You didn't exactly say it, but did it crank too slowly until you jump-started it? That would imply the charging system isn't working, but testing the generator off the vehicle is a poor way of doing it. If it has failed, you should have seen the warning light on the dash turn on, and the most common failure is worn brushes which almost always start out as an intermittent problem. Any testing has to be done when the problem is occurring, and you need the vehicle's dash light to know when that is.
Another problem with in-store testing is those testers only look to see if there's some output, not how much. It takes 5 - 8 horsepower to run a generator wide-open, and the motors in bench testers aren't that strong. On-car testing allows professional load testers to measure the maximum current output the generator can deliver. A less-common problem is a defective diode in the generator. When one of the six fails, you will only be able to get exactly one third of the unit's rated current, and that 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. A test bench will show that as "good". Output voltage will be okay too or even a little high.
Another thing you need to check for is a blown fuse link. Usually if it is burned open it's because of multiple shorted internal diodes, and the unit will not test good, even on a test bench. More commonly the splice for the fuse link is corroded so the current can't get back to the battery.
Your first test is to measure battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If it is low, measure the voltage on the generator's large output terminal. With the engine off you must find full battery voltage there all the time. A test light is more accurate for this than a voltmeter. With the engine running, if you find the voltage noticeably higher than battery voltage, there's a break or blown fuse in that circuit.
If the voltages are okay, the second half of the testing requires a professional load tester. If the generator has a bad diode, the common 90 amp unit will only be able to develop about 30 amps under full load.
Thursday, January 2nd, 2014 AT 8:42 PM