Hey all,

I ran into a tricky issue the other day, in which my 1995 Camaro Z28 would only blow through the de-frost vents. That would be the vent in the center of the dashboard up by the windshield, and the lower vent by your right leg in the driver's compartment. The air blew cold and blew strong (changing the strength dial did change the force with which the air blew), but I could not change the vents the air was exiting from. I could turn the A/C on and off with that dial, but that was all. Fearing the worst (as in many cases an A/C problem means digging into the rat's nest of wires in the dashboard), I did some research, and was able to solve my problem with a simple vacuum line fix! The following details how the system works, and why yours may not be working.

I'll start by saying that if you're a visual learner like I am, the following diagrams and images will help you understand this relatively simple system a great deal. The A/C system works using a system of vacuum actuators. This means that when you turn the dial, it sends an electrical signal to the actuator in your A/C system, which runs off of a vacuum. These actuators move to direct the airflow through the system. This also means that if your A/C systems isn't working properly, it could be either an electrical issue or a vacuum issue.
This leads me to ask, where does the vacuum come from? The answer is that it comes from two sources. The first, and primary source, is your intake manifold. While your car is running, the cylinders pull air from the intake manifold, resulting in a vacuum in the manifold. This vacuum is what's utilized by the A/C system. Well, what happens when you punch the throttle, opening the intake to the atmosphere? That vacuum decreases to near atmospheric pressure. That's no good for your A/C system! GM thought of this and added an additional source of vacuum to your system. Under the battery (front passenger side of the vehicle) lies a vacuum canister. When your engine is running, the vacuum from the intake manifold flows through a check valve, which is connected to this vacuum canister and your A/C system. This means that there's a constant vacuum in that canister, so when you punch the throttle, the check valve closes, and your A/C system pulls from the vacuum in this canister.

The image below depicts the actuator/vacuum lines, and how they connect to the vacuum canister and intake manifold through the check valve. This is followed by real images of the system (courtesy of Shbox).






So, now that I understand how the system works, I can hunt down my vacuum leak. My first instinct was to look at the check valve. I had just been running fuel pressure testing and thought I may have bumped one of these lines off on accident. These 90-degree rubber fittings are also known to degrade and come off on their own. Following this, I planned to check the line running to the vacuum canister. This line is known to degrade over time due to dripping battery acid. I would then check the vacuum canister for any cracks, and finally check the check valve by blowing through it one way, then the other, to see if it was my source of failure.

As it turns out, none of that was necessary! Following the vacuum line from the check valve to the intake manifold, I found my source of failure.


I must have caught the line on something while checking the fuel pressure and pulled it out by accident. Be sure to double and triple check your work before closing the hood! I'm now left wondering if my fuel pressure test was inaccurate because of the potential vacuum leak from the unplugged line... that would all depend on whether the line was pulled off before or after my test. In any case, this was a valuable lesson learned, and an opportunity for me to learn more about my A/C system!

I had seen several posts made across many forums, and though there was a great deal of information made available to the original poster, they never followed up on their own thread to say if the advice worked or not! I'm hoping this can be a one-stop-shop to help someone solve their issue quickly and effortlessly. Might not be as quick and efficient as the 2-55 A/C fix though 😉

 

Hey Yazz, thanks for the summary above - currently troubleshooting a vent issue on my 95z as well. Your post with pictures from shoebox has been super helpful! Looks like its between the check valve and caninster in my case, will need to go under and see whats going on next

 

Yep, that is the only fault that can cause your symptoms. Loss of vacuum and the system defaults to clearing to windshield. A good idea from the engineers to make sure you can see where you are going ;-)
Fuel pressure has nothing to do with engine vacuum.