I have a 99 f-250 CC v-10, and when my a/c is on, the fan will blow out of the vents in the dash and switch to the defrost vents when you really get on the throttle, and when you let off the throttle and just drive normal in comes back to the vents in the dash. Any ideas would really help.

Thanks
Tom

 

You have a lack of vacuum. The default is to the defrosters, vacuum moves the air to the other vents. Check to see if your vacuum canister is disconnected or has a hole in it.

Hope that helps,

Dan Harriman 84 F150 lwb
Orange, Texas

 

I'll check it out. Correct me if I'm wrong, if there is a vaccum leak anywhere in the system, could it act this way?

 

Hello Tommy;

The correct answer is yes and no. If there is a one-way check valve in the heater control vacuum line and the leak is before the valve (engine side) then no, it will not have an effect on the air distribution control. However, if there isn't a one-way check valve, then yes, any leak anywhere in the vacuum system can cause the air to default to the defrost under load, which causes a natural momentary loss of vacuum.

If there is a vacuum leak on the engine side of any vacuum system, then it might also light the MIL and cause a lean mixture trouble code.

Unless you have a reason to think that there is a leak somewhere other than the HVAC system, I would think that you would want to start looking in the heater air distribution system for the leak or loss of vacuum. Find the vacuum cannister, or the one-way check valve, and more than likely you will find the leak/loss.

What the cannister does is act as a buffer for the momentary losses of vacuum which occur normally under certain driving conditions, such as stepping on the gas, or under power, such as going up a long, steep grade.

Hope that helps some and good luck. Please post what you discovered.

Dan Harriman 84 F150
Orange, Texas

 

Thanks for the input so far. You guys are great. I haven't had a chance to look at it yet. Probably tonight. I'll be sure to let you know what I find.