Problem with three speed furnace fan

Hello all. I originally posted this in alt.hvac but they pointed me here.
Two years ago I had to replace my furnace but I kept the old fan. Last year I built a frame around the fan and installed it in my attic. I figured that this type of fan would be quieter than the standard whole-house attic fans you see at Home Depot.
It worked great for a year but now has a problem. It's a three speed fan with four wires: one neutral and one for each speed. I had wired it with a three speed fan switch. The problem is that now only the low speed works. It runs when I switch it to mdeium or high, but still on the low speed. When I switch it to high, the pitch is just slightly different. Does this mean that the other windings are fried, or is it possible to fix this easily? I know very little about how multispeed motors are wired inside.
Any help would be appreciated.
Thanks. Bill
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Bill,
You may have a weak run capacitor. If the other windings were fried, it would not run at all. If the windings of the high speeds were open, it would only run on low speed. There may be a bad connection in the switch as well.
Stretch
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