Another thought to pursue is one bad diode of the six inside the alternator. Based on your observation that the engine stays running until you turn on additional electrical loads, (AC and radio), that the alternator can't keep up with so the battery makes up the difference until it runs too low. Once the system voltage drops too low to run the Engine Computer, electronic fuel injection, ignition system, and electric fuel pump, the engine will stall.
A lot of the free testing at auto parts stores just looks for some output, then the assumption is made that the alternator is okay. In fact, with one bad diode, under a load test all it will be able to deliver is exactly one third of its rated output, meaning the common 90 amp alternator will only produce a maximum of 30 amps. That's barely enough to run the basics, the fuel pump draws around 8 amps by itself leaving not much for the engine systems and nothing extra for lights and AC.
Alternator output is three phase output like what's supplied to factories. With one bad diode you lose one entire phase. During the time that missing phase should have been there, the output voltage drops real low, then it pops back up to normal when the voltage from the next phase shows up. That change in voltage is called "ripple". Professional load testers measure ripple. Some display it as a voltage, some show it as a relative value on a bar graph. Ripple will be very high when one phase is missing due to a defective diode. I can send you to a web page that explains ripple better if you're interested.
Preliminary testing with a digital voltmeter is what I recommend too but in this case the results can be misleading. You should find 12.6 volts at the battery when it's good and fully charged and the engine is off. If you find closer to 12.0 volts, the battery is good but discharged. 11 volts indicates a shorted cell in the battery and it must be replaced.
With the engine running the battery voltage must be between 13.75 and 14.75 volts. Even with a bad internal diode, that is usually what you'll find. That's why our do-it-yourself test is misleading. The voltage can even be a little high from the voltage regulator trying to respond to the low voltage points from the ripple. Voltage is electrical pressure. The alternator can keep that up as long as the current demand doesn't get too high. Current is flow, and that's what it can't keep up with with a bad diode.
To put it in perspective, we had an old alternator bench tester with a 1.5 horsepower motor and that could be stalled by a 60 amp alternator running wide open. Most newer test benches have even smaller electric motors so you know they can not run the newer, higher output alternators at their maximum output to see if they are capable of producing that maximum output. The only accurate test method is to leave the alternator on your engine and test it with a large professional tester. Yours is rated at 130 amps. No auto parts store has room for a tester with a big enough motor to run an alternator with that much output. If testing shows your alternator can develop 120 amps or more, you have some other problem unless it's intermittent. I predict you'll find around 40 amps is all you'll be able to get.
I actually did have an intermittent problem on my '88 Grand Caravan. The hint was on some days the voltage gauge on the dash would drop a lot at idle and it would be fine on other days. The confusing thing was that diodes are rarely intermittent. In this high-current application they typically short, overheat, and burn open. After putting up with that for about six months my alternator failed completely due to worn brushes which is a relatively easy fix. The replacement assembly was only $9.00, but while taking it apart I found one of the bolts holding the heavy wire to a pair of diodes was loose. THAT'S why that one circuit of the three was intermittent for many months. It wasn't the diode, it was the electrical connection to it. You have a small silver Nippendenso alternator that also uses bolts to attach the three "stator" windings to the diodes. While it is certainly possible one of those bolts is loose, that is not very common. A bad diode is.
Saturday, September 1st, 2012 AT 6:25 PM