I have a 2002 ford ranger 4.0l 4x4 auto trans and it ran great until the other day when the battery light came on and later it would not start. I tested it with a voltage meter after I jump started it with my car and the voltage was bouncing between 11v and 15.8v. so I figured the voltage regulator in the alternator had went bad and bought another alternator to replace it only to find out that now its not charging at all. I took the replacement back and swapped it as if defective and still got the same results. I have a full 12 v running to the alternator post at all times, the yellow/white wire running to the regulator has a full 12v at all times, the white/black wire I tested for resistance between the connections and got none but the light green/red wire is only getting like 1.4v is this normal? and if so can someone please tell me what I'm missing here. I'm at a loss and don't want to have to dump a whole bunch of money into a shop because its not the best looking truck to say the least

 

There should be two fuses that are in the alt circuit. One is a large fuse located at the rear of the power dist center, the other should be in the fuse panel. Make sure they are both good.
To really test the output of the alt you need to supply a load like headlights + defroster.
The sensing circuit will sense the perceived load and instruct the alt to increase output to handle the load. This sensing system can be contained in the vehicle's computer which then controls the alt output. If the connections at the plug are good and tight (known for getting loose,) and you still have no increased output , it may be time to get an alt from FORD ! Not all , but many, rebuilt aftermarket alternators don't work correctly on Fords.

 

welcome on a board !!