1991 F-250 vent always blows hot air

Question

tl;dr;? see bold text

I recently inherited an old 1991 Ford F-250. It seems to run well, only has 80,000 miles, but it has two main issues (I'll post a separate question about the other one).

The one issue I'm trying to figure out right now is why the vents always blow hot air. I can turn on/off the fan just fine, but the vent always has hot air coming through it - with the exception of when the truck is not yet fully warmed up, I can get a little cold air from the A/C being activated. This obviously doesn't last long, as it turns hot when the truck is warmed up.

I'm not a mechanic but I've always done most of my own repairs/upgrades, usually from resources I find online. I haven't found anyone with this specific issue (but then again, my Google Results are contaminated with what appears to be a common issue with the F-250 vents that is different from mine).

Does anyone know what could be causing the truck to always have hot air coming out of the vents (when it's fully warmed up)?

EDIT: I doubt it's significant, but I updated the year to be 1991. Was wrong in original post, it's not a 1992.

Answer 1

Your truck should have a vacuum actuated blend door actuator which looks something like this:

One of four things (I can think of) are going on with it:

1. The vacuum line to the actuator is pinched or broken.
2. The actuator itself is worn out.
3. The linkage from the actuator to the blend door has become disconnected.
4. The vacuum actuator switch (the part in the dash) has gone bad.

From the sounds of it, the blend door in your truck is stuck so it is always forcing air through the hot side (ie: the heater core) and into the cab of the truck. You'll need to figure out where it attaches, but it should be fairly accessible from the bottom of the dash on the passenger side of the vehicle (this is the most likely place for it to be). If you switch the position from the temperature control on the dash, you should be able to hear the vacuum noise as it tries to move the actuator. This should lead you to where the issue is most likely at.