely it's coming from the Engine Computer. Voltages there will typically vary a little just due to normal resistance in the wires, along with the high-current loads they switch on and off.

Unless there is some other symptom, I think you're worrying unnecessarily about the dash gauge. Some newer-type of batteries today need a slightly higher voltage to get them to take a charge. In the case of "absorbed glass mat", (AGM) batteries like my truck came with. The fellow at the battery store refused to take it as a core return until I charged it at home for a minimum of three days at a slow rate. Sure enough, it charged up and has been working fine after letting it run dead over the winter a few years ago. Standard batteries can reach a full charge in just a couple of hours.

Your charging system will also adjust for a number of variables. To add to the confusion, many have "anticipation" events built into the software. Those are things that will bump up engine idle speed just before the AC compressor relay gets cycled on, or charging voltage when the delayed wipers are about to take a swipe. Charging voltage may be increased when the high-current rear defogger is turned on. A less-known event is the many self-tests the computers run while you're driving. Most of them require a special set of conditions before they will run, but when they do, people may report a slight stumble or hesitation while driving at a steady speed, or they hear strange sounds from under the hood for a few seconds, maybe once or twice per week. On some engines, they run a very high current through the sensing element in the mass air flow sensor to burn off any dirt or bug juice. That's usually done after the engine is stopped, as in the Corvettes, but things like that while the engine is running will drag system voltage down a little. The Engine Computer may deem that as acceptable and not try to get system voltage back up, especially if that event won't last very long.

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