So I've been working on fixing this window AC unit for a while, and when I started I knew next to nothing about circuits. At present today, I know a bit about circuits -- enough that i at least know what the various components are for, plus how to use the multimeter, but I am faced with a mystery I was wondering if anyone else might be able to put an educated guess on, because this puzzling event happened back when I DIDN'T know what I was doing, so the details meant nothing at the time. Now that i'm more knowledgable, I am unable to reproduce the same result.
Some background: Our apartment suffered an overvoltage event, which fried all of our surge protectors as well as the two appliances that were plugged directly into the wall: the AC unit being the more valuable. Upon taking the thing apart, i noticed that the board had a burn mark on the back side which corresponded to where an MOV varistor was mounted on the front.
Here is the point where I wish i knew at the time more about what I was doing, because when I first saw this I pulled out my cheap dollar store multimeter, which I had no clue how to use, and put it on SOME SETTING and put the leads to each side of the varistor, which suddenly caused the AC unit to jump to life and begin working! Without knowing what the purpose of the varistor was, I immediately ordered a set of new ones with the goal of soldering a new one on. I used wire cutters to remove the old varistor off the board.
Now about a month later, I understand what a varistor does, and so am at a loss as to how bridging its leads on ANY SETTING on the multimeter would have caused the entire unit to suddenly turn on. I also can no longer reproduce whatever it was that I did, and any attempts to bridge the leads now results either with A) nothing or B) it immediately blowing the 3.15A fuse placed in series next to it.
So here is the puzzle. What possible reason would it, when i original bridged the two leads of the varistor with the dollar store multimeter, have caused the AC unit to turn on? From my understanding of varistors now, it seems like the only possible results of putting something other than a varistor in between those two points in a protected circuit would either A) short it or B) do nothing, passively. I don't understand how the circuit would be turned on by doing something to the protected side in parallel.