How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask SuperBob Your Own Question
SuperBob, Ford Senior Master, Mazda Senior
Category: Ford
Satisfied Customers: 2609
Experience:  18 years Ford, Lincoln, Mercury, Mazda Dealer ASE master, diesel certified. Specialize electrical
Type Your Ford Question Here...
SuperBob is online now
A new question is answered every 9 seconds

Ford F150 Lariat 2003 ford f150 a/c blows on defrost when

This answer was rated:

2003 ford f150 a/c blows on defrost when i accelerate works fine in the driveway no matter how much i accelerate. the vacuum lines on the control head appear to all be hooked up and in good shape.

Hello, the engine vacuum is greatest at idle, when you are accelerating engine vacuum reduces and in order to keep the a/c from defaulting to defrost your vehicle utilizes a vacuum reservoir and a check valve to prevent the stored vacuum in the reservoir from being utilized by the engine under load. The vacuum reservoir on your truck is located between the battery and right front fender, the check valve is installed in the vacuum line between the reservoir and engine. I suspect that your check valve has failed. To check this, locate and remove the check valve (it is white on one side and black on the other, has two vacuum nipples and is only the size of a nickel and about 1/8 inch thick) look down in by your a/c accumulator (receiver/dryer). once removed try to blow air one way through the check valve and then try the other way, you should only be able to blow one way through this valve.



Customer: replied 6 years ago.
the check valve is good and when i removed i could here air escape from the reservoir
That sounds like a leak for sure. you may need to remove that reservoir and inspect it and the lines going to it for damage, I think you are close to fixing your problem
Customer: replied 6 years ago.
Do I need to take the fender off to get to the reservoir? Could the control head be leaking internally?

No, take the battery and battery tray out and you should have easy access to the reservoir and yes a leaking control head can cause this condition.

Customer: replied 6 years ago.
Is there a way to check the control head and if it leaks is there a way to fix it?

Under the passenger side of the dash are two vacuum lines, black one and a white one, there is a vacuum connector in this location, disconnect the connector and attach a vacuum pump to the black line that runs to the control head and plug the port to the white hose, turn the key on and select vent, apply vacuum to the black line and see if it holds and air continues to exit the vents. if it does go to vent your leak is in the dash or switch, if it stays on vent, then your problem is in the engine compartment. I have not had or heard of anyone who has had any luck repairing the control head, if that is the problem.

SuperBob and 6 other Ford Specialists are ready to help you
Customer: replied 6 years ago.
Thx I will check in the am and get back with u
sounds good, see you then
Customer: replied 6 years ago.
the resevoir leak checks good and i cannot find but one black vacuum hose under the passengers dash. I tried to leak check the black back to the control head but it leaked. I suspect this is because of the setting the control head is on. If I remove the black hose fron the control head and plug it at that end and it leak checks good does his mean the control head is leaking?

not necessarily, it just depends on what selection that the switch was on, I believe if you have the mode select in the off position it will fail in the way that you are testing. Are you testing with a vacuum gage?

Customer: replied 6 years ago.
Find the vacuum line that is supplying vacuum to the control head, black one. connect your vacuum gage to this port and drive the truck in the same way that it causes the a/c to divert to defrost, see if your vacuum gage goes down under the same conditions, this will tell you if it is in the supply line or not if the vacuum holds. the problem is in the control head or one of the actuators in the dash
Customer: replied 6 years ago.
Ok thx

keep me posted


Customer: replied 6 years ago.
Ok have to go to parts store for a fitting
Customer: replied 6 years ago.
at idle its 22 hg at a hard acceleration up to 80 mph it was a steady 21 when I slowed it went up to 24 hg and then settled back to 22. When I shut the engine off I leak checked it for 15 minutes no leaks were noted. I'm not the expert but this sounds to me like the control head and if it is were is a good place to shop for these ?
Customer: replied 6 years ago.
I leak checked the control head in 15 minutes it lost 5 hg so I guess I need t start shopping let me know what u think.

It sounds like you have found your problem.

Customer: replied 6 years ago.
thanks for your help
Your are welcome, let us know how it turns out
Customer: replied 6 years ago.
Thank you for the accept and the bonus
Customer: replied 6 years ago.
U earned it thx

Attachments are only available to registered users.

Register Here
Customer: replied 6 years ago.
Got a new module with a lifetime warrany put it in and it ops checks good. Thx and good job
Great news. and good job to you, you did all the work!!!