Electronic HVAC Fan Speed Control Issue

[EddieC]

In an -08 Tahoe with dual zone electronic climate control, the properly functioning fan speed varies with the cabin temperature vs temperature settings. When the cabin approaches the set temperature, the fan slows down.

Recently the fan is blasting full speed and does not vary automatically. I can manually turn it down, but it is not doing it on it's own.

Is there a diagnostic mode for the system? How would it be activated if so.

Would the blower motor control/resister unit be involved in that function?

 

[Doubeleive]

In an -08 Tahoe with dual zone electronic climate control, the properly functioning fan speed varies with the cabin temperature vs temperature settings. When the cabin approaches the set temperature, the fan slows down.

Recently the fan is blasting full speed and does not vary automatically. I can manually turn it down, but it is not doing it on it's own.

Is there a diagnostic mode for the system? How would it be activated if so.

Would the blower motor control/resister unit be involved in that function?

when you are sitting in the drivers seat look up at the headliner, right above your head is a small round grill with holes in it, looks like a small speaker, that is a environmental sensor that reads the temperature inside the truck, when they go bad they sometimes make a buzzing sound but not always. anyway that little sensor is about $25-30 online and is easy to replace by pulling the edge of the headliner down (carefully). If that sensor is going bad that could be your problem it may or may not have a bcm code that a tech2 could read. you will usually have 2 of these sensors one above the drivers head and one above the rear seat passengers head. if you have the bose audio system with anc there will be a microphone next to that sensor also but I don't think that was a option in 2008.

 

[Doubeleive]

that sensor has a small fan inside it and a thermal sensor, when the little fan blows air over the sensor, the resistance value based on the temperature is how it determines the temperature inside the vehicle and then adjusts the fan up/down when set to auto

 

[EddieC]

when you are sitting in the drivers seat look up at the headliner, right above your head is a small round grill with holes in it, looks like a small speaker, that is a environmental sensor that reads the temperature inside the truck, when they go bad they sometimes make a buzzing sound but not always. anyway that little sensor is about $25-30 online and is easy to replace by pulling the edge of the headliner down (carefully). If that sensor is going bad that could be your problem it may or may not have a bcm code that a tech2 could read. you will usually have 2 of these sensors one above the drivers head and one above the rear seat passengers head. if you have the bose audio system with anc there will be a microphone next to that sensor also but I don't think that was a option in 2008.

Thanks.
Is there a test for it? The cabin temperature sensor in Mrs's Subaru changes resistance when heated with a hair dryer is it's working.

 

[Doubeleive]

Thanks.
Is there a test for it? The cabin temperature sensor in Mrs's Subaru changes resistance when heated with a hair dryer is it's working.

yes it works the same by resistance, you could do the same test with a meter and a hair dryer, sometimes they just get dirty so you could try cleaning it and see if it works better
if the little fan is not spinning that could be a reason why also

 

[Doubeleive]

if you don't have passengers in the back often you could try swapping the front and rear sensors if the front one is questionable, there the same part number and the same sensor for a lot of gm's from like 03-14

 

[PG01]

What @Doubeleive said but also, your auto button is illuminated right?

 

[EddieC]

What @Doubeleive said but also, your auto button is illuminated right?

Yes, the auto button is illuminated.

 

[PG01]

Just making sure, you’d be surprised...

 

[EddieC]

I'll check it out. I found the part number if needed.

Does anyone know if the system has a diagnostic mode?
Are there other scanners besides Tech 2 that might scan HVAC systems if needed? (Just considering more general use units for other cars we might get in the future.)