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