No air from the vents?

Hi,

I performed the HVAC system reset as described above with one deviation. There are 2 fuses labeled HVAC; one under the hood (HVAC/ECAS) and one in the cab on the left side of the dashboard (HVAC 1). I pulled/installed both fuses as if the procedure specified 2 fuses rather than 1. For every action. After waiting 1 minute I started the truck. It came on in auto mode, and I switched it to manual mode because that is where I use it (I apologize that I failed to mention that in my original post). After doing so, I selected vent mode so I could monitor the temperatures out of both driver and passenger vents. The temperature on both sides started out cold but very shortly turned to WARM on the drivers side (I capitalized that for distinction because previously it blew full hot when it malfunctioned in this manner). I turned the truck off for about 60 seconds and restarted it. Same anomaly as the first time. And that is where I left it because it was getting late. Without knowing anything else and based on posts from the Duramax forums, I will next disconnect the battery for 5+ minutes and see what happens after I reconnect it.

Please let me know If only one of those fuses was supposed to be pulled and also let me know which one. I await your commentary on how to proceed from here.

Thanks for all your expert help,

Dion

Here is more information before before I answer the low on charge question.

The following day after my previous post I turned the vehicle on and the air blowing out the vents was back to being cold, and the system ran cold for the duration of an hour long drive. The following day on an hour long return drive, the system stopped blowing air at about the 55 minute mark, same as described in my original post. I will add that more specifically, what it does is slowly blow less and less air until eventually it is not blowing anything. As in my original post, once this happens I can still cycle through the different vents (vent, floor, defrost) and can hear air being diverted within the plenum AND hear the blower motor running, yet nothing comes out. And now my experience with this tells me that I will have to let the vehicle (more specifically the control unit) cool to ambient temperature before it will go back to working normal.

One thing I did not mention previously is that prior to performing the HVAC reset, I brought the control unit inside, completely disassembled it and multimeter tested the 100 ohm ceramic resistors (3 each) and the 2 large capacitors by desoldering 1 leg of each to remove them from the circuit. All components checked fine. But I did notice a browning of the white plastic back shell in the direct vicinity of where the ceramic resistors are located when the control unit is assembled. And knowing that ceramic resistors are used in high power/current applications, I know that means that they will generate quite a bit of heat. With the processor chip being on the same circuit board and less than 2 inches away, I am now thinking the most likely explanation is that after 14+ years of being too close to the ceramic resistors without any shielding or even so much as a heat sink on said components, that the processor chip is now suffering from latent heat damage and is malfunctioning whenever it reaches a certain temperature. To me, that is the most likely explanation of what is happening to cause at least 1 of 2 problems. The other problem, the blowing of hot air out of driver of passenger side, from what I have read up on could either also be the control unit or could be an actuator that has gone bad. Others have reported that same problem and it had turned out to be an actuator in some cases. But my HVAC system problem was bigger than just that one thing, which widens the number of things I need to rule out in order to pinpoint the problem. Long story short is I am fairly certain that a new control unit will fix 1 of 2 problems and will maybe fix both.

Now regarding the question about freon, the system blows plenty cold when it is not giving me these intermittent problems, so I have not included a low freon charge in my list of possibilities. However, in the spirit of ruling that out (or not:-D), I will rig up to check the low side pressure, and if low I will recharge the system and drive the vehicle until it either malfunctions again or does not malfunction again.

Thanks for your help with this.I'll report back with more details of this ongoing saga after I check low side pressure. If I do have to charge the system with freon, I will need at least a few 1 hour+ drives without a malfunction before I dare to hope it is fixed. And I am still not buying a new control unit until I rule this out because they just cost too much to to play "swaptronics"with. And merely plugging the unit in and starting the vehicle will likely void the return policy.

Thanks,

Dion

My A/C and heater will only blow onto the floor. I move the switch and dash vents will not work.