94' 3.0 Voltage Fluctuations (12v-16v)

[94' Ranger]

My voltage consistently fluctuates between ~12-16 volts at idle, I'm worried that this will eventually kill my battery.

I neatly stripped and re-did my battery top-posts and all connections to my alternator, no change. The alternator less than a month old and the battery is maybe 1 year old. I have a remanufactured Motorcraft alternator (Part# GL8719RM) in my truck and the battery is an Everstart 750cca battery.

My best guess would be either a bad/loose ground OR an exposed positive wire is intermittently touching ground somewhere in my charging system. Anyone know where I should be looking? I retightened every ground I could find and I've followed every wire connected to my batteries and was unable to find anything suspicious.

Perhaps if I were to introduce a solid external ground to different points on my charging system I could at least narrow down the search. Any ideas?

Thanks.

 

[RonD]

The voltage regulator in the alternator is bad

Very odd for a Motorcraft part but can happen
Get warranty replacement

Your meter looks like it off as well
Battery are 12.3v to 13.0v, 13.0v is brand new just "out of the box", normally they are 12.8/9v after a few days
12.5v after 3 years
12.3v after 5/6 years and time for new battery

Your meter was showing 13.5v
Test meter on a 9v battery or other DC power source
This is NOT the issue, voltage with engine running should be stable just under 15v after start up, and then between 13.5v and 14v after 5 minutes or so
Your dash volt gauge jumping around shows voltage regulator is not stable


Alternators are not that complicated
The Voltage regulator is connected to the the Rotor
It applies say 9volt to the rotor and that creates a magnetic field around the rotor
In the outside case of the alternator are 3 separate Field Coils
As the rotor is spun by the engine the magnetic field causes the 3 fields to get an electrical charge, but its AC volts not DC
Each field has 2 diodes that convert the AC to DC volts and that voltage comes out the B+ terminal on the alternator, the stud/nut terminal, larger wire or wires

So 9volts in get 14volts out
Voltage regulator adjusts the 9volts base on "push back" from vehicles electrical system
Just after starting the battery is drained so push back is low, so voltage regulator will send say 10v to rotor to get over 14volts out
As battery gets recharged push back goes up so voltage regulator lowers the rotor voltage, to 9v and under 14v out
If you turn on head lights push back drops a bit so voltage regulator responds by increasing voltage to rotor slightly

Push back is call Electromotive force, if you want to read up on it



I would test the wires on alternator just to take that off the table
Unplug the 2 or 3 wire connector
Key off

Test battery voltage, remember it
Test B+ with wire still hooked up, should read "battery voltage", if lower then bad connection

test the yellow wire on connector, should be "battery voltage"

Test green wire on connector, should be 0 volts
Turn on key
Test green wire again, should be Battery voltage BUT can be 0.1v or 0.2v less

If all 3 wires test OK, then vehicle system is PERFECT