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