4wd help help help

Navara D40 ...2007 auto.
light it just flashes dont know whats wrong with these D40's and the 4WD system or is it just the dealers. been sending it to the dealers and so far ........nothing ever seems to work
changed the switches 3 of them.... it didnt work
then they said speed sensor on rear diff...didnt work
then they said sensor assembly ....didnt work
now the said rotor .......?
about 3000$ gone and no end ...somebody out there ...guide please !

It could have been the ABS sensors (which are used to determine vehicle speed) as a functioning ABS system is required for 4WD to work properly.

Let's start with a correct engagement of 4WD mode.

To engage 4HI - just turn the switch. It will engage if you are doing less than 100km/h.

To engag 4LO - stop the vehicle, put the gearbox into neutral, press your foot on the brake and THEN turn the switch to 4LO.

If you're not doing it like that, then that's the reason for it not engaging. If you ARE doing it that way, it's going to be a fault somewhere. It could be a loose connection on the transfer case (that's happened before, dirt in the electrical contacts). It could be a faulty sensor/servo in the transfer case (this has also happened).

If your ABS light goes out shortly after turning the engine on then your ABS is probably fine and I'd be looking at the transfer case or the transfer relays which are in the engine bay. Faulty relays have also caused this sort of thing in the past as well.