Engine/Transmission (1998.5 - 2002) Alternator and AC voltage

[ROADWARRIOR]

My alternator is reading 14VDC at idle, but also 30VAC at the same time. Is this normal?



TIA,

Dan

 

[WestTN]

It means a diode has died. The alternator needs to be replaced.

 

[CKelley1]

not necessarily. if you are not having charging issues you may just have a sensitive meter. the alternator produces three phase ac and converts to dc using six diodes the ripple actually helps in the charging . that is also what causes alternator whine in some audio

 

[PBJ2500CTD]

Ah... but if all of the diodes are performing their assigned rectifier duties only the peak voltage (1/2 peak to peak), less diode bias drop of about . 6 to . 7 volts, should be seen at the output. Since this is a three phase event the output will exhibit a smoother ripple and less noise than a two phase circuit. If a diode doesn't want to function properly by allowing current flow when it shouldn't, the circuit will now pass the peak voltage (a negative wave in this case) causing a peak to peak voltage resembling the full input wave. The AC meter will read the voltage potential swing of the positive and (now unrectified) negative cycle - or appoximately 30 VAC. The best way to view the signal is with an o'scope. If a diode went bad you should see a spike (shorted) or drop (open) in the waveform every 120 degrees. Then there is the poor voltage regulator trying to figure all of this out and give you the 13. 5 to 15 VDC...
This site has a pretty good explanation - Alternator waveform - and this one - Sean's Jaguar Tech Pages - Diagnosing your Alternator with a DVM or Scope