I've got an 87 full size blazer, 350, 5.7 and a while ago the high setting on the blower went out. The lower setings work fine. I checked a few things at the hi blower relay and here is what I found: 12 v on red wire, voltage on orange when turned on, grounds check out, voltage on dk blue wire from blower resistors seems to be fine although there is voltage there when set on high also(don't know if this matters), there is 12 v at the connector on the blower end of the purple wire when disconnected but when i plug it in, bye bye voltage. If the blower is bad, that would cause problems for all settings right? If anyone has any other things to look for I'd appreciate it.

Replace the high relay.

On some of those older trucks there used to be an In-line fuse for the high speed that ran along the firewall. The fuse was actually inside of a big rubber bulb. It won't cost you nothing to look.

I tried the hi blower relay and same problem. Anything in the motor that might do this?