hi all, i've got a 2000 dakota, 4.7L, quad cab with just shy of 200,000 miles on it (so i can't complain too much). anyways, just the other day, my HVAC started losing its mind. it seems to be working in floor mode and defrost mode pretty well, but i can't get it to work in face mode. and, at idle speed with low blower, i've noticed that sometimes it switches to face mode even though i have the selector in floor mode. when i put the selector in face mode, nothing happens. the temperature and blower speed still work fine. even though defrost mode works at the moment, i'm worried that with winter coming up, the defrost mode will stop working randomly and will cause an unsafe driving condition due to fog or ice. plus, i gotta get face mode working again before next summer anyways (i will want it for air conditioning in hot weather, of course).

i'm thinking it's a vacuum issue, perhaps a bad check valve, but i'm not really sure. thoughts?

 

Look here this may help http://www.in30.net/ a web I made regarding HVAC in a Dakota.

 

there is a check valve on the brake booster vac line.

 

By default, with no vacuum, the system goes to defrost - to prevent that safety issue you're referring too.

Start with the vacuum supply to the control head in the dash. Make sure the check valves are functioning.

 

Sorry all my web sites are down I'm changing my setup with my hosting service and wanted my account to expire so I could change things.

So some pictures I post on this forum will not show for a week or more

 

update: it tends to turn into face mode (when it is on floor mode) only when i brake and only on low blower. as soon as i let off of the brake (or try to switch blower speeds), it goes back into floor mode.

also, i've already replaced the control head with a junk yard unit hoping for the best, and nothing changed. it hissed when i pulled the vacuum hoses off, so i know the check valve is working. i separately pulled the check valve in the engine compartment by the brake booster and it hissed there too (i just replaced with the same part). so that seems to leave the vacuum reservoir or something wrong with the HVAC unit itself.

thoughts?

 

"I replaced with the same part" does that mean you put the old one back in?

The check valve is only supposed to allow air to flow one way. Hearing a hiss doesn't mean it works as it could be stuck open and allow air to flow both ways. Manifold vacuum is highest at idle or when decelerating. It is lowest when you floor it.

It's easy enough to change. Give it a try.

The reservoir is just a tank, that's it.

Blower speed should not cause a change in where the air comes from, unless one of the actuators or a linkage has failed. That means pulling the dash.

 

brakes draw vacuum from the system. The fact that you are getting movement when the brakes are pressed exactly points that the check valve is likely malfunctioning- it's purpose is to prevent pressure changes due to the brake system from affecting the HVAC just as you are experiencing.

 

i had not considered that... yes, i put the old one back in. i'll see if i can get a new part. should be pretty easy from any auto parts store.

 

Similar problems with the 2000 4.7 auto 4x I just bought (Vacuum controlled HVAC)
Under low manifold vacuum conditions, the mode will often default to defrost (climbing a hill) and return to the chosen setting (face, floor etc) and remain there until a low manifold vacuum condition is once again created. I have yet to isolate the issue. Thoughts? I have yet to check the booster check valve.