84 F-250 6.9 glow plug system

[bennetr]

Replaced relay, control switch, plugs 6 months ago. No problems. Truck overheated (different problem fixed) and gradually it became harder to start truck. Got to the point that the only way to start it was to plug in the block heater. Now the block heater will not energize.

Question 1. Can the overheating cause the control switch to fry and cause the no start. Power relay seems to be ok, all lamps come on. Once the engine was warmed up. It would start all day. Once the engine cooled down. No start.

Question 2. What is the real reason for the block heater? How cold does it need to be? How does it cease to work? I left in plugged in for 3 days straight with out unplugging. How do I replace it. No mention in the manual.

That was more than 2 questions but need some advice before I spend 100 dollars on a new control switch.