First of all, please understand that you must never, never, ever disconnect the battery while the engine is running. I'll elaborate more on that later if necessary.
If the battery is good but simply dead, especially if it has been dead for a while, it is going to take a good fifteen minutes on a battery charger before it STARTS to take a charge. It takes time for the acid to become conductive so current will flow through the plates. On a slow charge, you'll see the charging current start near 0 amps, then rise to as much as 20 amps after about five or ten minutes. After about an hour or two on a slow charge, the current will drop back down to around five amps at which point you can consider it fully-charged.
Once the battery is sufficiently charged, the engine will stay running on the generator, OR on the battery if the generator is bad. On a lot of generators, their voltage regulators will not respond properly when the battery is disconnected, or, in effect, it appears to be "disconnected" by being dead yet. They will fail to run the generator, so the engine will die when your jumper cables are disconnected. That is actually a good thing, because many years ago mechanics who didn't understand how these simple circuits worked or how to diagnose them would disconnect a battery cable to determine if the generator was working. Some people today still do that, and a mechanic will usually be fired for pulling that stunt. When doing so, some systems WILL keep working, but system voltage can easily go up to over 30 volts. That will destroy all the computers and any bulbs that are turned on.
That won't happen on engines that stall with a dead battery or one that is disconnected.
Either charge the battery first for an hour, or use the jumper cables and another car. Once the engine is started, stop the engine on the booster car but leave the jumper cables connected. Measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it stays close to 12.6 volts or less, the generator is not working. If it is within the acceptable range, that only means it pays to continue the rest of the tests with a professional load tester.
Be aware that starting with '87 GM models, this is a very high-failure generator design. If you find that it's not charging at all, or your mechanic finds it's only capable of developing exactly one third of its rated output current, it will have to be replaced. 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. These generators develop a lot of huge voltage spikes that can destroy the internal diodes and voltage regulator, and they can interfere with computer sensor signals. The battery is the main component in damping and absorbing those harmful spikes but they lose their ability to do that as they age and the lead flakes off the plates.
Thursday, March 12th, 2015 AT 12:17 AM