I tried everything I could and read every forum and am just lost at this point. Coming here to see if I can get some help from you guys.
So starting about 3 months ago my 4wd started to act up. When I clicked the 4wd button the 4 tire green lights and the center yellow diff light all started flashing. Eventually it would go into 4wd after clicking the 4wd button a handful of times and the lights would eventually turn solid. Progressively things got worse to the point where once I clicked the 4wd it engaged the front axle and stuck on with the lights still flashing and never turning solid. The 4wd button became unresponsive, no more audible clicks when I press it as before. I went through a handful of forums where they said it could be the sensors on the t case actuator or front diff actuator sensor. I bought new sensors and did the continuity check on all of them before installing(put the meter prongs on the sensor contacts and push the plunger in to hear a bing) . Nothing helped. So I just decided I'll just replace the whole transfer case and t case actuator and that must solve it. Switched it out and same old problem. First run with the new actuator and case and as soon as I press the 4wd button it makes the audible click and the front axle engages but will not disengage. 4wd button again unresponsive with lights still flashing. I could manually disengage the front by unplugging the front actuator sensor and putting 12 volts to pin 2&3 on the rear t case actuator plug to reverse polarity. Once I click the 4wd button again it's the same old story. The audible click and its stuck on with the 4wd button unresponsive.
I also thought that it might be the front actuator but when I pulled it out I realized that it's on vacuum lines since it's the 1999 and it has no motor in it for it to go bad.
Any help will be GREATLY appreciated 😁

 

To get an idea of what you’re working with, the “new t-case” you got, was it new-new or used and new to you?

 

Oh sorry it was pulled from a 4runner that had 100k miles

 

The biggest issue with the 4wd system is lack of use. Lack of use allows condensation to corrode the electrical contacts.

You said you checked the position sensors on the t-case and front diff with a multimeter. When you had it in continuity test, you need to slowly push the plunger in and see if there is ANY dead spots. The 4wd computer has a reference table of “selector A = position1 switch on/off, position2 on/off, position3 on/off…” if that gets interrupted and the computer receives the wrong position signal it goes into error mode.

next I would pull the cap of the electrics on the t-case actuator and see if the contact trails are shiny and clean or dirty and corroded (that was my issue).

 

Yes I checked it with a multimeter and there was continuity throughout the whole press of the plunger. I will go through all my connections and check if anything is corroded. I will let you know what I come out with. Thank you!!

 

I checked all my connections and switched out all the remaining sensors on my transfer case (the 3 sensors with the pigtails that are extremely hard to reach.) Low and behold the lights aren't flashing anymore. It goes in to every mode perfectly. But when I get to 4wd, I hear an audible click of the front actuator working but then the car doesnt move, its like it is in neutral. I hear the front driveshaft spinning and I looked under as well to confirm but the car isn't moving. Im assuming that the front diff is toast. Do you think thats the case?

 

You’re going to have to get all four wheels off the ground.

You feel confident that the transfer case is engaging the front drives shaft and can switch from high to low.

now we need to determine if the front diff actuator is cycling it’s full range of motion. And if so, is it functioning.

it’s hard to believe that the front diff would be seized up and not making ANY noise will you drive in 2wd. Have you changed the oil in the front diff? What did it look like?

the easiest way I’ve found to cycle the diff actuator is with a mity-vac hand pump connected to either side of the vacuum ports. Then you can apply vacuum and jiggle the wheels or driveshaft until it shifts. Then once it shifts side if you can turn the tires by hand.

 

Yeah I did so, I raised the car in the air and only the rear tires are spinning. I hear the audible click of the actuator but the front wheels dont spin. I see that the drive shaft that goes to the front diff is spinning but all I hear is whining from the front diff.

 

Okay, Good Info.

next step, take the position sensor out of the front diff and cycle the actuator from 2wd to 4wd and while switching (it checking before and after) put your finger in the sensor hole and see if the rod is sliding back and forth. This should tell you if the shift for is actually moving. I found that sometime I would have to rock the tires back and forth to free up the gears before it would shift.

 

So after quite a bit I figured out the issue. I had to replace both my entire transfer case (bad actuator) as well as my front differential assembly. It was a load of work but I finally got it, thank you for your help.