When cooking Salmon on the Grill you should place the skin side down on the grill and then cover it.
Cook the salmon without disturbing it at a temperature of 325 F for 10 or 15 minutes or until the salmon starts oozing out the fat which is similar in look to mayonnaise.
Also look for signs that the salmon is flaking and when it is flaking and oozing out the fat then it should be fully cooked by then.