I have this thud that sound like a vent door slamming open or closed when the A/C is on. It does not matter whether i have it in Auto mode or I am manually controlling the A/C. The thud seems to be the most obvious when I turn a corner and accelerate heavier than usual, although it will do it under normal driving. I never hear it happen when the A/C is off.

Anyone else notice this? I think it might be normal but i just want to make sure.

 

Does this happen only when you turn the AC on or is it a constant THUD?

I have heard this in many vehicles, but, it is only the one time when initially turning the unit on.

 

No it happens while i am driving down the road with the A/C on. It will thud sometimes in the morning but that is normal as i have had several dodge/fords truck do that at first startup. I dont think it is normal to be making the thud while driving though. I have though it could be the compressor kicking on and off. It seems to happen most when i turn a corner and give it more throttle than usual. almost like the computer senses it needs the power and cuts the A/C off temporarly.

the guys on ford-trucks think it is something to do with my vacuum. i am throwing no codes and the truck runs great though.

 

anyone else?

 

The thud you hear could be caused by the panel/defrost door slamming to the defrost mode when there is a loss of vacuum and slamming back when vacuum returns. Check your vacuum reservoir and the check valves as well as the lines for a leak. The motor itself could also be the culprit.

Regards

Jean Marc Chartier

 

Could you point me in the right direction as to what the vacuum resivour and check valves look like on the 5.4? Also when I find them what do I need to check?

 

The first thing to do is to confirm that this is what is really happening. If you can, if not ask some one to help, while driving with AC on hold your hand over the defrost vent. If you feel the air flowing out of the vent but id does when the thud happens then that is what is happening.

Regards

Jean Marc Chartier

 

Is a loss of vacuum normal? So your saying when I hear the thud I should feel air out of the defrost vents, that would mean it is the defrost/panel door? I never feel any loss of air from the panel, and there is only 1 thud not 2.

 

I also get a thud out of mine on a cold start-up. sounds like it comes from passenger side floorboard area?? Could that damper make that big of a thud to feel it in the floorboard a little bit?

 

A loss of vacuum is not normal. There are vacuum reservoirs that store vacuum for when the engine is not producing much vacuum like during acceleration. That is why I questioned if the air was changing flow. The panel/Defrost door is vacuum actuated.

Regards

Jean Marc Chartier

 

So your saying when I hear the thud I should feel air out of the defrost vents, that would mean it is the defrost/panel door? I can check this next time. I never feel any loss of air from the panel, and there is only 1 thud not 2.

 

Anyone alse

 

The air flow is not changing. And there is only 1 thud, I would think you would hear 2 thud when it got vacuum back and repositioned. Could it be the auto climate control changing the position of the cabin recycle/outside air vent?

 

If you have electronic climate control, I believe there's a self test you can do - check the HVAC forum.

 

i cant find anything, so i started a new thread. does anyone know how to perform the diagnostic test on my 07?