Let's say, I draw a wheel, cut in half. It is pointed with the flat part towards the viewer.I only want to dimension one half of the wheel, the other half goes into an assembly.I want to dimension the diameters, not the radii that interest me.Is it possible to start the dimension from the axis line towards the outer edge (so pointing half the desired distance), and getting the ∅ value in the dimension?

Which version of AutoCAD are you using?You want to cut the wheel in half, so that you're left looking at half of the wheel, i.e. a semi-circle.  Then you want to dimension from the centre, to the circumference but indacate the diameter not the radius?  It sounds a little "dangerous" to me as you'll be indicating a measurement that isn't true but it can be done, easily.  Just use text override and enter whatever value (and text) you want.  I would indicate that it's the diameter as well, just to clarify things.HTH