Digital multimeters are cheap. Invest in one if you're going to work on your truck yourself. Harbor Freight Tools has a perfectly fine one for less than ten bucks and it goes on sale often.
Start by using a test light to check for voltage at the large output wire on the back of the generator. With the engine off you should find 12 volts there all the time. The light should get brighter when you start the engine. At that point you really need the voltmeter to know what's going on. You must find exactly the same voltage at the battery and the output terminal, that is between 13.75 and 14.75 volts. If the two voltages are not the same there is a break in that wire between the output terminal and the battery.
If the voltage does not increase with the engine running we have to look at the input circuit which mainly consists of the voltage regulator. Turn on the headlights or connect the voltmeter or test light to the battery so you have something to tell you when the voltage goes up. Start the engine, remove the four-wire plug from the voltage regulator, and use a stretched-out paper clip or cotter pin to jump the two outer terminals, "A" and "F". That will make the generator run wide open so do not raise engine speed. Doing so can make the output voltage go high enough to cause damage. Only connect that jumper long enough to see if voltage goes up. If it does, suspect the voltage regulator. If it does not we have to do further tests.
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Tuesday, March 26th, 2013 AT 8:27 PM