Blower only in default mode, but check valve is good

My air is coming on but only in default mode, which is defrost and floor vents. I checked the lines and check valve and they are good. Anyone know if that would mean I have to replace the control switch panel? or if it could be something else?

The mode door vacuum motor may be faulty.


thank you. Ill have to figure out where it is on my car

you didn’t specify what year Chevy, but many of these are electric blend doors, and the one that controls the air direction is likely up above the steering wheel. It’s a pain to get at, but is replaceable without removing much of anything but the blend door motor itself.
These are made with plastic gears that get brittle and break over time. I have had to replace 2 of my 3 on my 2000 GMC, and the 3rd one is currently unplugged until I get up the gumption to pull the dash out.

depending on what year Chevy you have, there are videos on youtube about this.

it’s a 2001 chevy express 1500 1/2 ton cargo van. Thank you but for some reason the one in this van it’s all in one little box that isn’t meant to be opened. I had that happen in my old nissan and it was easy to switch in the back like you said so I checked the check valve, hoses and the switches and I can only take out the switch module box all in one. Im wondering if that means that it’s just the panel box I have to replace

sorry, I missed that you said express in the description above. As tester said, the vans tend to be vacuum operated. Check for a vacuum junction block under the dash, maybe on the passenger side, and make sure there is vacuum there.

ok. thank you

Good advice above. Another idea, if you can see a vacuum hose running from engine compartment through fire-wall, make sure it is in good condition and connected securely to vacuum fitting. No E1500 experience myself, but if your E1500 is more than 10-15 years old, might want to bite the bullet & replace all the vacuum lines, they tend to deteriorate and start to leak over time.

thanks! I followed all 3 lines, there were no breaks and checked the check valve. Ill have to check them again with better light because I followed them with my hands but I cant see where it goes into the firewall. I felt it but cant see it. Right now I have half the engine parts out to change the water pump and I still cant get to see it. Ill have to try sticking my phone camera down in there.

Another area to check is if the vacuum reservoir isn’t cracked, or if the nipple on the reservoir for the vacuum hose isn’t cracked.



thanks, Im going to have to stick camera down in there. This engine has everything hidden for some reason on things that seem they should be visible. Even the bolt on the tension pulley is on the back instead of the front. Never seen that before.

Just a fyi. My 50 year old Ford truck uses vacuum motors, two for the distributor advance/retard mechanisms, and another for the mechanism to regulate the engine inlet air temperature. Over the years I’ve had problems from time to time with the rubber diaphragms (necessary part of vacuum motor ass’y) developing a leak. In my case the most obvious symptom was the engine didn’t run correctly due to overly lean condition caused by the vacuum leak through the diaphragm. I connected a hand-held vacuum pump to each vacuum motor, one at a time. The one that wouldn’t hold vacuum got replaced.

Have you tried a hand-held vacuum pump to activate the stubborn vacuum motor?

One other idea, the brake booster is a sort of vacuum motor, and when it fails it could cause vacuum-related symptoms in other parts of vehicle. Are you noticing anything unusual happening with the brakes?

Thanks, we just bought it after it was sitting a few years, no problems with brakes but had to so far replace a few hoses, water pump, alternator, rebuilt transmission, new battery.
I dont know much about the vacuum hoses because my other cars were simple air systems, easy to fix.

Vacuum motors are both simple & robust beasts. No need to be a afraid of them. Plus they can be made pretty compact in size b/c they use a little of the engine power to create the force. Those are the motivations car designers have to use them. The most common failure mode is they don’t have the correct vacuum input (b/c a vacuum hose has a leak or fell off), or , less frequently, their rubber diaphragm has sprung a leak. Both failure modes are pretty easy for an experienced mechanic to identify quickly. For a diy’er to tackle the job, pretty much requires having access to a hand-held vacuum pump.

Here’s one example

There are less expensive versions available also. Usually the same basic pump, but fewer attachments. IMO, the ones with fewer attachments are all a diy’er would usually need. If special attachment needed, I’ve always been able to home-brew what I need.

Thank you, I see one on there for 24.99 Ill have to try that.

1 Like

Hand held vacuum pumps can remove fluid from awkward to reach places too. I’ve used them in that mode on occasion, very useful diy’er gadget. With older vehicles (like my truck), when engine doesn’t run correctly and no obvious explanation, my first test is a complete assessment of the vacuum system. Surprising how often just doing that turns up the culprit.

Not quite as useful for newer cars, less dependence of vacuum systems.

1 Like

Yeah I agree but once I was waiting for a train and all of a sudden all I had was defrost and floor vents. When I got home I took a long vacuum hose hooked at the engine and the diaphragm. When everything worked I looked around and found the hose had dropped down on the manifold and burned a hole in the hose. So yeah look around but the parts are cheap.

1 Like