Alternator on ‘84 720 went from putting out 14-15v at idle to 6v! HELP!!

[Large_pepperoni]

I have an ‘84 720. Had some problems with alternators recharging battery. Fixed up my grounding system and bought a brand new (not reman) alternator. Truck has been driving fine for the last month, until today.

 

When new alternator was first put in, voltage would read at 11.8-12.2v when first starting, and at 14.5-15.2 after a drive. After every drive, battery has always been right around 12.4-12.5v.

 

However, there has been one weird issue. Since replacing the alternator and adding a ground wire from engine block to chassis, the under-hood light bulb (which never turned on before) now will not turn off, even with the key out of ignition. No problem, I just disconnect the battery every night before bed. But last night I forgot to unhook it and woke up with a dead battery. Jumped it up, drove home and parked for a few hours, with battery on a trickle charger.  Started it up and drove around for 15-20m, and the battery is at 6.5v at idle. Got a jump again, but the reading was the same. 

 

This is is the 3rd alternator I’ve had in the car. At worst, the old alternators had been putting out 11.5-12v at idle. Not enough to keep starting over and over, but never anything this bad. 

Can anybody tell my why this might happen? All connections are clean and tight at the alternator. Belt is tight and in good shape. I have visually inspected all the wires under the hood. I AM LOST! Any input is appreciated. Thanks!

[]2eDeYe]

What type of battery are you using and have you had it load tested?  

[Large_pepperoni]

Can’t remember that type. Brand is superstarter. Battery has been tested and is in good shape. Charged is fully and is back up from 6v to 11.8v at idle. Still not good.

[BambooU]

This could be out there, but what is the condition of all the connections? As in, are the grounds clean, connectors corroded, or any loose / sloppy connections?

 

I was pulling 12.01 volts out of my brand new alternator and relatively new battery. I cleaned my contacts and vinegar dunked my fuse block and it jumps to 13.95 volts. 

[datzenmike]

Removing the engine light is easier than the battery cable. Or look at the contact switch that turns the lamp on, it may be stuck or bent out of adjustment. If the engine is running you should still have a 14.3 charging voltage even if the cable is disconnected. If you have been removing the positive cable you may have damaged the fusible link(s) connected to it.

 

You need about two volts more than the battery to push a charge back into it by the alternator.

 

A charging alternator should read (about, more or less)  14.3 volts.

 

A good charged battery at about 60F should read 12.65 (give or take)