I have 97 ford F350 Diesel with factory engine block heater. Worked fine until now. When I connect heater plug with power cord to house 120 current, it trips circuit breaker every time. Power cord tests fine. Sounds like short/ground somewhere/heater? How to repair and approx cost?

Do you know if this is a freeze-plug or "knockout" type block heater? If so then they are fairly simple and strait-forward. It is a basic water-heating element that slides into a freeze-plug hole on the side of the block and cinches down with a nut, squeezing a rubber grommet to seal.

Have you traced the wire all the way from the plug to the heater and verified there were no pinched points or wear spots in the insulation? If the wiring absolutely checks out, then my guess would be that the element has broken, and is touching (grounding) inside the motor. I have seen them break before. Hopefully yours is still in once piece when it comes out or else there may be a piece floating around in one of the water jackets. Pay attention to that.