I only have 2 wheel drive (rear) when 4WD is selected. The front propshaft is engaging when 4WD is selected but the front diff is not driving either front drive shafts.

The electrical switch on the transfer box tests Ok and the Vac pump behind the front bumper is working and applying 14inHG to the pipe going into the front diff but only for around 15 seconds and then the pump stops and the vacuum is released. I have vac tested all the pipes and the front diff - all good.

Does anybody know what is inside the front diff to engage drive? Do they fail often or just stick? What is the normal operation of the vac pump?

Any info appreciated

Thanks
Graham

This vehicle had been repaired elsewhere recently following frontal damage.

When I first enquired with our local dealer about this fault, I was told it was usually the vac pump behind the front bumper that was the problem. Given that the pipe from the front diff had been fitted to the vac side of the pump I thought this was correct and could not find a fault on the system.

I have since found out the "Vac pump" is actually a "Pressure pump" and should supply around 6psi of pressure to the front diff to engage the lock.

I tested the other side of the pump and with 6psi the 4WD light came on and the pump stopped and held this pressure, so I swapped the pipes around and all is working fine.

Graham

Now thats worth knowing.

Good to see a Fix posted.

Regards mark