Measure the battery voltage with the engine off, then with it running. Off you should find 12.6 volts if it's fully-charged. If you find 12.2 volts, it's okay but discharged. With the engine running the battery voltage must be between 13.75 and 14.75 volts. If it is low, measure the voltages on the two smaller terminals on the back of the alternator. That has to be done with the engine running. One will have full battery voltage. The other one must have less but not 0 volts. If you find 0 volts on the second terminal, the brushes are worn and must be replaced. That's the most common failure. If you find exactly the same voltage on both terminals, there's a break in the wire going to the voltage regulator inside the Engine Computer. That's not a very common problem.
If the second terminal has between 4 and 11 volts, measure the voltage on the large bolted-on output wire. That must be the same as what you found at the battery.
If all of those voltages are correct but the battery is running down over hours or days, suspect a failed diode. That will reduce the alternator's maximum output to exactly one third of its rated output. 30 amps from the common 90 amp alternator 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. You'll need a professional load tester to measure maximum output current and "ripple" voltage. Ripple voltage will be very high when one of the six diodes has failed.
Wednesday, February 12th, 2014 AT 12:58 PM