IIRC the extra pin on a 4 pin motherboard header may report if the fan has totally stopped, like a failure line, to tell the mobo to shut the CPU off. I've got an old 4 pin fan here from a '94 vintage ...

This post is a quick sequel of my previous article jam-packed with my personal experiences and observations while I was fiddling with standard 4-wire PC cooling fans. Today I will be talking about the ...