Chassis is a 92 SI. Fan is a 12" Spal. The 15A fuse keeps blowing when the fan switch goes off, but I put a 20A fuse in instead of the 15A, and the fuse doesn't blow.

I know that's not the right way to fix this problem, but someone please explain why this works and why the 15A fuse is blowing in the first place. Thanks!

 

The fuse rating is typically half of the draw of the circuit. So a 15A fuse is protecting a 7A circuit. (more or less) Apparently your circuit is now loading up more than 15A,but less than 20A.

If you use a larger amp,you run the risk of starting a fire, as the fuse is designed to protect the wiring from overheating/melting/burning.

 

So to properly accommodate a larger draw, I should replace wires with larger gauge?

I replaced the stock rad fan with a Spal fan... Would this be the cause?

 

Yes.
I dont know. I'd have to troubleshoot....

 

most likely the new fan draws more amperage, which makes sense since its a larger more powerful fan. Ensure the wires to the fan are the necessary gauge.

 

Awesome, thanks for the info!