That doesn't tell much but it sounds like that might have been a load tester.
The symptoms you're describing is what happens when one of the six diodes in the generator fails. You will lose exactly two thirds of it's capacity. That means a common 90 amp generator will only be able to deliver 30 amps. You need 6-10 amps to run the fuel pump, a maximum of three amps to go right back and run the generator, and by 2002 there were enough computers in use to need another 5-10 amps to run them, the injectors, and ignition system. That doesn't leave much to recharge the battery. When you want to run the power windows, wipers, or head lights, current is drawn from the battery to make up for what isn't coming from the generator. That's where system voltage starts to drop.
In addition, one of the three things required to produce electrical current mechanically is movement, (that's why we spin something with a belt-driven pulley), and that movement goes way down at idle. Generators are not very efficient at low engine speeds so its capacity drops even more. You'll see the lights dim at low speeds and when extra things are turned on.
Two tests will show if there's a defective diode in the generator. The first is that load test. I have an entire page on testing Ford charging systems on my web site, but you need the load tester to measure the maximum current. You don't even need to know the current rating of your generator because with a bad diode you're going to find a maximum output considerably lower than the smallest generator available.
When you find 30-40 amps maximum, suspect a defective diode. The second test is the "ripple" test. The tester measures the variation in output voltage. AC generators put out three phase output. With one bad diode, one phase is lost. During the time that phase is missing, output voltage drops considerably. That's what the ripple test measures. Most testers don't provide a value. They just have a bar graph showing the relative amount of ripple from "normal" to "excessive".
Without access to a tester, you can often find another clue that there is excessive ripple with an inexpensive digital voltmeter. With a properly functioning charging system, battery voltage with the engine running must be between 13.75 and 14.75 volts. The voltage regulator responds to the drop in voltage from the missing phase by bumping up the charge rate a little. That results in a battery voltage that's a little higher than normal, but that can only happen when most loads are turned off. As soon as the head lights or heater fan are turned on, the generator can't keep up and the voltage drops.
Many generator testers at auto parts stores just look to see if there is some output and they base that on whether they see a rise in voltage when it's running. Very little load is drawn from it. It's when it's on the car that a higher load is drawn and system voltage drops. You need to have your generator tested with a load tester that has the ability to draw at least 100 amps and can measure ripple. Holler back with the maximum current it can produce.
Thursday, March 1st, 2012 AT 2:47 AM