Noticed the battery light on my DII wason today.WhenI gothome I checked the battery, it's good. Checked the mega 150A fuse (which caused me lots ofprobs justthis past feb.), it's good.So this lead me to believe that it's the alternator.Checked the voltage at the megafuse (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, whenI fireditup, to move itinto the garagethe 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 alternatorjust asit 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 thebattery 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 couldverify.. My disco is supposed to stall anytime the battery cable is disconnected, good, or bad alternator, correct? Akid 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.