How many volts will a alternator put out

[Crumm]

I am diagnosing a strange problem on a Ford truck but instead of going into a long drawn out explanation of the problem I will try and keep it to one simple question that will help me in the diagnosis.

If you disconnected the main lead off the alternator then checked the voltage at the alternator power stud with the engine running how many volts should it read? In other words what is the maximum voltage a 12v alternator will put out when asked to give all it has.

 

[JLDickmon]

I know full fielded under load they're capable of around 18 volts..
whether or not the regulator would let them, is another matter...

why?
What's going on?

 

[Crumm]

why?
What's going on?

This is on a very old SuperDuty(1975 lol) with a external regulator. Everything keeps blowing including light bulbs and electronic ignition modules. I took the alternator to a reputable electrical shop and they said it tested out OK. They figured it was the regulator so I put a new one on it. Fired it up and once again things seemed a little bright just before the dome light bulbs shattered as sparks flew out of them. I put a test meter on the alternator after disconnecting the main charge wire which has been disconnected all summer since it blows everything and melts the fuse links. Reading was 48 volts at idle. I think I have a problem...

 

[JLDickmon]

This is on a very old SuperDuty(1975 lol) with a external regulator. Everything keeps blowing including light bulbs and electronic ignition modules. I took the alternator to a reputable electrical shop and they said it tested out OK. They figured it was the regulator so I put a new one on it. Fired it up and once again things seemed a little bright just before the dome light bulbs shattered as sparks flew out of them. I put a test meter on the alternator after disconnecting the main charge wire which has been disconnected all summer since it blows everything and melts the fuse links. Reading was 48 volts at idle. I think I have a problem...

external regulator as in on the firewall/fender?
umm...
I Ate Sally Fields..
dammit.. been so long I have to dig up a diagram..
it's got a grounded wire, but I can't remember what one..