Hey there, 1st post here, so let me throw out the details.

1990 4Runner SR5, 3L v6 5 speed, with the ADD system (no manual hubs).

Basically whats happening right now is I shift into 4hi or 4lo, and the light pops up on the dash, but I only have RWD.

Now my understanding is that the ADD system works on vacuum, and has a couple actuators on the passenger side for sending the vacuum to the proper hose down below, I have not inspected these as of yet, wanted to get on here and find out from ppl who have worked on these things more than I, LOL. I read somewhere that there might be a connection running to the exhaust? My exhaust is rusted out and has holes everywhere, might this be an issue for the ADD system? Other than vacuum connections, any other possible issues?

I tried a search, but got very little info with anything close to my symptoms, as most seem to have the 4wd light not working or blinking.

Thanks in advance,
Jon.

 

Each to his own, but I would start with the manual:
http://www.ncttora.com/fsm/1990-1995...f/addcontr.pdf
The manual shows that the 4wd light is triggered by a switch that senses the movement of the actuator rod. Most likely, your electrical and vacuum system is fine, but when the actuator moves into position your front differential is not engaging.

As a start, I'd get under the truck when someone else works the transfer case, to confirm that the actuator is moving. You might also be able to see what turns out to be the problem.