I have a 2006 Ford Taurus in which the "Check charging System" warning light comes on although the baterryreads 12.5 Vdc without the engine running and 14.5Vdc with the engine running. I also looked at the AC voltage with the engine running, 32.9Vac. Is the alternator bad or is it a faulty sense circuit?
Load test the alternator if okay -could be the computer controlled circuit of the alternator field generation, also inspect wiring
June, 4, 2011 AT 7:48 PM
Nice to know you understand about ripple and AC voltage, but are you sure that was 39 volts? .39 volts would be typical. 39 millivolts would be better but is unrealistic. If the AC voltage really is that high, that would point to a defective diode. When one of the six diodes is open or shorted it will reduce the generator's capacity to very close to one third of its maximum rating. That means you'll be lucky if you can get 30 amps from a 90 amp unit. 30 amps is just enough to run an electric fuel pump, ignition system, head lights, and radio with little else left to charge the battery. Most charging system load testers today measure ripple along with maximum output current.