APPLIES TO:
The NV233 transfer case mounted on 1st gen Blazers 1989-1994 (because that is my model and I do not know if it applies to later gen. - please expand).
4x4 Blazer with 4.3 litre V6 WINZ or WINW and electric NV233 transfer case (switch on upper left dash above lights switch)


SYMPTOMS:
Using the 4HI switch driving or in standstill with transmission in any position does not shift to 4HI (4x4 will not engage) - green light is flashing (eventually), then extinguishes. Shifting to 4LO "normally" works fine and back to 2HI as well. There are cases where either the 4x4 (front axle) will not disconnect or it takes several shifting attempts to go back to the other position. Normal indications for successful shifting to these positions will, again "normally" be given by the indicator lights in the switch.


DIAGNOSTICS:
Best way for diagnosis is to read out of DTC. Mine shows a DTC value of "2" (encoder fault). As from talking to other guys and dealers this might be the most common problem associated with the 4x4 transfer case. Though it might be the switch itself as well. To eliminate the dashboard switch itself it may be jumpered accordingly.

DTC associated with the TCCM. The status lamp flashes the same amount of times as the DTC or it can be read out directly with a TECH tool. (for manual DTC retreival jumper A and J pins in the DTC connector.

DTC 1 RAM STANDBY POWER LOST
DTC 2 ENCODER FAULT
DTC 3 TCCM MOTOR CIRCUIT
DTC 4 RAM/ROM FAILURE

According to the service manual this happens when the encoder switch which is located on the shift motor itself encounters an error (increase error memory by 4). Once enough errors (32) are accumulated the TCCM stops shifting to 4HI and reverts to a basic shift pattern called "rail-to-rail" shift. This means only 2HI and 4LO shifts are possible.

Conclusion: Replacement of the motor-switch assembly seems the only way to make the thing shift again according to dealers.
But replacing the da.n thingy is expensive even more so if you live in Europe as I do. (1'500 $ OEM with official dealer)
As it was broken anyways I dared opening the thing up. Not so easy but I even found a way to get the assy out from the transfer case without disconnecting the transfer shaft.

A couple things I found out about the NV233 transfer case and the encoder switch in particular:
[ul][*]Q: How does the motor shift the transfer case?
A: It turns the shift axle by about 50° from 2HI to 4LO[*]Q: How much turn or shift is needed for the different positions?
A: Some positions only 10° of turn[*]Q: Anybody ever repaired such a switch?
A: Yes, I did it - and it worked a couple of weeks to fail again [8D]
[/ul]
MY SOLUTION TO DTC 2:
I was able to open and fix the problem with the switch temporarily. (probable cause of malfunction: contact grease in the switch assy got old and hard, leading to bad contacts)
Opening of the switch itself was done using a DREMEL to cut out the original plastic weld. 2mm wide 1.5 mm deep (1 inch = 25.4 mm - get metric - makes live easy)

The switches brushes are very fragile. Been measuring the whole switch assy and got weird and incorrect readings.
I cleaned the printed circuit from the old grease and gave it a shave with a 400 sanding paper. Same treatment for the rotor arms and brushes.
Fine tweezers to bend the tiny rotor arms into position again.

I put it together using plastic glue (REVELL modellism) and ARALDIT (R) 2 component glue to fill the original weld I cut out.
I ground the ribs holding the connector, by about 5 - 8 mm (1/4" - 1/3") to be able to put it back on without removing the axle. To unbolt it I fixed a cutting edge to an old screwdriver to cut the ribs down which interfered with the front axle shaft. So with my parts no need to disconnect anything.

I hope this general information is of use to others as well as i have dug into the issue a bit and expanded with some pics, links, tips, hints from others into a comprehensive thread. Thanks Kyle for the very good reference links. They explain to perfection how it works. I have added some pictures which show the whole thingy and as well a clarification and details on the 3arm wiper. It was never clear to me how it would work as drawn in the GM service manual - simple answer - the 3 arms are "not" interconnected.

And last but not least for the U.S. - No warranty on my explanations - try at own risk as you may brake it completely, injur yourself at own risk etc. I deny any responsibility!

TIMES:
Get it out of the car - about 30 minutes
Open up - repair - put together about 90 minutes (plus 24 hours to let glue dry properly)
Put it back in car - about 30 minutes

Guy with experience in repairs - very well equipped pro tools - garage with lift - service manual (but not of much help)


Cheers and good rides as it is now snowing mixed with freezing rain


P.S. Kyle, I do no longer believe that the motor itself is a stepper motor but rather a normal geared DC motor. Once I have mine working again and I have taken the other Blazer apart I might set up a testbed to check

 

The transfer case shift motor is a precision stepper motor which is monitored by the encoder. The encoder is what the TCCM uses to determine the location of the transmission sector shaft that controls what mode the transfer case is in. The sector shaft is rotated to select the various modes. Over all, I think that the sector shaft turns around 60* total so all of the mode detents would be present in that 60* movement. The encoder itself has 4 sensors which use a gated ring to determine the precise location of the sector shaft.

Here are two excellent articles to read through:
Troubleshooting the NV233 Transfer Case - rsgear.com
The first is an excellent article detailing the steps for troubleshooting the NV233 transfer case which is in your truck.
Understanding the NV243 Transfer Case Part 1 - rsgear.com
The second is on the NV243 transfer case which is used in the fullsize trucks, but has the same encoder design, just heavier components. I am referring to this article to show the detent locations and to show the explanation for how the encoder motor actually works.

You can check out the other transfer case tech articles that Rockland Standard Gear offers by CLICKING HERE.