A/C only flowing through defrost and floor vents

Tiny
FLORIDAVE
  • MEMBER
  • 2016 CHEVROLET EXPRESS
  • 6.0L
  • V8
  • 2WD
  • AUTOMATIC
  • 14,000 MILES
The vent knob does not work, air only flows through defrost and floor vents, when I turn the knob to floor, floor + defrost or defrost the air becomes warm. What should I be checking and where?

Regards,
Dave from Florida
Tuesday, July 28th, 2020 AT 6:57 PM

6 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,477 POSTS
Hi, Dave.

What determines the air flow direction on a vehicle is called the mode door. This door has an actuator which moves the door to different positions allowing air to flow different directions.

Now, since the vehicle only has 14,000 miles on it, I would be contacting a dealer to see if there is anything they would consider doing as a courtesy. However if you would rather check it, you will need to remove the actuator and check to see if it is moving when you request the vent setting.

The actuator is located under the dash of the vehicle and not the easiest to get to. Here are the directions. The two attached pics correlate with the directions.

Do me a favor. Before you begin, let me know if the air flow ever changes or if it is always coming from the defrost and floor vents.

2016 Chevy Truck Express 2500 V8-6.0L
Mode Valve Actuator Replacement - Lower
Vehicle Heating and Air Conditioning Air Door Actuator / Motor Service and Repair Removal and Replacement HVAC - Manual Mode Valve Actuator Replacement - Lower
MODE VALVE ACTUATOR REPLACEMENT - LOWER
Document ID: 1748516
Removal Procedure
Remove the I/P carrier. Refer to Instrument Panel Carrier Replacement.
Release the tabs from the lower mode actuator.
Remove the lower mode actuator.

See pic 1
Installation Procedure
Install the lower mode actuator.

See pic 2
Press lower mode actuator on until you hear tabs engage.
Install the I/P carrier. Refer to Instrument Panel Carrier Replacement.

_________________________

Let me know if you need help or have questions. Also, here is a link you may find helpful:

https://www.2carpros.com/articles/air-vents-stay-in-the-defrost-position

Please note that your vehicle does have an engine vacuum operated actuator. There could even be a vacuum hose off or damaged not allowing the unit to function. With the engine running, pay attention to see if there is a hissing sound from around the dash.

Take care,
Joe
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Tuesday, July 28th, 2020 AT 7:22 PM
Tiny
FLORIDAVE
  • MEMBER
  • 4 POSTS
Hi Joe, thanks for responding, I will let you know if the air flow changes before I begin, before I realized there was an issue with the vents not changing I noticed that when going downhill without acceleration the air would blow much stronger out of the vents and when I pressed on the accelerator the air flow seems to decrease a lot. I'll check on it again and let you know soon.
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Tuesday, July 28th, 2020 AT 8:19 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,477 POSTS
Hi,

Based on what you just indicated, it sounds like there is a vacuum leak. When you have the throttle closed (going down hill) engine vacuum increases. When you open the throttle, pressure drops which can affect the actuators if there is a leak.

Please keep in mind the leak may not be in the vehicle. It could be the supply line or any vacuum on the engine which will cause a loss of vacuum. Here is a link that you may find helpful. It explains how to find a vacuum leak.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know what you find or if you have other questions.

Take care,
Joe
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Tuesday, July 28th, 2020 AT 8:37 PM
Tiny
FLORIDAVE
  • MEMBER
  • 4 POSTS
Hi Joe,

This morning I inspected all the vacuum tubes I could see in the engine compartment and under the driver and passenger dash after removing those covers, I did not see anything amiss or hear any hissing with the engine in idle or rev'd up. I have a few questions before I proceed to check for vacuum leaks:

1) Can an HVAC vacuum gauge be used to test the intake vacuum pressure?

2) Where is the intake vacuum hose located for connecting the vacuum gauge?

Also I cannot find the following components from the engine compartment, do you know where they are located?

1) Vacuum brake booster location.

2) Throttle body location.

3) Vacuum solenoid location.

Kind regards,
Dave
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Wednesday, July 29th, 2020 AT 9:51 AM
Tiny
FLORIDAVE
  • MEMBER
  • 4 POSTS
Hi Joe

Good news, I found the vacuum leak, I found it by removing the engine cover from inside (1st pic) exposing the engine, I found a vacuum hose on top of the engine that was very brittle and disintegrating (2nd pic) and I ended up buying a new vacuum hose for $3.50 (third pic), I followed this hose to some kind of three way vacuum splitter (4th pic), one of the hoses from this three way was just hanging in the engine compartment (5th pic), I looked nearby and found an open vacuum port on some device I'm not sure (6th pic), when I turned on the engine the AC started blowing out of the right vents, when I change the knobs the vents change correctly and I notice a sighing sound that I presume is the vacuum actuators. I'm very satisfied with how things went and I appreciate your help, this problem has been solved.
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Wednesday, July 29th, 2020 AT 3:34 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,477 POSTS
Good job, Dave, and you are very welcome. I'm glad you got it fixed. The last pic you attached is a vacuum reservoir. That is specific to the HVAC system, so you certainly found the issue. That reservoir is to prevent the actuators from changing direction when you turn the vehicle off. It only allows vacuum to go one direction.

Regardless, I'm glad it's working for you. Please feel free to come back anytime in the future if you have questions. You're always welcome.

Take care,
Joe
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Wednesday, July 29th, 2020 AT 8:17 PM

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