If it comes off the thermal valve on top of the thermostat housing it must be a vacuum switch for the AC. It's meant to kick up the rpm when the AC compressor bogs down the engine. That's my educated guess cause I couldn't afford AC so I don't have that set-up, but that's one purpose of the thermal valve.

'89 P'up, 2.6 I-Tec, 488,000 miles and done... gone to the great beyond