dont know if this is proper place but i have a issue..my '89 suburban has a crank no start. I can jump it and it will fire nicely but take the box off and after maybe 1 more start or driving round the block all i get is click-click or a slow crank. Its chargin at 12.8 at the battery and 13.00 at the alt. If i let it sit for a while it might fire..any ideas?

 

Weak Battery and weak Alt. Should be closer to 14.4 with the battery low.