Pour new concrete slab over old cracked slab?

The old slab is obviously on an unstable foundation. Anything you pour on it without correcting the problems will end up like the existing slab.

You'll need to watch the size of the aggregate in the new concrete. And I assume the old slab your pouring over isn't cracked and your putting in expansion joints in the same locations as on the existing slab. You also need to prepare the old slab by cleaning off any loose material, and either soaking it with water so it doesn't try to pull all the moisture out of your new concrete you pour, or applying an epoxy bonding agent.

If the slab was too thin or the mix was poorly designed then that would cause it to crack. If your using it as a garage - parking a car on it, then you should dig it up and lay it properly, but if its just for light use, then you could get away with going over the old stuff.

by what you are describing,i would bet my last dollar the cement is thin and you have no rebar.

its not the right way to do it,i think you know that,but a person can drill the old slab and add short ft long rebar into the holes then tie your rebar grid to the uprightslike i said it isnt right,but i know you already have planned to do it,so if your not removing the slab,this is a cheap alternative.

No, don't do it. Remove the old first. Then pour new over a bed of sand.