Overdrive throttle switch caused meltdown

[RickPA]

Just got my BJ8 back from resto. One thing I noticed was that the overdrive throttle switch was not delaying coming out of overdrive like it used to (should). When I talked to the resto shop about it he said the the new switches were calibrated to come out instantly. I decided to look for myself and found an article describing how to adjust the amount of throttle needed to disengage the O/D. When I made the adjustment it seemed like it worked, once, but was joined by a lot of smoke coming from the wiring harness and the overdrive relay also on the firewall. Of course the fuses were of no help. But what in the circuit could cause a meltdown like this?

I have a call in to the shop but was wondering if anyone has replaced their throttle switch recently (this one was from M***) and had any unusual happenings with it.

 

[Bob_Spidell]

This doesn't make much sense because the OD throttle override ('kickdown') switch opens a circuit when the throttle pedal is depressed a specified amount (usually, the smoke gets let out when you close a circuit). I've disassembled a kickdown switch and all it is is a set of points opened/closed by a cam connected to the shaft going to the throttle, and the relay is just, well, a relay (SPST I believe). The relay would mostly fail on OD engagement. My guess is something in the wiring is making contact with ground somewhere; neither the switch or relay would be likely to cause this (but it's not impossible).

 

[Michael Oritt]

Bob is probably right as going out of OD de-energizes the circuit. I wish you luck on finding both the source of the problem and the place in the harness where the wire insulation is melted. Without being critical of either you or your rebuilder an incident like this is a great argument for separately protecting/fusing as much as possible each circuit.