I have a negative global variable driven from a design table, which is used in an equation to generate a sketch dimension. This scheme breaks when going positive to negative or back and does not cause the desired direction reversal in the sketch dimension and causes an equation error as well. Any suggestions to make this work?

P.S., I know that I can use a positive only sketch dimension scheme, but do I still have to implement this as of SW2015?

This is still the case as of SW2015 SP5. You have to create a proxy plane sufficiently far away from your geometry driven by variable X. Then your dimension can be equated to X+Y, where Y is the variable that goes from positive to negative.

It would be nice if dimensions could automatically flip if a negative was used, but I can understand why that would be a challenge to implement. It would require that every element being dimensioned also included a direction for reference. This is simple for planes, since it's just the plane's normal direction. But for any other element (points, lines, etc) that reference isn't so simple.