The mode selection is done through a series of air doors that are operated by vacuum. When there is either a partial or total loss of vacuum, the system will default to either defrost or on some vehicles, heat. Acceleration can also have an effect on the doors for the same reason.
Look under the hood for a small black plastic hose coming through the firewall. Follow it to the vacuum source looking for any breaks, cracks or leaks. Also follow any other lines that branch off because they may go to a vacuum reservoir (storage tank) and a leak there will cause the same problem. Using a vacuum gauge to determine if you have full vacuum at any given point is the best way to find it. You can cut into the line anywhere and splice it back together with a piece of vacuum hose. Find the broken line, you found your problem.
August, 20, 2010 AT 4:40 PM
We have checked all of that and there are no breaks, cracks or leaks. I was told that it could be the blend door actuator, but am not sure if this would solve the problem. I don't want to guess at the parts, cause I don't have a lot of money to replace parts that doesn't need replacing. Thank you so much for your prompt response!
August, 20, 2010 AT 4:43 PM
No, the blend door is for temperature control, not mode control. The mode doors are vacuum controlled.
If no one has had it apart to create a " man made" problem, then it has to be in the vacuum actuators or switching.
August, 20, 2010 AT 5:47 PM
Thank you so much for your prompt answers! If it is the switching what do I ask for at the parts store? I am ignorant about car parts and wouldn't know what to call it. : ) I will ask about both parts, just need to know about the switching part. Again, thank you so much!
August, 20, 2010 AT 5:59 PM
I didn't say to replace any parts. Repair isn't always throwing parts at the problem. The problem has to be inspected to find out why it's doing that.
August, 20, 2010 AT 6:26 PM
He is right about finding the problem before you get the parts BUT most NEWER vehicles have 3 blend door actuators and yes one of those could be the problem or it could be in the climate control. You definately want someone to check it out to seee which it is because all these parts are pricey.
August, 20, 2010 AT 6:49 PM
This vehicle only has ONE electric actuator and it controls temperature.
August, 20, 2010 AT 7:09 PM
Ok, thank you both for your help! This is how it all started. My son took the car up to " Pep Boys". They told him it was the " module" and that it needed to be replaced. They would do it for him if he go and buy the part and bring it back to them, BUT my son didn't ask him if that was what the part was actually called. So, when I called the parts store to get this module that's when they gave me this spill about the " blend door actuator" and that this module didn't come up, but the bda (shorten) did. That's why I was confused and wanted to get more info from you to see what exactly this thing is called. Seeing now, this mod could be anything. Again, thank you very much! You ARE most helpful and I do appreciate everything you have told me!
August, 20, 2010 AT 7:13 PM
This is from the service manual for that vehicle
AIR DISTRIBUTION AND FILTERING
NOTE: The air distribution system of this vehicle cannot be equipped with a cabin air filter.
There are 2 sources of air available to the air distribution system: * Outside air
* Recirculated air
Recirculated air is only used during MAX A/C, AUTOMATIC and OFF modes if equipped with EATC, or MAX A/C and OFF if equipped with manual A/C.
Air distribution within the vehicle is determined by the function selector switch position, if equipped with manual A/C. If equipped with EATC, air distribution is controlled by the electronic automatic temperature control system in the AUTOMATIC mode, but it can be overridden by the driver if desired. Airflow control doors are used to direct airflow within the heater air plenum chamber. Vacuum control motors are used to position these airflow control doors.
The air distribution system is designed to provide airflow from the defroster duct when no vacuum is applied to any of the vacuum control motors. This is done to make sure defrost is available in the event of a system vacuum leak.
Air enters the passenger compartment from the: * instrument panel A/C register.
* floor duct.
* defroster duct.
* side window demisters.
The center panel duct, defroster duct and side window demister hoses are supplied together as a single air distribution assembly.
Passenger compartment air is exhausted from the vehicle through open windows or luggage compartment air vents.
August, 21, 2010 AT 3:27 PM
Does this vehicle have EATC or manual controls? Eatc is automatic with an electronic control head. Might be what the shop was talkiong about. Or does it have manual controls? Three controls in the dash. One for temperature, one for direction of air and one for fan speed. If EATC, they have " O" rings that seal in the back of it and they will get dry and leak. Now guys, lets get all the information before getting into any jabs at each other.