Idle issue

[whomadewho]

1997 Nissan pickup SE 2.4 5 speed. Runs perfect,charging good. The issue I am having - when I turn the fan on high,after driving for about 4 miles, when I come to a stop the engine will not idle and there is smoke coming out of tail pipe. If I leave the fan on a lower speed the truck runs perfect. Checked the voltage with the fan on high,checks out good. After I shut the engine off and let the truck sit for awhile, when I restart the truck runs perfect. I thought maybe the resistor that controls the fan switch or the heater switch maybe going bad,but if that were happening one would think there would be a voltage draw. Any thoughts would be appreciated.