EDIT: for folks stumbling across this thread looking for answers, my problem ended up being a misaligned neutral safety switch.


Howdy folks! I'm working on a 1999 chevy blazer. It's a four door v6 4.3L automatic with the NP-236 transfercase.

I was taking it out for a test drive after doing some recent work on unrelated systems. Everything was working fine, until I dropped it into 4 low and couldn't get back out.

Now I've been reading quiet a few posts on this 4x4 system- but I haven't found anyone else who happened to come across the same issue, and also posted what the fix ended up being. Nore have I read so far as to completely understand the system myself. Nonetheless, here's an overview of what I've got going.

Okay, so upon getting stuck in four low I tried all the suggested holding X while shifting Y combos for the four button 4x4 shift control. After intitial attempts the buttons would no longer respond. Luckily I was close enough to home, so I limped back and tossed it up on blocks.

with everything up in the air I played with it for a bit to see if it would change its mind and decide to play nice - but to no avail. With the TCCM reset, it'll make a few attempts to shift, then give up and disable the button. It was during this time that I noticed that even though the 4 low selector was lite, I wasn't actually getting power to the front right.

Now I wasn't getting any 4x4 service lights, but I tried grounding pin 13 to get the TCCM DTC codes anyway... but I got no response on the selector lights.

I pulled the battery and verified that there was vaccum at the shift actuator, but I wasn't convinced it was pulling the cable far enough. I removed the actuator body from the cable and attempted to pull it by hand without success - I even went and put it in drive to see if I could pull it hard enough to lock the axel in and... nope.

but upon removing the shift actuator cable completely, I found it's not bound up on either end and moves freely.

This leaves me to believe something mechanical for the lock ring in the front diff is the culprit.

but what leaves me confused is, even if the axel lock is nonfunctional.. it's currently not engaged so why can't I shift back up to 2hi?

Checked the level of the autotrak 3 fluid and its about 1/2 - 3/4" below the fill hole - but surely being down such a small amount of fluid wouldn't be it?

My next item of suspicion is the encoder motor, but I can hear it make attempts sometimes. Also if the encoder motor does fail - that will trigger the 4x4 service light cause the TCCM runs a check on it on startup no?

Also checked the electronic vaccum switch by the firewall and it seemed fine - had vaccum on the one line as expected.

I pulled the main plug on the TCCM and there was no corrosion on the pins.

Gonna pull the encoder motor tomorrow and test it. What else can I prod to try and narrow down an exact diagnosis?

 

Grounding pin 13 on a NP236 will not work for codes. That's only for a NV233 with the three button switch. Your tranfer case system will actually set a code you can read with a capable scan tool. If at all possible, that would be a good place to start. Realizing you're not getting a service light, I'd still want to check it for codes. Again, if possible for you.

 

Just to update the post, pulled the encoder motor and gave it 12v direct - no response. No surprise given that there's nearly a hole through it from rot. Gonna pick up another one and report back next week 👌

 

Well the new one's in and that wasn't it. Apparently these motors have a brake inside that needs to be powered aswell when you bench test it, and I didn't do that when I tested the old one.

Going to throw it back into the air tomorrow and take a closer look at the front diff. The fact that the actuator cable moves freely in its sleeve but not at all once connected makes me suspicious. The actuator itself sucks in just fine, no cracks in the rubber or nothing.

What I'm struggling to peice together... when I had the old encoder motor off, I tried indexing the shifting shaft through its positions, and found I could only get 3 possible positions- but there's four modes is there not? 2hi, 4hi, neutral and 4low right?

Or are 2hi and 4hi actually the same?...and I ****ed the tranny when I flat towed it didn't I? I only disconnected the rear drive shaft and not the front, and if the front drive shaft is normally still spinning in 2hi.. then I was still spinning the tranny wasn't I?

 

Got back up in the air, changed the fluid in the front diff- pretty black but no metal shillings so thats good.

Hooked up my scan tool and read the transfercase + transmission datastreams. Everything agrees the vehicle is in four low, but nothing is throwing current codes. Transfercase had some historical codes, but I've never had the 4x4 service light come on, and once I cleared the codes they didn't come back. However the scan tool reads the current position as "4 low - invlaid"

Currently the vehicle just flashes four low forever, pressing anything else will make the selection flash for a bit before it goes back to flashing four low.

when dropped Into drive both driveshafts are getting power, but only the rear wheels turn.

here's some pictures of the scan tool results I got, maybe someone can read these better than me. All I'm understanding from them is that the vehicle is 100% in 4 low but the front difflock isn't engaged and that's it 🤷‍♀️

 

So I was looking at the historical codes, and I found this Post talking about C0327. It mentions that in the event this DTC trips it activates the Transfercase Engagment Lock. I noticed that when I was reading my scan tool but It didn't mean anything to me at the time. This post was the first mention I found of the system.

So what exactly is the Transfer Case engagement lock and how many systems does it tie into? Is it purely bound to the output of the TCCM? Is there any way this system could be triggered without throwing any codes?

Because another thing that is causing me confusion... the next line on what the DTC does after being triggered is "turn on the service light".
Infact ALL the DTC codes say that. So what gives?!
shouldn't SOMETHING somewhere being throwing codes out at this point? It's been run plenty so codes would've had time to populate if any were gonna come up!

Ugh I need to find the nearest wall and put my namesake to use.

 

Does that tool have "Hot Functions" or "Special Functions" or whatever Matco calls it for bi-directional controls? If so it may have commands that allow you to control mode and range shifts of the system. That could help point your focus. I'll be in my garage later and have some time to list some troubleshooting. But in the meantime take a look at the link below, specifically starting at post #5

https://blazerforum.com/forum/engine...zer-4x4-89481/

 

Wow that's alot of good information which I needed! Thanks!

I've still got to finish reading it all, but after reading the part about the transfercase having its own neutral position.... I did not follow that procedure when I flat towed this vehicle nearly 12km at speeds exceeding 50kpmh at times. I removed only the rear drive shaft, I left the front driveshaft on...so rotation was still making its way to the transfercase... that's it there isn't it?

I'll crack the drain plug on the transfercase and see if my brand new fluid had been changed to sludge- if it's clear, I can most likely rule out mechanical damage from the tow - but I think that's probably it.

 

Based off the codes, I don't know if I would immediately jump to that conclusion. Again, if your scantool has Special Functions ability, you may be able to command the system and if it reacts correctly, it may be more of a circuit issue between, and to include, the switch circuitry to the TCCM. Then it's a matter of testing to locate the short, open or high resistance.

 

Okay, well after giving that all a good read, I've atleast got a good plan for approaching further diagnosis. I'm going to source a beefier scan tool from a friend over the weekend so I can send commands to the TCCM like you were saying, and I'll print off those circuits so I have a reference on hand when I bust out the multimeter. I'm also going to check to make sure all the speed sensors are showing. I'll be sourcing an extra TCCM and 4-button switch from a local yard that has a matching model, so I can swap those out and see if I get lucky.