Battery light?

Noticed the battery light on my DII was on today.  When I got home I checked the battery, it's good.  Checked the mega 150A fuse (which caused me lots of probs just this past feb.), it's good. So this lead me to believe that it's the alternator. Checked the voltage at the mega fuse (it's easier to reach than the alternator) it's the same as the battery, running or not. The truck sat for a few hours, when I fired it up, to move it into the garage the battery light was off? So, I drove it around for a few, and the battery light didn't come back on until the truck warmed up to operating temp. I down shifted, the rpms went to about 2800, the light went off.  Two minutes later, the light was back on and stayed on this time.  Any Ideas?  Did I catch the alternator just as it was going out? 

The Alternator is on the way out its still putting out voltage but not enought, thats why at high rpm the light whent off.

Lets get specific. How did you test the battery, what was the exact voltage?
Did you check the belt, is it tight?
I would suggest you go to your local auto parts store and have them do a load test on both the battery and alternator.

OK,  Took the alternator off, and took it to several parts stores, and an auto electric specialty shop, all said that the alternator is flawless??  Load tested the battery and it is perfect.  The belt is tight, and was changed in february along with the battery and mega fuse. Any other ideas???  In the meantime, i'm gonna re-install the alternator and see if the light is on when cold, and comes on again at operating temp.
 
Also, if someone could verify.. My disco is supposed to stall anytime the battery cable is disconnected, good , or bad alternator, correct?  A kid at one of the parts stores suggested the "remove battery cable alternator test" to me, insisting that it worked on every vehicle.  

Your engine should not stall when disconnecting the cable as long as your battery has between 13.6 and 14.4 volts.
How many volts do you have at the terminals when the engine is off, and when running?

Battery voltage is 12.5  my cousin (who is a ford diesel mechanic) brought a load tester, which can be best described as battery cables, connected to a space heater equipped with a volt meter, and a toggle switch.

12.5 v at the terminals, running or not.

12.5 is too low to handle the ECUs, need to look into this further, you need at least 13.6 volts.

Alright..  Mike, i'm gonna need your help.. I put the alternator back in, cranked the truck and the battery light was still on. I only had a test light handy, so I checked all fuses, and connections, including the two wires on the back of the alternator. The large wire had power, but the small wire had none. Went back inside the vehicle, rev'd up the motor to about 4k rpms. Battery light went out and stayed out.  And now there is power on the small wire on the back of the alternator. I let the truck idle for about 10 min and the light was still out. Any ideas??
 
What exactly is the small wire's role? I know it passes through a 10a fuse and to the key switch. But i'm not quite sure what it does exactly?

Are you looking at the tack lead?
Drive down to a local auto parts store and get the system load tested, I know you have done it out of the truck now do it the correct and easy way, and it's free.

On my way there now. I'll post back in an hour with the results.
 

went to AutoZone, battery has 12.5 volts, and shows good.  Charging system is putting out 13.9v and 13.5 under a load. But the battery light wasn't on at the time, it was on upon cranking, and stayed on until I rev'd the motor up in neutral at a stop light. It hasn't come on since then.

13.5 under load is under spec's, granted, not by much but under.

Alright, it's got to be the regulator (new alternator time?).. As I went to leave my dad's house, the battery light was on again.  He has a Schumacher system tester in his garage. With the battery light on the system's only putting out 12.5v (the same as the battery), Got the light to go out, same as before, and then it was putting out 14v.

Sounds like its time to bite the bullet and buy a new alternator.

make sure that your small wire to the back of the alt is not getting shorted to ground, if it does, the light will be on and it will not charge, have any work done reletivly recently before the light started comming on?

quote:

ORIGINAL: Spike555

Sounds like its time to bite the bullet and buy a new alternator.

 
 
Most times alternators do not go out all at once,,,so Spike is right.