Defrost only blows luke warm, not hot

[jeremytn86]

So, I have searched here, and the web, trying to find the same issue and/or an answer. I have a 99 K2500 Suburban 6.5 diesel. Front and rear heat is nice and hot. When I put it on defrost though, it is barely warm. Some backstory, I had the blower motor and resistor changed out by Midas when I was working in Michigan. They said a lot of stuff in there was melted. I was working 7 days a week, 12 hours a day and staying in an Airbnb, so I didn't have time to fool with it. Later, I replaced the little circuit board with the control knobs for heat and position on it in the dash. Also, very very seldomly, the blower will stop completely, no matter the position, but then start back up within a few seconds. I fear the resistor or motor is going bad again, though that was within 2 years ago, or they did a horrible job. Also, if someone is fairly bored and likes to teach, I would love to know about the HVAC system as a whole. I have been trying to read up on it, but I am confused. There are three gear assemblies, with three separate actuator modules? One in passenger side, one in middle, and one in driver side? What does each do, what is where? The blend door is by the driver's side and controls the temperature? The mode door is in the middle and controls where the air is being blown out of? The recirc door is in the passenger side and controls if the air is fresh or recirculated? Or is all that wrong? Anyone have any pictures, schematics, diagrams, videos, or write ups relating to any of this? Thanks in advance.

 

[GoToGuy]

Do you have the service manuals. If not, in a sticky above there is a thread to the OE service manual links, free download pdf. The HVAC section is not complicated. One section is heating and one is ac. If your heated air is plenty hot but defrost is not hot. That means the flapper valve is not in correct position allowing unheated air blown in. The gears and motor are contained in the heart shaped module that drives the air valves/ doors. Having the OE manual breaks down into understandable chunks.

 

[AK49BWL]

The mode door actuator is on the left and controls all the vent doors - floor vent directly, face and defrost vents via a set of extensions attached to the actuator. The center is the temperature blend actuator, and right is the recirc. Most everything else you have correct. Also, when the defrost is selected, keep in mind it will run your A/C compressor. So even if you have it all the way over to heat, you'll still get some of that conditioned air because the evaporator is before the heater core. That could be why you feel the air coming out is not quite as hot. The blower cutting out could be a wiring or connection issue... These trucks are known for melting the connector at the blower speed select switch at the very least.

 

[jeremytn86]

The mode door actuator is on the left and controls all the vent doors - floor vent directly, face and defrost vents via a set of extensions attached to the actuator. The center is the temperature blend actuator, and right is the recirc. Most everything else you have correct. Also, when the defrost is selected, keep in mind it will run your A/C compressor. So even if you have it all the way over to heat, you'll still get some of that conditioned air because the evaporator is before the heater core. That could be why you feel the air coming out is not quite as hot. The blower cutting out could be a wiring or connection issue... These trucks are known for melting the connector at the blower speed select switch at the very least.

Thank you for your time and the info. I knew about the auto AC on defrost, I unplugged my AC because of this, no difference. The difference in temperature between defrost and all other modes is VERY profound. I think I will try and replace that middle temperature blend actuator and see if that fixes it. I just thought it was very odd to only have a problem on defrost.

 

[jeremytn86]

Do you have the service manuals. If not, in a sticky above there is a thread to the OE service manual links, free download pdf. The HVAC section is not complicated. One section is heating and one is ac. If your heated air is plenty hot but defrost is not hot. That means the flapper valve is not in correct position allowing unheated air blown in. The gears and motor are contained in the heart shaped module that drives the air valves/ doors. Having the OE manual breaks down into understandable chunks.

I don't know how I missed this earlier. The flapper valve on which of the 3 motors/actuators? The one in the middle or the one on the driver's side? I have downloaded those now, thank you.

 

[GoToGuy]

In the HVAC section , there is a page with the operation description, since you the manual it's easier to read and if you don't understand, ask away.