under a full load what should the alt. voltage read. do i turn on radio, fan, air,lights, wipers, etc to read this. if its under a ceartain voltage will this effect trans.? what else could keep trans. from not engaging at all? thanks.

You need 13.8V to charge the batt. Under a major load it might drop down around 12V, which is OK if it's only for a short time, as you're not charging the batt.

It's hard to say if low voltage will cause a problem, you didn't state what year or tranny you have.

YES, low volt WILL cause retarded symptoms through out the whole truck.

OUTPUT volt from alt should be nothing less then 14v while truck is running WITH accessories on, if it's less then this, it's dying.