Mazda5 charging issue

This is a weird one.

Puts out a steady 14 volts at idle and up to 3000 RPM. At a steady 3000 RPM or above (which is where Highway cruising is on this car) it dropped to 11.3 volts according to the OBD2 data.

Battery light comes on.

Battery tests fine on the parts store tester.

Cleaned the grounds and the terminals at the battery.

My only guess is bad alternator.

Does that seem like the correct diagnosis?

Don't want to throw $150 at an alternator if it's not the right diagnosis.

That sounds really odd.  Could the belt be slipping as the load increases?  Alt pulley issue?

Is the voltage regulator separate from the alternator? 

Any belt squeal? Kinda sounds like the belt is slipping under load maybe?

I know sometime in the 90's Mazda went to externally regulated alternators, with the PCM handling voltage regulation. Sounds like the issue, but I know nothing else about Mazdas..

Belt doesn't appear worn, and the tensioner us still stiff. No polishing or rubber buildup on yhe pulley. Or areas that i can see.

Issue does not vary withe electrical load. I turned everything on max/off trying to see if that was it.

I have no idea how hard it is to pull a alternator on a Mazda 5, but our parts store will test an alternator for free.  That might narrow it down.

Bad diode in high-speed charging circuit of regulator.  Stay under 3k rpm until you get a replacement?

AngryCorvair said:

Bad diode in high-speed charging circuit of regulator.  Stay under 3k rpm until you get a replacement?

My thought exactly.  I may pull for a bench test, but its NOT an easy one to get to....