What is this thing???

Can someone please tell me what this is and where does it go? I found
it just dangling around under the hood, so I taped it off to something
so it does flop around.

Thanks!

http://www.flickr.com/photo_zoom.gne?id=99026460&size=o

1987 Volvo 740 GLE

 

That's the aux. fan thermal switch, it turns on the fan in front of the
radiator if the coolant gets too hot. It goes where that hex plug in the
radiator is right above the tranny cooling line. Normally there's a
rubber fitting there it fits in.

 

Aha! Thanks!

 

Last I remember, there was a plastic plug seemingly screwed in that
hole, as if it were removed on purpose.

 

Someone probably replaced the radiator and didn't have the right fitting
to put the sensor back in. The original rads had plastic end tanks, the
pipe fittings tend to snap off those when they get old, bad idea from
the start.