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.
I don’t know how you use the truck, but you might bypass the heater core by connecting the two heater hoses together. When winter comes, reconnect the heater core. This may be your cheapest way out for an old truck.
On my dad’s 1939 Chevrolet and his 1947 DeSoto, there was a valve under the hood to shut off coolant to the heater–which in each case was a box under the dashboard. He would turn off the heat in the summer and turn it back on in the fall.
Under the dash, inside the heater/ac box, there is a flapper operated by a vacume motor that will shut the door and direct the air flow through the ac unit instead of the heater core. If the flapper is still open to the heater core, air from driving will still direct heat into the cab. The reason for the flapper to not operate could be as simple as a hole in the vacume line from under the engine to the heater box, a bad vacume motor, a disconnected hose at the hvac control, etc. I’d check under the hood first and then progress to the hvac control and heater box. When you start finding the vacume motor or line, you can test it by applying vacume to it, like from a long hose running from the engine to see if it works or not.