04 Suburban, no heat??

Hey guys,

I just noticed this afternoon that my HVAC system (that I hate by the way) on my Suburban doesn't appear to be working correctly. If I put it at say 75* or warmer where warm air should be coming out of the vents it blows ambient air or cool air. Then every now and again it gets warm. I know I have hot water in the system, but the stupid heat controls appear to be FUBAR. Is there a common failure point on said dual zone stupid digital display HVAC system? Can I swap in the more simple three knob system that has worked on cars for 100 years or so? This HVAC control is the only thing I have hated about the truck since I got it, and this is the reason why? I anticipated troubles with it, and they have manifested themselves.

I know it sounds condescending, but make sure the system is full with no air bubbles.

My 03 Silverado had a misbehaving HVAC system too. Turns out the outside temperature sensor had fallen out of its mounting hole (below the passenger DRL) and was dangling behind the bumper. Once that was put back in place the air was cold and heat was hot again.

It wasn't an obvious issue either, as the outside air temp displayed on the rear view mirror comes from a different sensor, which was working correctly. So I don't know if this is your issue, but its real easy to check.

DrBoost wrote: I know it sounds condescending, but make sure the system is full with no air bubbles.

It is not a cooling system issue. It is with the stupid HVAC system.

The hot water is the H of the HVAC system. I'm not saying it is low water for sure, but I've had more than one customer (any my wife's van) that had no heat. Filled the coolant reservoir and heat came back. Then I fixed the leak that caused the low coolant condition. I agree that it could very well be a stuck blend air door or some electronic doo-dad that's messed up, but this is a cheap, easy test/fix.

My Silverado has random issues with the driver's side blend flap/valve once in a while. It only affects one side at a time, but a coolant issue would affect both. Might help troubleshooting?

Yeah... the hot is from the engine, so make sure the rad is full. Second, does it get cool on both sides? There are two actuator motors to run the blend doors, and its not unusual for the position sensor to fail. They generally still make hot/cold, but only at the end of travel on the slides. Anywhere in the middle of the temp adjustment it gets confused. If its the drivers side thats bad, its easyish, as the actuator is on the bottom of the heater box above the treans tunnel. The passengers side is on the top, and involves removing the heater box to the tune of about 6 hours and an A/C discharge/charge. If its cool on both sides, and the coolant level is up, is the thermostat stuck open on the engine, or is the heater core restricted?

We have a F250 at work, it has a very slow leak in the coolant system. When it starts blowing cold air it's about 2 gallons low.

What happens if you set it to a higher temp?

i don't know if they use the same system with only a different user interface, but the dual zone HVAC in my mom's boyfriend's '06 Chevy WT truck doesn't blow extremely cold or extremely hot with the levers at either extreme- if you want cold in the summer, you need to put it about 1/8" off the bottom or you will get HOT, and if you want hot in the winter you need to put it about 1/8" below the top or you will get COLD...

backflush the dexcool mud out of your heater core.

replace whichever of the blend door actuators that has gotten lost or broken.

I tend to believe it is a blend door problem, as it gets hot, and then cool, and then hot again. Not something I think would happen with a coolant problem. Coolant system is full. How hard is it to repair the blend doors?

novaderrik wrote: i don't know if they use the same system with only a different user interface, but the dual zone HVAC in my mom's boyfriend's '06 Chevy WT truck doesn't blow extremely cold or extremely hot with the levers at either extreme- if you want cold in the summer, you need to put it about 1/8" off the bottom or you will get HOT, and if you want hot in the winter you need to put it about 1/8" below the top or you will get COLD...

This system is a digital display that is adjusted with a knob that controls a temp displayed on a screen. No adjusting it a little off one extreme or another, like say with a old fashioned reliable cable system. I hate this system!!

No you can not swap control heads to the ancient 3 knob design. I would JY another control head cheap and see if it does the same thing. If it does, it is one of the actuators most likely. If it doesn't, it is the control head. I have fixed a few with new control heads and others with new actuators.

Edit: Even though the knob and slider head looks different, it is the same electronic system kinda.

Mazdax605 wrote:

novaderrik wrote: i don't know if they use the same system with only a different user interface, but the dual zone HVAC in my mom's boyfriend's '06 Chevy WT truck doesn't blow extremely cold or extremely hot with the levers at either extreme- if you want cold in the summer, you need to put it about 1/8" off the bottom or you will get HOT, and if you want hot in the winter you need to put it about 1/8" below the top or you will get COLD...

This system is a digital display that is adjusted with a knob that controls a temp displayed on a screen. No adjusting it a little off one extreme or another, like say with a old fashioned reliable cable system. I hate this system!!

the 06 Chevy doesn't have any cables- it's all electronic, but with sliders that go from blue to red instead of a digital number readout.. it would be way too convenient if the rest of the HVAC was the same with just a different control head...

How hard is it to repair/replace the actuators if the control head doesn't fix it?

A cable system requires no expensive dealer service and reeks of a simpler time when things (and people) actually worked. A gee-whiz computerized system with a digital display is a golden goose for the dealership that will provide a steady flow of golden eggs whenever it breaks.

Those control heads are horribly unreliable. You really need a good scan tool.

Try resetting the system by disconnecting the battery or head. Also see if there are any thumping noises when you move the doors. Its really just the chevy part that makes it suck.

I can get info from the HVAC system on a scan tool?

Mazdax605 wrote: I can get info from the HVAC system on a scan tool?

As long as it is a Tech2 or SnapOn Modis level scan tool.....

1988RedT2 wrote: A cable system requires no expensive dealer service and reeks of a simpler time when things (and people) actually worked. A gee-whiz computerized system with a digital display is a golden goose for the dealership that will provide a steady flow of golden eggs whenever it breaks.

the simple cable control system on my brother's '04 Cavalier resulted in the dealer having the car for almost a week when they had to replace a bunch of broken parts under warranty less than 6 months after he drove it off the lot... fast forward 6 years, and i got the pleasure of replacing the mode select cable when it broke.. i figured out how to do it without taking the dash out, but it sucked.. $17 part that took me 4 hours and several bandaids to replace.

The snapon solus will always read it. Modis is a pos but thats another thread. The maxidas csn read it as well. Really just a scanner that can go into the hvac system.

The heat worked great last night on a very cold night. I drove it for about 35 minutes each way, and once the engine got warm the heat worked fine. I have no idea what happened to it on Saturday.