05 Denali HVAC problems

[Kgp1004]

Hi, I recently bought a 2005 Yukon Denali with 131k miles on it. When I bought it it did not have the ac controller in the dash. (Unit under radio). So I bought one of eBay, in great shape, looks brand new. I installed it and found many issues. The temp control doesn't work. One side blows cold and one side hot. No matter what you set temp on. And the blower (fan) runs constantly on high. Even with the key turned off and out of the ignition. Please help! Where do I start looking for issues. I have the tools and knowledge, just need some direction. The dealer is useless, unless their computer tells them what the problem is they don't know anything

[trottida]

Did you try to re-calibrate the HVAC system with the vehicle off by pulling the 10 amp HVAC/ECAS fuse in the underhood junction box for a minimum of 10 seconds, starting the vehicle and waiting a minimum of 40 seconds before touching anything else? It will do a lot of weird things while re-calibrating so don't be alarmed. This needs to be done every time you change an HVAC component.

 

The other issue could be that one or both of the front HVAC blend door actuators have stopped working or the blend door is broken. The actuators control the flow of air from the heater core to the AC evap core. When they stop working you're stuck with heat, cold or somewhere in-between. There is one for the drivers side and one for the passenger side and one for the rear AUX system; so if your getting hot on one side and cold on the other when both are supposed to be cold then it is highly probable this is your problem. The drivers side and the rear AUX is easy to replace and the passenger side not so much.

 

There's lots of info out there on this issue in both written forums and in YouTube form.