We had a burner that would only heat to high so my husband replaced the rotary switch 6-7 months ago. Its worked fine until now. We were thinking it was the heating element this time, so we bought a new one. As we opened up the stove to replace it, we saw the rotary switch he replaced has scorched marks and the white wire insulation on the wire is melted.
Any idea what could have caused this or what we should do now?
Scorched Switch
Normal Switch
One likely cause of switch failure could be a short in the heating element to ground. Inside the metal spiral of that element is a piece of high-temperature, high resistance wire, such as nichrome, surrounded by ceramic insulation. If that insulation fails anywhere other than exactly in the middle, and the wire contacts the metal shell, additional current could flow to ground, drawing excessive current and damaging the switch. In that case, replacing the heating element should have resolved the issue.
Another likely cause of overheating at the terminal is that the QC (Quick Connect) terminals were loosened when removing them from the old switch. In the picture below, the gap marked with orange lines should be reduced.
Adapted from https://www.welchome.org/electrical-quick-connect-terminals-k.html
If the female QC connector can be pulled off the QC lug (male) connector easily, then that indicates there might have been a high-resistance connection, causing local heating at the connector. Give the QC connectors a squeeze with pliers before reinstalling. The connector should be a tight fit, taking some effort to force it on again.
Check the wire inside the white insulation and the QC connector. If the wire looks damaged (grayish, rather than shiny, or dull copper, or strands are broken), or the QC connector has lost its temper (softened do it no longer holds tightly), then fix that.
If the wire is long enough (you may pull it a bit from the sheath to extend it) then cut back to undamaged wire and put a new high temperature QC, for that gauge wire, on it. This is an example of that type connector from Amazon, but might not be what is needed.
If the wire is a bit too short, another piece can be attached with a butt connector.
In either case, insulate the wire and connections with high-temperature tubing or tape, such as glass fiber tubing.
BTW, even though the switch has some scorch marks, if it still works, I'd just tighten the QC connectors a bit and keep using it.