Expert: Rob Painter - 6/16/2011

Question
QUESTION: I have a 1996 Cadillac Concourse.  The heater will not turn off so the air condition can blow.  Even when the heater is in the off position, hot air is blowing.  Is there a part that needs replacing, if so what part?

ANSWER: Hello,

You have a blend door not operating correctly. In the venting there is a door controlled by vacuum that opens and closes that controls the difference between cold and hot. I cannot tell you why this door is not operating other than it is possibly a vacuum problem. Most cars have a heater control valve on the engine which in the automotive world we called a dole valve. This valve supplied vacuum to the blend door. These valves would fail and the result would be to blow only heat because the blend door under the dash would have no vacuum.
You could have a bad heater-A/c selector or you could have a kinked vacuum line under the dash.
Without being at the car, I can't tell you the exact problem and instead of guessing on this one, it might be best to get an exact diagnoses. I can't see a huge expense on the diagnoses because now you know what is causing your problem and it is only a matter of finding out why.
Finding the problem for a competent tech should take less than an hour. It should be obvious to the  exact cause. The first thing I would ask to be checked would be the heater control valve on the engine.

Good luck and please follow up and let me know what was found.

Good luck!

http://www.autotheftexpert.com

---------- FOLLOW-UP ----------

QUESTION: Would the heater/ac selector or a probleme with the vacum present a diagnostic code?

Answer
Hello,

You didn't mention having a code, but yes it would. That is why they are called diagnostic codes. Please understand, I am not bring a smart guy, but a code could lead one to the same issues I address. the problem I have in assisting you is this car is that the car is 15 years old and I no longer have books on cars this old.
However I do highly recommend to anyone attempting to fix their car and many want to do this because of the economy, you really need a factory service manual. Nothing else at a book store comes close whether its Chilton, Haynes, Motors or even Mitchell on demand which many shops use.
This is one of the cheapest tools you can possibly buy and there are diagnostic flow charts that will direct you to a problem.
People commonly ask me what I do when researching a problem in a specific vehicle. I tell them I go to http://www.alldata.com. The first down load costs $30 for a year. It is immediate and will not only assist you on your heater a/c problem, but anything else you may run into over time. I do not know how far back they go, but you need a factory manual. It will also tell you what the codes mean.
If you have an additional car that you want to order another manual, down loads for the other cars you have is $16 instead of $30.
Check out the site and scroll the list and see if they have the manual for your car.

You would be surprised as to all the problems that can be fixed with the owner's limited mechanical knowledge. Knowledge is power and if you want to do your own repairs, this is the same manual a Cadillac tech uses, thereby saving time and taking most of the guesswork out.

Good luck!

http://www.autotheftexpert.com