Alternator B+ not reaching battery

[mooty]

Recently had a battery or alternator dilemma, battery charges fine but alternator failed bench test at parts store. However it was delivering some charge, just very inadequate. I suspected that the battery was not charging at all due to the way voltage dropped on a multimeter but neglected that notion and bought a new alternator anyway. Welp, same problem. All connections to alternator seem fine and it's receiving input power as required.

I'm still at a bit of a loss but I understand there's a "mega fuse" beyond the B+ lead from alternator somewhere. Any clue where that might be on a 99 ford ranger 4.0? and if that's the problem, should I worry about why this happened or do these fail sometimes?

 

[mooty]

Just an update. I located the mega fuse and am getting 0.5 ohms between it and battery as well as directly on the fuse itself, indicating that it's fine. Back to the drawing board unless I bought a bad alternator.

 

[RonD]

With key off, and all wiring hooked up.

Measure battery voltage, say it is 12.5v

measure voltage at B+ and alternator's case(ground), should be 12.5v, if not fuse is blown or B+ wire is bad

Unplug voltage regulator wire connector on alternator
There is a Yellow wire, it should also have 12.5v
This is the reference wire, it has a fuse in the engine fuse box, usually 15amp and labelled ALT

On the same connector will be a Light Green wire, this is the ON/OFF switch for alternator.
It should have 12.5v but only when key is on, so turn on the key and check it's voltage.
This voltage comes from ignition switch circuit and passes thru Battery Light bulb.
When not spinning alternator is a "virtual ground", so with 12.5v on one leg of light bulb and alternator(ground) on the other leg, the Battery Light bulb comes on.
When alternator starts to spin and generate power, it now has voltage so with voltage on both legs bulb goes off.

An alternator is an electric motor, but used in reverse, so if Light Green wire had voltage all the time the alternator would try to spin the engine via the fan belt and battery would drain, so 0 volts on light green wire(key off) turns alternator OFF, 12v turns it ON