I have a 2002 screw with the same problem 90% of the people are having, ac to defrost on acceleration, back to vents on deceleration. I also cannot find any leaks.
What I want to know is if any tinkerers or curious people have taken the control head apart to see if it can be repaired? I am thinking if it is man made, it must be repairable.

Thanks for any and all input
devilsrighthand

 

More than likely, you probably have an issue with your check valve...
part # XL3Z-19C828-AA. I have replaced alot of them for the same
issue as you describe. List price is $24 and change.

Do you have a vacuum gauge? If so, we'll help you diagnose your issue.

 

Where is the check valve located?

 

check valve is located rh side of engine compartment near the firewall.
There are 3 vacuum lines; one on each end and one near the center.
I want to say it may have a blue portion on it..
Only way to be certain of exact fault is to properly test with a vacuum gauge at
various points under seperate conditions..

 

 

I want to thank everyone for the replies. I have not had time to work on this for the last few days. I plan on installing a vacuum gauge in different locations this weekend, I hopefully will have the 3 days, since it is a holiday.
I myself really feel it is a vacuum leak, it switches back and forth very readily with acceleration and deceleration. I thought I heard a leak the other day after I shut off the engine, from around the right front wheel well, there are a couple of lines going down along the frame there, not sure where they go to. Just haven't had time to slide under to trace them.

Thanks
macchi

 

Keep in mind when testing for vacuum, that there is a vacuum reservoir behind the battery. Be sure to vacuum test it to ensure it is holding vacuum and doesn't have a crack in it. (it is plastic)

FordMasterTech

 

Quick question for FordMasterTech, I have checked my reservoir and it will hold vacuum. I pulled the lower panel from the drivers side and located the vacuum motor that switches from defrost to vents. I installed my vacuum pump on the motor and it will change to vents and hold pressure. I hooked the vacuum pump on the red line from the motor, and it shows it is getting very little vacuum. Is the next step to go into the control head?
I had to quit for now, it is getting to dark to see very good.

Thanks for any info you can give me.
devilsrighthand

 

Sorry if I am late getting back to you, I have been at the in-law's this weekend...

If you remove the control head, check the black supply line first to make sure that adequate vacuum is reaching the head. It is best to drive the vehicle as the problem you are having shows up (acceleration, etc.) If that shows fluctuation, replace the check valve and recheck it... If the supply is good, reconnect your vacuum line to your vacuum motor and pull a vacuum on it at the control head harness. See if it holds. If both of those are good with no leaks, sound to me like a control head issue..

If you have any other questions, lemme know...
Good Luck!

FordMasterTech

 

I apologize for taking so long to do this, but time is not on my side.
I hooked up my vacuum pump to the black supply line at the back of the control head, and drove down the road trying to duplicate the problem of vacuum loss. No such luck, the vacuum gauge holds steady at 20 inHG. So, I guess my control head is faulty.
Is the control head repairable? I have fixed some items that were not supposed to be repairable, but the control head I have not been into. Has anybody taken one apart?
I almost have to think there is a diaphragm inside somewhere that could possibly be repaired. I am hoping someone here has been inquisitive enough to peek inside. I am just about ready to try it myself just out of curiosity.

Thanks for all the help and info.
Devilsrighthand, AKA Mike