Strange FWD Problem

Hi everyone i'm a newbie on here looking for any help in what I think is a unique problem with the 4WD system I will try and explain this problem we are having here goes, first of all i'm very conversant with the system, secondly the 4WD drive system works perfectly however the problem is when in 2WD and at random the L/H front hub tries to engage, sometimes this noise of the gear trying to engage with shaft lasts for a few seconds where it will engage and other times you have to stop and engage 4WD and then disengage to carry on.
There's a lot been done here already to eliminate most obvious things, both front hubs were removed and refurbished with new seals and bearings, new vacuum pipes both rubber and steel new vacuum tank fitted, so the obvious thing that's happening here is that somehow the 4WD system is being activated at random while in 2WD that being puzzling enough but what is really baffling is why the L/H hub that rattles never the R/H which it should be the first hub to engage because it has the shortest pipes as when you select 4WD and listin for the hubs to click it's R/H first then a second latter the L/H clicks in, so why is it in this bizarre fault on the road the left hub tries to engage never the right, we have swapped hubs and I have tried running with the pipes off the left and the problem never occurs with the pipes off which would confirm there is nothing mechcanly wrong, any help would be greatly appreciated on this
Alan

This sounds very mysterious!! Could it that what you hear is not the hub engaging but something else?

I would test this by stopping when I hear the engaging, turning off the motor, lifting the wheel with a jack and the turning it by hand. If engaged, then the prop shaft could also turn or you could feel that the shaft also turns and if you cannot turn it, then also the other wheel and the transfer box are engaged. And if the wheel spins freely, then the noise is something else.

Depending on the result, I would think about the next steps.

Have you checked the magnet which retains the 4wd engaging collar inside the hub assembly. It may be damaged and unable to hold the collar retracted. Its the ring in no.3,

Done better than that when it happened I was ready with my 8mm socket and removed the hub assembly at the side of the road to confirm it was indeed in the engaged position, also the hub was replaced as a precaution and even swapped side for side and still the fault remains, dam mystory

O yes I have and the drive hub has been replaced with a S/H one and when the appeared again I then swapped from side to side remembering that the R/H side never tries to engage

Hello, this was quick reply!!

So you checked and found out that one wheel hub was actually engaged.

Did you find out if the gearbox was engaged too or do you have a manual gearbox?

So the next thing I would try to find out is if it is vacuum issue or not.

The easiest way to do this would be to disconnect and plug the LH vacuum lines and check if the hub randomly engages or not. After this it should be obvious if you have to start exchanging parts in the vacuum system or in the LH hub.

Another question: When you said that you exchanged the hubs, what exactly did you exchange? (The answer would only be relevant if it still switches with the vacuum lines disconnected.)

Yes all been checked and cleaned, remember what I said the 4WD system works PERFECTLY every time when called upon

It's a switch system and works PERFECTLY every time difficult to tell if the gearbox has actually engaged 4WD as when the fault happens you stop the vehicle Jack up the left wheel and you can tell that it has engaged the hub Jack up the right wheel and it's free wheeling so if the gearbox was in 4WD it still wouldn't drive.
Remove the hub to confirm it has indeed engaged then physically load the spline against the magnet refit hub and the same fault will occur sometimes within a mile or it could be 20 miles but it will happen.
It does not happen if you pull the pipes off confirming that there's nothing wrong with the hub