First - sorry about my english, but it's not my first language
I have Blazer 4.3 4-door 1993 with oldest "electronic" system, so I have one button with only "4LO" and "4HI".
4LO works good, but I have a problem with 4HI. And... I read hundreds of posts about "4HI not engage" and
I know all about this vacuum problems and this acutator in transfer case, but in my car it looks litlle different.
I can engage 4HI and it works, but after... sometimes 500 metres, sometimes few kilometres it disengages when I drive.
And then... green light is blinking, there is terrible noise under the car (some gear grinding/chain slipping) and you
loose drive... there is no drive, like on "N". But, if I press this button again "on the fly" it sometimes engane 4HI again or I have
to stop, press it again, 4HI disconnect and I can drive 2HI. The same effect is when I try to engage 4HI when car is
not moving, on N (like 4LO). I know, that I shouldn't do it, but I just tried and then there is the same grinding noise and car is
not runnnig, unless you switch to 4LO or disengage 4HI.
So, what is dead? Trasfer case? Acutator in transfer case? And most important question - why it works on 4LO,
but not on 4HI? It's the same drivetrain, just on 4LO you have one gear more

Gab

 

Has happened to me 2 times.

The positioning electrical motor has a mechanical switch geared to the switching axle on the transfer case.

When so and so many faults have accumulated on sensing the position it goes into a "limp mode" where it is able to only switch from 2HI to 4LO directly.
For the reason of the inner workings of the transfer case this works only in standstill and with the transmission in "N".
Else it could make that ugly grinding gear noise, because it's unable to engage the 4LO or switch back to 2HI.

The part is called "encoder motor".

This is an old thread with nearly all the information required.

https://blazerforum.com/forum/tech-a...-switch-29170/

http://tcase.rsgear.com/articles/1998_03.pdf