[Solved]Sonoma/S10 - Air only blows through defrost - Try this first
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Early model Sonomas and S10’s have vacuum-controlled actuators that move the air doors inside your HVAC unit to specific positions. The different positions direct airflow to the vents, defrost, floor, or a mixture. A loss of vacuum due to a broken or loose hose can cause the actuators to not function.
This case study talks about a GMC Sonoma with a customer complaint of the vents switching too slowly. We will see what the actual cause is and how to fix it.
What is a vacuum-controlled actuator?
An actuator is a device that uses a command signal to move an object. In this case, the actuator is commanded by the HVAC(Heating, Ventilation, Air Conditioning) control unit to move the HVAC doors or air doors. Most newer vehicles use electric actuators, but on this vehicle, it uses a vacuum provided by the engine.
Any time the vacuum is lost, the actuators do not work.
HVAC Control Unit
There is another problem that can occur with vacuum-controlled actuators. That is a slow movement of the actuators.
When switching from defrost to the vents or from vents to the floor, the movement of the air doors to those positions should be relatively quick. In some instances, a low vacuum can cause the operation of the actuators to be slower than normal.
What can cause a low vacuum?
Different issues can cause a low vacuum, but some main ones are:
A broken vacuum hose.
A cracked vacuum hose.
A loose vacuum hose.
Weak vacuum from the engine.
Of course, special equipment may be needed to check the vacuum from the engine.
For this case study, the only step I had to perform was a visual inspection of the vacuum hoses.
On this GMC Sonoma, the vacuum hoses run on the back of the engine bay.
Cracked Vacuum Line
With a visual inspection, I could see that the main vacuum hose was severely cracked due to old age. This crack caused the vacuum to not stay in the hose and not make it all the way to the actuators.
What fixed this vehicle?
After repairing the cracked hoses in the engine bay, the GMC Sonoma’s air started working normally!
It’s important to replace all the hoses that a severely cracked for the best result.
I hoped this case study helped get your HVAC working properly. Comment below if it did!
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