My jeep has suffered the famous blower motor switch meltdown at some point in the past. When I bought it, fan speed 3 was intermittent and so I tore into the dash to find a unique problem....
Both the fan speed switch and the 3 prong switch next to the vacuum control have been melted. Someone added a relay to the input node of the blower switch by tapping into the 12V socket (I assume for power and ground since two of the relay wires went to each of the socket wires) and also tapping into the middle prong of the vacuum control switch. Needless to say I don't quite understand what they did by adding a relay only to the input node, but whatever they did, it resulted in two of the relay socket prongs melting. I tested the fan on high and found the relay to get extremely hot as well as the two wires that corresponded to the melted socket prongs. The relay they used was a 30amp max relay... Also, I actually have a 4 prong plug that isn't plugged into anything and one wire is cut on it. I have no idea what this plug is for but it has positions 1-4 marked on it and the words "3 amp" on it.
I was able to get the whole system working by installing a new blower switch and various new spade connectors for all previously melted connections. So here I am a few months later and I get the ole acrid smell through the vents and sure enough the relay is melting its way into oblivion.
I've decided to truly fix the problem and I need help with a few things. My plan is to add 40amp relays to each fan speed and I understand how to do that. I've already disconnected the old relay from the 12V socket and rewired the socket so that it is separate from anything else, the way it came from the factory. So...
Where does the input node of the blower switch need to be connected?
Where does the middle prong of the vacuum switch need to be connected?
And out of curiosity, what is the extra plug for?
I've cleaned my resistor and it works fine (since I had all four speeds). I removed my blower motor and am probably just going to replace it since it's obviously worn out enough to melt the wiring once before... I'll post pictures of everything in a little while.
My Frankenstein setup! Notice the unused plug with one wire cut!
Melted Relay. Yellow wire (30) went to input node. Red wire (87) went to 12V socket (prob positive). Blue wire (86) went to middle prong of vacuum switch. Brown wire (85) went to 12V socket (prob ground).
I've only had to redo the wiring on an older TJ which is different so I can't help on the above, but I can offer that since your wiring has melted again, the likely root cause is the fan motor. Once the fan motor starts going bad, it starts drawing a lot more current which starts melting wiring, the switch, etc.
Fortunately, the fan motor is an easy swap from the engine side of the fire wall once you remove the battery.
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Does anyone know what the "input" node for the blower switch is for? I'm kinda thinking it just powers the switch and so maybe I can just run a relay on it and 12V straight from the battery??? Everybody always adds relays to each speed node but nobody every says anything about the input node...