I'm having a problem with my heater box air flow. (no A/C in the car.) Here's what happens...the directional control and fan control work OK. When you move the first lever to ON and run the fan, the air flow will flow, to either the DEF or HEAT, only if the Temp lever is in the HOT position. As I move it to COLD, all air flow then shuts off. The fan runs, but no air comes out. Move the TEMP lever back to hot, and the air flows out. The previous owner had the box out for a new core and I have had this problem since I had the car. Any ideas? It's as if the path around the core is totally blocked.

Perhaps one of the doors isn't opening or closing when you switch from hot to cold? If hot air blows, but cool air does not, then perhaps the cable came off that runs from the selector or where it attaches to the box?

I thought about that, but it's not a temperature thing. The airflow actually cuts out, so something is causing the airflow to be blocked when COLD is selected. I wonder if a flapper is in backwards?

It sounds like the cables are swapped on the heater control.

The Air / fan control is supposed to open the "air" door with one cable, and the temp control should open the "hot / cold" door with a different cable. I think someone reversed the cables.

Jim...you may be right!! I will check it out!

Thanks!

Either jim is right or when u assebled the heat box flapps u put them in in the wrong order, which stops one of them operatng correct.....
i have done this several times over the yrs.