It is a split system we have changed the computer board,the defrost thermostat,the contacts and the unit won't stay in to defrost long enough to melt the ice off the out side what can I try next.it is 17yrs old and I don't have any more ideas of what to do HELP ?

Either another bad defrost board or its low on refrigerant.

Have you checked the charge? Properly charged units will defrost in the time allotted. It was -12 here the other night, and mine defrosted without incident.