My f250 FX4 AC started blowing through the Defrost vents only, yesterday. I think I have narrowed down the issue, but I need some help. There are two components on the right side engine compartment, One looks like a mini vacuum pump and the other looks like a switched diaphragm. I can apply vacuum using my hand pump and the AC will switch to the vents. Question is, how can I tell which is is not working? I don't seem to be getting any vacuum from the mini pump, should it come on when the key is on or AC engaged? I tested the diaphragm to see if it would hold a vacuum and it does. Is there a fuse that contols this pump? I looked in the manual, but did not see anything that jumped out at me. Thanks
Also my 4x4 hubs won't lock in, which one of the vacuum lines goes to those hubs.

thanks

 

Your AC system is blowing defrost only because you have a vacuum leak. Defrost is the default for the system.

Your 4x4 hubs (ESOF) won't lock for the same reason, a vacuum leak. You need to find the leak, but chances are you already have (your 4x4 hubs).

 

When you turn the key to the on position, you should hear the pump run for a few seconds, building vacuum. Have someone turn the key to the on position and you stand near the pump to listen to the pump. These pumps have been known to go out. Also, when you turn the **** to 4x4 does the light for the 4x4 come on? When you stand outside the truck and turn the **** to 4x4, can you hear the motor engaging the transfer case?

Let us know.

 

As your hubs do not have vacuum supplied at all times most likely they are not the source of your problem. As for the components you are asking about, one is the vacuum pump for your diesel and the other one is the vacuum control module to your hubs. You need to fire up the pickup and see if the pump is running and pulling a vacuum. Most likely problem is a broken line from the pump to the system or a bad vacuum can, but it could be that the pump has failed. Good Luck

 

Ok, thanks for the info, I will check this afternoon. As far as I can tell, the pump is not running. I hooked up my vacuum gauge and was not getting any pull from the pump. Of course that truck is very loud, so I will have someone turn the key on to see if it come's initially. Also the 4x4 light does come on when you flip the switch, I have the hubs in Auto and I can tell that they are not engaged.

 

True, your hubs don't have constant vacuum but there is a pulse to engage and disengage the hubs. Think of it as a Bic pen. One click to open, another to close. If the vacuum pulse to engage or disengage fails to hold vacuum the pump will keep trying, thus act like a vacuum leak and the ac vents will default to defrost. Not to say that there might be other leak points, but I would strongly look at the hub seals and lines first. Here's a write up on the vacuum seals.

Welcome to guzzle's 4x4 Needle Bearing Maintenance Web Page

 

You should be able to hear the vacuum pump for a few seconds when you first turn the key on (without cranking the motor). I can hear mine fine from the drivers seat.

I'm not sure which fuse the vacuum pump is hooked to but check all fuses having to do with 4x4 such as :

#31 under the dash (4x4)
#24 under the dash (EATC)
#3 under the hood (electronic shift on the fly)

 

Ok, I had someone turn on the key while I was standing outside next to the vacuum pump and it did not come on. I did check fuse #31 the other day, but did not check the others, I will check those today, if good, then looks like I'm off to ford to get a vacuum pump. Does anyone know if there is an official name for that pump?

thanks for the help....

 

If you have a 12volt test light then unplug the vacuum pump and make sure that you are getting power to it before buying a new one.

I believe it is just called a "Vacuum Pump"

 

Status update - removed the old pump, hooked up a 12V power source, and the pump would not come on. So I puchased new pump from Ford 150 bucks, installed and all is working now, pump powered on, ran for a minute or so and shut off, so the pump was the issue. Thanks everyone for your quick responses.

 

Thanks for letting us know what the fix was. Might help someone else down the road. Glad you fixed it also.

 

Here is what my 2004 F-350 SC 4x4 is doing:
AC or the heater air blowing through the proper vent by the selector ****.
Switch the 4x4 **** to 4 wheel drive and in about 30 seconds the air will start blowing out of the Defrost vents. Switch to selector **** for the 4x4 back to 2 wheel drive and in 30 or more seconds the air will switch back to where it was.

 

It sounds like you have a vacuum leak, either at the hubs or in the line going down to the hubs.

 

Update for everyone, ended up going to the shop, found out the problem was the 4x4 vacuum switch that is located on the transfercase. $70 part. Thanks

 

My problem was repaired with the replacement of the Vac. Pump & a Vac. Regulator about $350.oo. No cost to me. Ext. Warrenty took care of it.