A/C or Vacuum Issue ????????

[ladiabla74]

Hello,

I have had a vacuum problem for a while now. I own an '02 F150 Lariat crew cab. The prolem I have is that as the vehicle goes uphill it will switch A/C from vent to defrost. I checked before and was told that if the
A/C switches from vent to defrost its because it has a vacuum leak. Well, this is what I've done so far. I checked the vacuum lines under the hood and found all to be good. Checked the diaphrams and seem to hold vacuum except one that is located above the brake booster. By the way this may sound stupid but I was actualy checking the lines by sucking on them. On the diaphram above the booster, is that supposed to work like that and close up during operation or is it bad if it does not hold vacuum? I believe it is the return or some kind of exhaust that goes back to the tank. Is a tiny vacuum leak there enough to cause the A/C to switch or is there another thing I need to be looking at? The vacuum canister is holding good vacuum too and so is the A/C system in the cab since both the canister and A/C sytem are connected together on the same side of the check valve.

I don't notice any problems on the way the truck runs at all. It seems to run good.

Thanks in advance for any help...........

[gbadgley]

There is a vac. check valve in the heater control circuit. Most are on the firewall where the heater box is. Replace it, you will be fine. Greg

[ladiabla74]

Is this the one that connects the vacuum reservoir canister and the heater/ A/C circuit? That one seems to be working fine but will it fail under working conditions? Thanks for your response.......

[NumberDummy]

1999/2003 F150

F6SZ19B888AA .. A/C-Heater Vacuum Selector Control Valve Assy. (Motorcraft YH-1450)

MSRP: $24.46 / ftepartsguy.com price: $14.09

Mounts to the backside of the A/C heater control panel. This is where all the vacuum lines connect to.

Move the lever, the valve closes one door in the plenum chamber and opens another.

10's of 1000's have been sold since the 1970's, when this little darlin' was first introduced.

[ladiabla74]

I have the electronic version (Lariat truck). I did find one solenoid that was leaking but plugged it up with silicone and worked for a while then all of a sudden it started acting up again. What I notice is that when I first start the truck it takes about 3 to 5 minutes before the vents start working. Meanwhile, it blows up the defrost. But will revert back to defrost every time I go uphill. Usually once the truck senses a load. How does vacuum work at this point?

[projectSHO89]

You are bleeding off vacuum somewhere. Time to get a vacuum hand pump with a gauge and check it properly.

I've seen EATCs that leaked vacuum internally. Hope that isn't your case - $$$$$.

Steve

[BigTimber]

Same problem here on my 03 lariat Supercrew with the electronic controls. Ac blows out defrost vents when under load, then it blows out front vents when coasting. Mechanic says I need a new master control unit....$567 new, ~$350 salvage. Surely there's and easier solution other than replacing the whole unit?