IÆve been thinking about it last night and drawn up couple diagrams. At least that's how I understand it.
In fact itÆs quite simple. System consists of 2 solenoids, check valve, vacuum reservoir, some hoses and actuator. Vacuum is supplied through suction from intake manifold and kept in check with check valve.
With properly functioning system there are only two modes:
2WD. When electricity is supplied to both solenoids. Right side of actuator is connected to negative pressure right side to barometric pressure. Actuator moves from right to left - free wheeling clutch is disengaged.

4WD. Turn power off and springs return valves to original position. Now left side is equalized with atmospheric pressure and right side is connected to vacuum. Whoosh and free-wheeling clutch is engaged. Even when engine is turned off itÆs still going to lock up. That why when you leave your truck in 4WD for some period there is no waiting time for it to kick in.

So it's just "atmospheric pressure and vacuum"? What about viscous coupling? How does that fit into the Active Trac system?

The VC is what allows the Pajero/Montero to be driven on tarmac in four wheel drive much like most car-based cute-utes nowadays.

I was referring to front axle disconnect system. I guess I should correct the title. VC adds limited slip function to center differential. It's two separate units. Differential allows to drive Monty in 4WD on hard surfaces and VC prevents problems when one of the wheels looses traction. Otherwise it would take only one wheel spinning to stop the progress without center diff locked.

that coupling that you speak of takes place within the transfer case

Of course. Part #30421 couples with center diff via tabs.

That's good stuff Alex. Every now and then its hard to move the lever to put the Montero in 4WD. I guess it's because it has been seldom used these last four years, no snow.
Let me ask you this. Does the front drive shaft spin when in 'Free Wheeling' Two wheel drive? Looking at the transfer case diagram I would guess yes?

Yes good stuff - I think I'm getting it

...So practically speaking - a case problem...

I have not yet put in a new lower butterfly plenum on my DOHC. I have it disconnected.

When I am on a flat stretch of freeway doing about 70-75 on a windless day, (slower speed in headwind) my 4wd lights will start flashing. If I let off the gas and then step on it again to maintain my original speed - the lights will go off. Only to come on again over a short period of time or if I put a slight bit more load on the engine.

My theory is I have a vaccuum leak from the bad plenum, and this is somehow affecting the system. But isn't there less vaccuum at higher RPM's? So would it be a bad check valve?

I dont understand what the resevoir is doing either <img src="/forums/images/graemlins/confused.gif" alt="" />

Yes, go buy a plenum...somebody spare me $400. <img src="/forums/images/graemlins/barf.gif" alt="" />

I'm not willing to spend money, I'd just like some free answers <img src="/forums/images/graemlins/smile.gif" alt="" />

No.
As to 4WD they actually recommend to engage it every now and then just so it's kept in working order. Say every month or so.

Yes you need to engage or cycle through all the modes reagularly so it dont stick. My buddy has a Gen 3-2001 model and he has had problems with the Diaphram stickin on the front diff and not wanting to engage. Just gives you and excuse to get out and go 4wheelin <img src="/forums/images/graemlins/wink.gif" alt="" />

Bret, I would think you need to go through and check you vacuum lines to the front for leaks as well as the diaphram to see that its workin... <img src="/forums/images/graemlins/smile.gif" alt="" />