Assuming you measured at the lug on the rear of the alt.....If you are getting zero volts on the charging lug and the alt is checking out good, IMO, you have a couple of areas to look at:
1 - the exciter wire isn't making a good connection with the pigtail or there isn't 12v coming thru the exciter wire.
2 - the sense wire isn't connected into the elec system, therefore it's not telling the alt to produce juice or the pig tail isn't wired correctly (wires reversed)
You would only need a diode in the exciter wire if you had run-on (truck wouldn't shut off with the key)
you stated that the truck would run without the battery sometimes........ so your system is working? but just not as good as you'd like?
BTW that puts a huge load on the alt and is very bad for the alt, the alt wasn't designed to handle the entire load of the elc system all at once etc....
Last edited by EOD Guy; 02-21-2013 at 05:02 AM.