Hello I am trying to help my father with his 2003 Ford Ranger he has replaced the alternator twice back to back thinking the truck won't charge both alternators tested fine he replaced the battery as the truck drained the old battery. He can start the truck and it will run but as soon as you remove the positive cable from the battery it dies. Also the truck has 13.7 volts coming out of the back of the alternator while the truck is running, and he has a battery light on inside the truck. We have checked the fuses inside the cab of the truck they appear to be fine what can it be guys ?

 

Welcome to the forum

You need to test the battery voltage with engine running, it should also show the 13.7v, same as B+ terminal on alternator, if not then in a 2003 the fusible links are blown

Simple test
Key off
Test battery voltage, remember it, say its 12.8v
Test voltage on the B+ Terminal, should be EXACTLY the same, 12.8v, if not then fusible links are blown

Also unplug the 2 or 3 wire connector on alternator, test the Yellow wire, it should also show 12.8v exactly, if not then its fusible link might be blown, but most likely just the 2 12gauge links are blown

Fusible links are used as "slow blow" fuses, they are wires that will melt and separate without starting a fire, they are about 6" long

These will be near the engine fuse box not the alternator, you will need to unwrap the wiring harness at that end to access the fusible links

2003 and 2004 charging systems were the same, have a read here: https://www.ranger-forums.com/genera...arging-155666/

Post number 8

It was just a corroded connection on the yellow wire not blown fusible links