i have an 81 pickup that the batterie dies while it is running and then it wont start. the alternater tested ok the battery tested ok, it has a crack job of a wiring harness but everything looked ok what else could i check? if the battery is charged it will sit fine but die while running.

 

Check the alternator,it can test good but still be bad, if its bad your truck will run until the battery runs out of power, once the battery is dead the truck will die because there is nothing left to fire the ignition. does your truck have an external voltage regulator ?, if so that could also go bad and cause your battery not to charge.