I’m having a frustrating problem with my 1991 Chevy 1500 pickup truck. The heater is ALWAYS on, no matter what I do. I suspect that some sort of mechanical controller or vent shut off device/air mixer is malfunctioning because I don’t think the heat is actually coming from the heater core, I think it’s just engine heat. I pulled the fuse on the heater which turned off the blower and controls, but the heat is still coming out the vents, especially at highway speeds. So this leads me to believe that exterior air is pulling engine heat into the cab.
I don’t know how I would even begin to fix this. I need a starting point. Where would I look for the vent or flap or whatever that mixes the cool air with the hot air? The AC hasn’t worked since I bought the truck about 18 months ago.
Any help or ideas you can offer would be great.