Do not disconnect a battery cable while the engine is running! That stunt will get a mechanic fired. It used to be done many years ago when mechanics didn't understand how these simple systems worked, but back then the only thing that could be destroyed was any light bulbs that were turned on. Today it is real easy to destroy every computer on the vehicle.
The charging system is made up of the generator, the voltage regulator, and the battery. The battery is the key component that helps the regulator hold system voltage to a safe level. Without it, system voltage is capable of reaching 30 or more volts. Computers won't tolerate that. What's more, due to the "ripple" voltage, depending on how the voltage regulator responds to the pulsing dips in voltage, it could stop field current, and therefore generator output current, and make a perfectly good generator appear to be not working because the engine will stall. Also, an engine might continue to run on the output from a generator with a failed diode. With one failed diode of the six, the most the generator will be able to deliver is exactly one third of its rated current. That is not enough to meet the demands of the entire electrical system under all conditions. The battery commonly will run down over days or weeks, while system voltage will be okay, or even a little high.
You have an additional problem that only applies to GM vehicles, starting with the '87 model year. The engineers redesigned their generators and developed one that is highly prone to failure. Due to their design, they develop huge voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. It's the battery that dampens and absorbs those harmful spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. Aged batteries are the main reason it is common to go through four to six replacement generators in the life of a GM vehicle. To reduce that number of repeat failures, always replace the battery at the same time you replace the generator, unless it is less than about two years old. The old battery will work fine in an '86 or older model.
Dash voltmeters are never very accurate, so don't use that to make a diagnosis. What they're good for is recognizing when something is different than what you're used to seeing. Now that you've noticed that, the correct way to tell if the charging system is working properly is with a full-current load test, but that requires a professional load tester. You can start the test with a simple digital voltmeter. Measure the battery voltage with the engine off. It will be 12.6 volts if it's good and fully-charged. If you find it's closer to 12.2 volts, it's good but fully-discharged. Charge it at a slow rate for a couple of hours.
Next, measure battery voltage again with the engine running. It must be between 13.75 and 14.75 volts. If that is what you find, that only means it is okay to have the rest of the system tested professionally. That will include the maximum output current, and ripple voltage. Most testers just show ripple voltage on a relative bar chart as "low" or "high". High ripple voltage indicates a diode has failed and one of the three output phases is missing. This is where you'll only be able to get 30 amps from the common 90-amp generator.
In my opinion, GM's '86 and older generator was the world's second-best design. It was easy to disassemble and replace defective parts. That is not true of the newer design. It is just about impossible to get apart without damaging the flimsy metal tabs on the diode block, then, if you do get it apart, there's no way to test the voltage regulator and no way to measure continuity through the brushes. Those are the two more common failures besides the diode block, and with no way to test them, you'll want to replace all the parts because you sure don't want to go through taking it apart a second time. There have also been some problems with the bearings, so you'll want to replace those too. By this time you'll see it's a far better value to just buy a professionally-rebuilt generator with a warranty.
Check a number of parts stores for prices. My friend has gone through this at least three times on his '99 Suburban. He found a replacement generator for $200.00 with a one-year warranty from one store, and one with a lifetime warranty for only $150.00 from a different store.
SPONSORED LINKS
Tuesday, June 25th, 2019 AT 4:34 PM
