4WD vs AC?

[slopitch]

Ok, just bought a 2000 F350 7.3PSD. When I switch to 4WHigh with the AC on, the damper switches from the dash air vents to the windshield air vents. After about 30 seconds to a minute, it switches back to normal. I think that the 4WD is activated by vacuum but I don't think the AC damper is. If the AC damper is, could the 4WD vacuum be borrowing from the AC damper vacuum?

[super_stroke]

you have a vacum leak some where. the place i would start is the lines that go from the frame to the hub's on the back side. thats the cheapest thing to replace and a pretty common pont of leaks. there's also the yellow o-rings in the axle that seals the hub to the axle i think. marc and some one else can be more specific for you. i just replaced mine with super winch manual hub's. no problem's

[slopitch]

So, you don't think it is even going into 4WD? What about the vacuum canister? Could there be a leak there?

[mschn99]

The AC and 4WD both use vacume provided by the little electric vacume pump. The most common place for a vacume leak causing one or both to be inop only in 4x4 is the backside hub seals, or excluder seal. It is a press fit seal and they deteriorate over time...

[slopitch]

Some say just manually lock'em in and forget that you ever had a problem. It may hurt your mileage but you won't have to worry whether there is enough vacuum pressure or not to lock them in automatically.

[super_stroke]

thats true. but why put undue wear and tear on the front drive line?? just order a set of after market manual hubs. it'll save you money in the long run.

[slopitch]

Maybe I am not understanding something about how the system works.
I have hubs that I can lock in manually or allow the automatic system to lock the hubs and diff. at the same time. I thought. Maybe the diff is always locked in and the switch to 4Hi just locks the already-turning axle to the hub, I am not sure. If I already have the option to manually lock my hubs, why would i need aftermarket lockers?

[super_stroke]

maybe im not understanding you. are you talking about locking the hubs and leaving them locked or just lock them when you need them? if you leave them locked all the time it will hurt mileage and wear things out in the front end over time. if want to replace them dont spent a couple hunred dollars a piece for factory locks. you can get a pair of warn, super winch, or mile marker locks for about half the cost of one factory lock. but you wont have th shift on the fly feature with those after market locks.

[mschn99]

Bottom line is that i personally dont trust the auto function on those hubs, and more importantly, there is a lot of stuff to catch up to speed if you engage the 4 by at any decent speed in auto mode. With them locked in, you dont have that and you KNOW that it is locking in....

Run them locked when you need 4x4, unlocke (auto) when you dont......