Had a leaking heater core in my '99 dak., switched out the heater core with no problems. Took it out for a drive to test out my new heat and got no heat. Control switch wouldn't even switch to different vents. Both water lines from the heater core were at equal temps. Took everything apart very carefully to try to see what the problem was. Only thing that I've noticed was the corrigated (SP??) vent was not connected from the elbow coming off the top of the air-box. Thought it might be the blend door, but with the heater switch attached to the cable the rod for the door moves fine. Back traced the black vacuum line going through the fire wall and it 'seems' to be ok. All other vacuum connections were tight. I'm stumped. Any thoughts or suggestions on what else to check????

 

Did you check all the wires connected to the heater core box, you have to remove them to get it out.

and when you mean no heat, does it BLOW air, or nothing at all? You could have jammed the fan some how.

 

All the wires were connected. I could here the blower changing force as I increased the fan speed, but it seem as though the vents were constently blowing at the same low speed.

As far as the heat was concerned. There was no heat coming out of the vents even after the engine was up to full operating temp. Bought a new blend door to install, but the old one appeared to have work properly. When I pull the dash back out the first thing I check in doing so was the heater control switch and the vacuum plug looked fully seated. So there is the problem of no heat and the problem of the vents not being directed to the proper location???