71 Dart A/C car heater issues

The ac on these cars blows harder than the heat. If you look at the wiring diagram the blower, it never gets full power on heat, only ac. I think this was designed this way because cool air is harder to move than hot air. It is likely that your fan is operating properly, but there still may be an issue, could be the fan resistor.

If the fan is blowing and you are not getting heat, there could be a number of issues. As Kendog has pointed out, the easiest thing to start with is the heater valve under the hood. First see if the lever moves when you change from heat to ac. That will tell you if it is even working.

Take it out and put a barbed hose splicer in and see if that helps. The valve can be tested by moving the lever with your finger and blowing through it once you have it off. If it is moving with vacuum and you can blow though it when it is in one position or the other, the valve is not your problem.

The rest of the process gets a lot worse. since these cars are 50 years old could be a number of things. Some ideas:

1. Heater core could be plugged. You could try flushing it out. Do it both ways. The bad news here is that the rust may be the only thing keeping it together and it may start leaking after you flush it.

2. The heater box could be full of leaves & trash blocking the air flow. There is no filter, so anything that makes it through the cowl vents goes into the heat / ac box.

3. The vacuum operated doors in the heater box may not be operating properly. This is kind of complicated on an Ac car, and will likely require removing the box from the car to service. If the air is coming out of the right places when you move the controls, the vacuum actuators are probably working, but the seals in the box may be shot, or the doors are not moving full travel.

Removing and rebuilding an ac box isn’t brain surgery. Just like rebuilding an engine, you need to pay attention to detail. Detroit Muscle sells good rebuild kits. There are a lot of threads on here about it.