Excess resistance in the circuit is what causes it to burn the harness. Wiring is to small to handle the heat when the plug doesn't use the amps up to heat the tip. Heres a simple explanation:
Ford/Nav VC gasket/harness was redesigned with better conductors several years ago. Here's a basic way to explain it without math at all:
If the glow plug (load) were to short, the burn or weld will happen at the point the short occurs. If you use a welder it welds at the electrode (load). It won't burn up your weld cables.
So why does the harness connector melt? Excessive Resistance causes heat. A GP has little resistance or ohms. It's nearly full power that warms the tip using up power to make heat. What little power isn't used returns thru ground to battery completing the circuit. Excess resistance or poor conduction from green stuff or corrosion prevents full power from flowing to the load (glow plug). The wiring gets hot and melts because you still have a complete circuit back to battery ground, but no load to use the power up and keep the wiring cool as power flows thru it.
Now think of the welder again. What happens if you forget to put a ground clamp on the metal to be welded and touch the electrode (load) to the metal? Nothing. The circuit is not complete, so no power flows at all. No weld or short.
Best way to prevent the burned wiring is a good conductor to your GP's. I pack Di grease in all underhood connectors. The best way to keep the connection moisture free and clean is silicone dielectric.
If your harness burn't from the GP connector back, then you may have a shorted plug. Ohm it, compared to the others. If it ohms out higher than the rest it's also BAD. The extra resistance is inside the plug, not allowing full power to the tip. Since it still conducts, heat builds and melts wiring. If the internal GP circuit doesn't conduct, no power flows--no heat. Just like a welder without a ground clamp. We would be better off if our GP's burn't out like light bulbs do.
