On my 84 F250 IDI, the battery is being killed if it sits more than a day. I hooked a test light between the negative post and cable, it lights up, but goes off if I unplug the little plug on the alternator, not the bigger wire. Is this the alternator or the voltage regulator?

 

That's an alternator diode gone bad.

 

since you'll be buying an alternator anyway, consider upgrading to a higher output unit (many threads on that here)

 

That sounds like the regulator to me, quick test is to do what you just did but unplug the regulator and see if the light goes out.

 

I've actually never checked this on my diesel but all the other Fords have an external voltage regulator on the driver's side of the radiator support. That's where I would start looking. Should be a square box about 3"x3"x1" tall.

-Matt

 

If you are discharging thru the alternator, it means the regulator is stuck. On mine, it's on the inner fender on the passenger side, between the battery and the evap core. Mine is an '88, YMMV.