When I turn my heat on, my temperature guage maxes out and then of course my "engine" light comes on the dash. Truck is a 91 IDI. it doesnt seem to be overheating to me because the heat coming out of the vents isn't scalding hot. any tips?

 

It sorta remind me of how when cranking the engine the engine temperature gauge will move towards hot. I think it may be related to that somehow.