Battery light staying on?

Good afternoon,

The first thing I would do is check the battery voltage. It should be 12.6 volts.

https://www.2carpros.com/articles/how-a-car-battery-works

Then with it running, check the voltage at the battery again. It should be 14 to 14.6 volts.

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Then change you voltmeter to read AC volts and tell me the reading. You may have a bad alternator causing all these issues.

https://www.2carpros.com/articles/how-to-replace-an-alternator

Roy

Check the fuseable link between the alt and the battery. If it is bad, the alt can't get power to the battery. Also, check grounds. Make sure the battery terminals, ground to engine block, and all body grounds are clean, tight, and not rusted or corroded.

Does it crank over or it cranks over and refuses to start?

Get a carb cleaner and spray into the carb or the throttle body on an EFI. Did it start and die? If not disconnect a sparkplug wire or 2 and ground it to the engine -have helper crank engine over-do you have a snapping blue spark? If so-you have a fuel problem, check the fuel pressure to rule out the fuel filter/fuel pump/pressure regulator and listen to the injector/s are they pulsing or hook up a noid light. No snapping blue spark continue to troubleshoot the ignition system-power input to the coil/coil packs, distributor pick-up coil, ignition control module, cam and crank sensors-

Gauge repair, now we know why it's a banking business? How long does the replaced gauge last?Were they new, used, or repaired?

Could be coming from the computer controlled of the alternator's field generation circuit-

Any applicable trouble codes? There are codes listed for the instrument cluster. The instrument cluster receives serial data from the pcm to operate gages. Any wrencher is the best at what make and model they work on the most, that's just how it is.

If vehicle still functions and drives your instrument cluster needs repair. Ebay has several low cost repair services.

The "Battery" light hasn't changed since the 1960s. It means the generator isn't recharging the battery while you're driving. What HAS changed is GM redesigned their generators for the '87 model year and it's a very poor design. They develop huge voltage spikes that can damage the internal diodes and voltage regulator, and can interfere with computer sensor signals. It is real common to go through four to six replacement generators in the life of the vehicle. To reduce the number of repeat failures, replace the battery at the same time unless it's less than about two years old. As they age, the lead flakes off the plates and they lose their ability to absorb those voltage spikes.

You already noticed the charging voltage is too low. Measured at the battery with the engine running, the voltage must be between 13.75 to 14.75 volts. If it is low, suspect the generator. If the voltage is okay, you need a professional load tester for the second part of the test. That will test for maximum output current and "ripple" voltage. If one of the six diodes has been damaged, you'll only be able to get exactly one third of the generator's rated maximum current, and ripple voltage will be very high.

You didn't list any symptoms now after the generator was replaced. The battery is likely still okay, so leave that alone for now. The old generator from 2013 worked okay for two years, so replacing the battery the next day was good enough to avoid another generator failure. The failure now was just "the nature of the beast". Every GM owner replaces their generator two or three times if they keep the vehicle long enough.

My recommendation now is to have the charging system professionally tested by a mechanic, not at an auto parts store. In particular, what I want to see is "regulation voltage" between 13.75 and 14.75 volts, full-load charging current very close to the generator's rated value, and "ripple voltage" should not be excessively high.

The common generators for your truck were a 105 amp, a 124 amp, and a 140 amp unit. Full-load current will be very close to one of those values, or, ... If there's a defective diode inside the generator, you'll get exactly one third of that value. A generator is physically incapable of developing more current than it's designed for.

If ripple voltage is high, that is caused by the generator developing harmful voltage spikes, but it's the job of the battery to dampen and absorb them. Consider replacing the battery if ripple voltage is high/