04 Venture voltage jumps all over the place
 

I have an 04 chevy venture. Since replacing the battery and alternator the voltages jumps between 12.7 to 14.6. It never stays at one voltage. Idling it jumps all over the place. I have turned everything off I can and still makes no difference. There are 3 different alternators for this van. I have the 125 amp one in the van now. Is that the right alternator? What else could it be?

At idle the voltage should be ~14.3-14.4 and ~12.6 at rest (engine off). It may vary by 0.1 volts or so but should be more or less steady. I would suspect the voltage regulator, but that's probably built into the PCM/ECM. I'm not as familiar with GM products having owned MoPar's for the past 30+ years (recently acquired a '14 Equinox). Check and clean all wiring and connections. Are you getting any fault codes? CEL on?

im with doug the alt seams like the issue. take it off and have a parts store test it

IMO it's the voltage regulator, that's probably electronic controlled. Does it have a two prong connector? One side should be marked field, that's the field voltage from the regulator that controls the output from the alternator. Dosen't matter the output from the alternator as long as it can maintain the system voltage under load, i.e. mega lighting, stereo system. If you have a voltmeter, one side to frame or actually if it has a two prong connector one side of the vm to each connector set on dc volts. If the regulator is erratic that will cause the symptom you have the field voltage will also be erratic, be sure to test at idle with no "on off" load such as ac on, off, the regulator will maintain field voltage to keep the system at around 14 volts, so idle with a constant, or no load.

I should also specify an analog volt meter if possible, you can watch the needle swing if the regulator output is erratic.I have a digital with an analog meter scale.

No codes but it keeps taking out my battery and starter

It was tested and works fine. Even the guy at the shop is completely stumped