I've been working on my mom and step dad's rodeo for years now, literally. Over the last couple of years it has started eating alternators. I figured it was just the cheap aftermarket alternators the local parts houses sold me. But it's getting really annoying and there has to be something else going on here. The last one only last 3 months, and i just put another confirmed good alternaotr on it, and now it's not charging at all. I'm looking for a wiring diagram to show me if I have all the correct voltages on the plug going into the alternator. I ran a complete new wire to the battery, so I know it's not that one. It's a 6/96 and has the four wire plug. PLEASE HELP!! They're using my truck and I need it back!

 

Make sure that your Alternator/Battery light is not burned out in your dash. The charging system uses this bulb to complete a circuit that creates an electric field so that the alternator can start generating a voltage. If this is burned out, it can be hit or miss whether the alternator will generate a voltage.
I just had this problem on my 2001 Rodeo.

There may also be a fuse in line with the bulb that could be the problem.

 

The light is working fine, and i actually checked every fuse in the truck just to be sure, and everything is good there. I found the alternator plug up near the battery, and all the wires going to the alternator checked out ok, but i don't know what wire does what to check and see if i have 12v, ground, etc.

 

If it's like my 2001, The biggest wire is for the 12V system and charging the battery. The small wire goes to the alternator/battery light circuitry, and is the one that also supplies the initial electric field needed to start the alternator generating voltage. Follow this wire. If I remember correctly, it goes through the fuse box under the hood. Maybe something in there, but if the light comes on when you first turn the key, probably not.
The ground is going through the case.
If this doesn't help, I once used some online help from JustAnswer.com. They ask for a nominal payment, but it's voluntary and you don't have to pay them until after you get your answer.

 

Did you have the alternator checked to see what failed in it?

 

Clean ground wires from engine back to battery!
If there is a high resistance connection /loose, dirty, broken conductor...
Then the alternator can try to generate a abnormal high voltage to overcome said resistances which can
result in abnormal wear and tear on alternator components!
Most strange electrical problems can be cured with clean (ground) return wires back to the battery!