Question about Aluminum & Copper Wiring & a fan w/a defective switch

Hi everyone, electrical newbie here. :-)

I have very basic electrical wiring knowledge. I can safely replace a light switch, install a ceiling fan, etc. But except for an Electrical Wiring 101 Do-It-Yourself type book I bought at Home Depot, I have no formal training. Most of what I've learned has come from tinkering with and repairing the clipping machines I use in my animal grooming business (not the same as house wiring, I know, but that's what I have to draw from).

So here's an overview of my question (I'll probably give too much detail, but if I don't feel free to ask me for more info).

I have a very large 30" oscillating fan in a horse barn. It is a pull chain on/off. Three speeds. There is an unfortunate defect in the design of this very expensive fan that I won't go into, but suffice it to say the beaded pull chain has snapped off inside the motor housing more than once, necessitating the replacement of the three speed switch. In 8 years this will be the third switch.

I don't know what you call the type of connection, but you install the switch by forcing the wires past a metal clamp under pressure inside the four wiring holes in the switch. So, in other words, once you install the switch, you can't remove it w/o damaging the wires, you have to cut the switch out. (Yes, I tried inserting a metal pin to release the wires, doesn't work well in this case). So, anyways, the switches have been cut out so many times, there is barely any wire left to connect a new switch to.

It's a special fan that is rated safe to use in environments with "explosive fumes" like a mechanic's garage. Because of that, the entire motor is permanently sealed, the wires that run into the motor disappear into what looks like a block of silicon. There is no hope of hard wiring new wires into the motor itself. So, I am left with these little tails of wires to work with.

Now, after that lengthy introduction, bare with me while I detail today's adventure. As I said, I've replaced this switch before with no problem, so I thought I could easily do so again. I replaced the switch, tested the fan and discovered an odd thing. The fan had Hi speed and Medium speed but when I pulled the chain a third time (which should be low speed) the fan remained at medium speed. A fourth pull turned the fan off as it should. As curious as I am as to how this could be possible- (I can see the fan not having HI speed if a wire is loose and not providing enough voltage, but I don't see how it could not have LO speed unless the replacement switch itself is somehow defective- which I am now beginning to suspect) I concluded that I just must have not inserted the wires strongly enough, so I decided to remove the switch and reinstall.

I did my best to work the wires out of the switch without damaging them using a pin in the holes, but these are threaded wires, not solid, so it still frayed the wires quite a bit and they had to be cut shorter to clean them up.

The red, yellow & blue wires which run from the fan motor to switch holes 1, 2 & 3 respectively were ok to use again, but the black wire that goes into the cord plug was just too short. So I spliced in a little bit of copper wire that I cut out of an old cord plug I had laying around that used to power a pair of heavy duty sheep shears. The gauge of the extra wire I used appeared identical in size. Sorry, I don't know gauges, but I counted about 20-25 or so little wires. All other wiring related to the fan is aluminum. I don't know if this makes a difference or not. I understand copper carries different current than aluminum, but it's not as if I was running copper all the way to the outlet, it was just a splice in between the aluminum cord plug and the aluminum wired switch.

Anyway, I reassembled the switch, plugged in the fan, turned it on, everything seemed to be working fine. Except STILL no LO speed. I resigned myself to having to put up with it being that way and went about my business. About 5-10 minutes later the fan just shut off for no apparent reason. I have a very sensitive GFCI in the barn and it will trip if someone sneezes. It did NOT trip. The fan just went off. I could smell a faint smell of what I call "hot wire" smell (not quite burning, but you know). I took the housing apart again and looked inside. No sign of burning or melting or anything. But all attempts to get the switch working again failed and the hot wire smell scared me off.

I have ordered yet another replacement switch from the company. So, here is the question at the end of my very long story: Do you guys think the switch was just defective? (seeing as how it seemed to have the low speed problem) Or did my copper splice burn something out?

Any explanation of the do's and don'ts of mixing copper & aluminum is greatly appreciated.

Can I just go ahead and install the new switch as planned when it comes (using my copper splice)? Or am I asking for trouble? The entire cord plug could be replaced, of course, which would do away with the need for a splice (there is plenty of white wire running into the motor) But again, all the cord plugs I have laying around for fixing my clippers are copper. If I went that route, would I have to track down an aluminum cord plug?

Thank you to anyone who had the patience to read this ramble and can offer any advice.