This one is bleeding frustration. No "high" on fan setting, and other settings result in blower running for a few minutes, then stopping for a few minutes, then running for a few minutes, etc. Have replaced control switch, resistors, relay (next to resistors), and blower. Have swapped relay under hood with one known to function. No blown fuses. Thinking resettable breaker somewhere but can't find it, and that doesn't explain no high setting. Next may be the double barrel, but don't want to shoot her yet (only 190K miles).

Any "try this next" would be appreciated.

 

Wow you have definitely eliminated all the "usual" culprits especially the blower resistor. I would run it and see if the wires going to the blower or the resistor are getting hot - the wiring is the only constant in this scenario. Have you owned the truck long enough that it was working right at some point or did you inherit this issue?

 

Bought it new, and have done everything myself so I know her well, and its why this is so frustrating (may be first trip to shop in 11 years).

Only wiring observation was connector for hot wire on motor was a little crispy, but thought that might have just been from running the thing for fifteen straight hours multiple times over her life.

 

I would suspect that wire being "crispy" has something to do with it. I know on some of the newer trucks there was a harness issue that would cause weird blower issues. I have installed a few replacement harnesses on them, but this was a bulletin on some of the newer trucks. It sounds like a similar situation though. On those trucks it was the harness going to the blower resister that would get REAL "crispy". The fix was a harness with thicker wiring and a new resistor.

 

Ok, will give that a try next. Thanks for the suggestion.