This is so bizarre. I haven't been able to figure it out.
I have Dave McKeen's fan fail box (the blue unit) installed, along with Hervey's low-current fans. Fans work great, come on and off as they should. The fan fail unit also works properly - if I run the fans with the car idling and unplug one or both fans, the dash light blinks with one or two flashes exactly as it should. And around town, the light is always off - just as you'd expect.
But, when I get on the highway - the light blinks double as if it detects both fans being failed, any time that they're on. It doesn't do it unless the fans are running, and the fans are definitely still running even when it's blinking.
I had thought it might be something with engine RPM (alternator outputting different voltage or something, perhaps?) so I did a test with the car parked and the revs held high - no flashes. Only at actual highway speeds.
The only (far-fetched) idea I can come up with is that the forward air pressure on the fans is causing them to draw much less current when spinning and the fan fail unit thinks there's no load? Like I said, it's far-fetched.
Anyone have ideas?