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 it tasted a little like gasoline. 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...........

 

It's a vacuum problem, the most likely cause is the three way check valve, located on the right fire wall just below the main electrical terminal. The left side of the valve goes to the engine, check valve is located behind that port. The right side goes to the vacuum cannister behind the battery, the bottom port goes to your AC door,"damper" Those two ports are open to each other at all times. Port to the engine when working correctly is closed for vacuum flow from damper and cannister side of the valve to the engine. There is a little vac motor under the dash, they rarley go bad. Leave your AC turned to maxi cool and shut engine off, a half hour later start engine, if air blows out of defrost it's almost a sure bet the check is bad. Since this is an old post, did you replace your threeway check valve? If so what is the part number, as the dealer couldn't find the part for me?

 

I got the check valve, Ford Part number XL3Z - 19C828 - AA it came with the long vacuum line to the vacuum canister, and the short adapter hose to the engine. PS I cured the problem by cleaning the check valve with rubbing alcohol and spraying a tiny bit of silicion into valve. I'll still replace the valve when I get a battery teminal puller.

 

I have the same problem on a '03 F150 as Ladiabla. I have replaced the check valve but it did not help.
I have used a hand operated vacuum pump to apply vacuum to the line that runs though the fire wall but it will not apply or hold a vacuum to that line.

I was looking for it to a least pull some vacuum (because the engine does pull vacuum there to open the AC door). How can I check things under the dash without pulling everything apart and what & how is it checked. Where is the vacuum motor at that Aftrmidnite is speaking of & what is it for?

Thank You in advance

 

I believe the Damper motor (vacuum operated) is accessible by emptying the glove box, pushing a tab on the side and you swing the glove box past it's normal open position and it will lift off the hinge. I believe the vac motor is just left of dead center of the glove box opening. Also there is an oval panel in the top of the dash. Before you remove it have some place to put, suggest a towel on the hood that is larger than the panel. Put blue masking tape on one end of the panel. Work your fingers around the panel to disengage it. It has metal clips all the way around it, they are as sharp as razor blades. So you don't want them contacting your fingers, dash, seat or paint. When reinstalling it put it back with the masking tape in the same location as you removed it. I don't think the vac motor is under this panel, but behind the glove box.

 

Thanks for the reply ! I will do as you have said and get get back to let everybody know what I find.
again, thanks for the reply

 

Also the Vacuum canister is in the battery box, fender side. It's a black rectangular box that slips into the battery box fender side. You have to pull the battery to get to it. I went ahead and cleaned everything up, before putting it all back together, which let me replace the vac hose that came with the check valve.