Help me understand this 4wd control system

So the new to me 2003 Avalanche 1500 4WD has ..... issues. I am picking them off one by one and this one has me stumped.

When I start the thing it will randomly be in 2wd, 4 high or 4 low. Sometimes the Auto 4wd button will be lit

And quite a lot of the time the N light will be the only thing lit

Usually I can put it in neutral and select 2wd. Sometimes I have to restart the truck to get it to respond.  When it starts in 4 low and I hit the button the hubs unlock with 2 distinct clunks even though I never hear them engage. 

Isn't the N light supposed to be a reminder to be in neutral before changing modes? 

Not sure if I should replace the button module or look deeper. What else is involved? Is there a separate transfer case module? 

No codes on the older Mac reader I have, nor the torque app. 

I don't know about this car specifically but I do know about 4wd transfer cases.

It shouldn't shift the TC by itself, sounds like the control system is screwed up. The N light should indicate a neutral TC position - in this position, the vehicle shouldn't be able to drive in any forward or reverse gear. From being stopped in neutral you should be able to select any mode. You may need to move the vehicle a bit in forward or reverse (in a straight line) to change from a 4WD to a 2WD drive mode, the light would indicate when it's come out of gear. And of course you have to come to a full stop (or lock up all 4 wheels ) to change between high/low ranges.

If that transfer case is actually shifting itself into neutral, make sure you use your parking brake, or you could find it somewhere other than where you parked it!

I would assume that “auto” should be the default setting, but have no real help.

it is shifting the transfer case to neutral, not the transmission. that just uncouples the front axel and wont allow the truck to roll away.

Rusnak_322 said:

it is shifting the transfer case to neutral, not the transmission. that just uncouples the front axel and wont allow the truck to roll away.

TC neutral usually disconnects both axles, is this vehicle different? 2WD mode just disconnects the front axle.

I have a Suburban with the same system that works fine.  The light is to tell you that you are in neutral on the transfer case.  

It will switch between 4wd high, 4wd auto, and 2wd while in gear or in motion.  To shift to or from N or 4wd low the transmission must be in neutral if working properly. 

That transfer case is the NVG 246 EAU.  There is a transfer case shift module.  I have a GM data file on my Suburban that describes how this all works, as it is on my work computer here are some pics of the info on my screen, but you can go and download this yourself from GM directly, it is full of useful info.  If they don’t have the avalanche then pick the Suburban on as they are mechanically the same. 

Saron81 said:

 

I would assume that “auto” should be the default setting, but have no real help.

"Auto" usually means an AWD mode, where both axles are connected but there is a differential between the axles. In any 4WD mode, that differential is locked.

Edit: From the docs above, on this vehicle, "Auto" means an active diff sending power to the front wheels when rear wheelspin is detected.

I have the same setup in the Suburban (edit: mine is a 2003 as well). It will sometimes show neutral when I start it, but it isn't in neutral and will drive normally. Put the trans in neutral, push the 2 hi button and it will show correctly. I think it has something to do with the encoder, either it is sending a false signal or the communication between the PCM and it gets jumbled.

Three drives this morning.  Start it N lit. Hit 2hi and after 30 seconds of flashing the 4hi lights up. Hit 2hi again and it lights up in a second. 

I know that in tight parking garages I can hear the front wheels fighting the rears when the N or auto is lit. If it will go into 2hi that goes away. 

Yeah that system is totally berkeleyed up. Sounds like it's driving in locked 4WD modes on the street, which will massively accelerate drivetrain wear and tire wear, and could create some "interesting" handling characteristics on wet pavement.

You are going to need a Tech II or a TechIIWin emulator to go any further.  Basically, you need something that can read and interpret the signals in the powertrain control module to determine if its a faulty swith, t-case sensor, or PCM.  

A Tech II or MDI clone is $350 or so from the bay, I would go that route.  I ordered one off AliExpress and their return policy sucks (long story but ended OK for me) go Ebay instead.  

There is an encoder motor on the transfer case that has a bunch of location contacts in it.  If the dash shows N but you are still moving, something is busted, either the encoder motor or the module, or the wiring, or...

My Silverado has an iron bar connected to the T-case, with a big old knob sticking through the floor.  I like that, but I have to fix electronic crap all day, every day.

And, don't run in AWD unless your tires are very, very close in diameter, or you are on slick surface.  You will burn up the transfer case clutches way faster than a dreaded AWD Volvo will eat its angle drive, and the internet knows how awful they are.

In reply to Streetwiseguy :

There is no AWD mode in that system.  Closest thing it has is "auto 4wd", which just slams it into 4wd when you spin the back tires too much...

Went to a buddies shop and he hooked up a big fancy snapon tablet reader and found a transfer case encoder code. He still recomended replacing the dash button unit first though. 

Cheap trucks are expensive.....

I want to 2nd the suggestion of using the parking brake until you have the transfer case situation resolved.  Not avalanche specific, but have seen neutral positioned transfer cases result in vehicle rollaway.

seen people die that way

Ah the good old days when we shifted with a simple lever.

rslifkin said:

In reply to Streetwiseguy :

There is no AWD mode in that system.  Closest thing it has is "auto 4wd", which just slams it into 4wd when you spin the back tires too much...

Semantics.  It is a progressive clutch pack. Theoretically.  Whether it works well or not is a question for another day.

Had a similar problem with a GMT 360.   (Trailblazer, Envoy).  It was the Encoder Motor.   It may also be the transfer case position sensor (in the encoder assembly).    First check all the wires and connectors, drop the skid plate for access.