4X4 prob and now no forward or reverse...help

[duluthsnow]

My 4WD has worked flawlessly on my 02 Trailblazer LTZ (133,000 miles) up until a week ago. During a snowstorm that dropped 30" on Northern MN I had a complete failure of all systems. I selected 4WD LO while at a stop and I heard a clunk. After that I had no forward or reverse. First, I unplugged the #8 fuse and waited 30 sec...nothing. Then, I had it towed to my local mechanic. I purchased a TCCM from an 03 without any change. My shop in th mean time got my 2wd working again. I installed a new front axle actuator, encoder motor and today a new front axle disconnect. My shop called today and said he was driving my vehicle to check if everything was working after the disconnect install....it seemed to only be in 2wd and then HE lost everything....no forward or reverse...the same thing that happened to me initially. He also changed my fluid in the front diff with Mobil 1 75w-90. I am at a loss as to where I go from here. My mechanic also tried a spare selector switch he had with no result. Now the indicator lights flash instead of going solid. Should I take it to a tranny shop? One other thing I noticed both when it originally happened and now is that when I try and put the selector in park slowly I can hear gears grinding. Jim

 

[Sparky]

Ouch!

Sounds like maybe something in the transfer case... Was the autotrak fluid replaced in the case on schedule?

When the lights are flashing and you are changing modes, is the encoder motor moving normally?

 

[The_Roadie]

Welcome! Sorry to hear about the failure, though.

I would be taking it to a professional tranny shop for a diagnosis. More likely to be the transfer case, depending on if the fluid was changed every 50K as required. But progressing failures now in 2HI mode surely could be the tranny, again, with little to go on except possible fluid or filter change history.

 

[AtlWrk]

My first guess would have been the encoder motor but since you have replaced that the next logical step would be a mechanical failure in the tranfer case. OffroadTB.com has a great article that explains the functioning of the transfer case:
Offroadtb.com Transfer Case Operation: NVG 226 (NP8)

Inside the transfer case is a coupling that connects the input shaft directly to the output shaft. When you switch to 4LO, the encoder motor moves the coupling to disengage from the input and engage the planetary gears to give you an extra stage of reduction (extra torque). This is done with splines like the front disconnect and not clutches--this is why it can't be done on the fly. To get the transfer case into neutral the encoder motor moves the coupling so that it is not connected to the input shaft or the plantary gear set (i.e. the output shaft is not connected to anything).

Since you're losing forward/reverse (effectively in neutral) either the coupling is not being moved/held in position properly because of broken linkages or it is broken.
ALSO! If it is a transfer case failure like that putting the transmission in park will NOT stop it from rolling!

The clunk you heard may have been the normal enagement/disengagement of the coupling in the transfer case or it may have been something breaking Also, losing forward/reverse would have nothing to do with any of the front wheel drive components. The gear grinding noise you hear is because the transfer case has effectively disconnected the transmission from the rear wheels meaning that the transmission internals are free to rotate from the residual torque coming from the engine at idle. It's like putting the trucking in park while you're still rolling.

Point of all this is that your symptoms point to something mechanically wrong in your transfer case.


Agreeing with Sparky, what has the fluid maintenance been like?

 

[duluthsnow]

I have not replaced auto-trak II fluid to the best of my knowledge since I got it with 57K. It is definitely in park when it's in park. I did hear the motor noise after my mechanic originally got the 2wd working again and the selector switch lights stayed solid. Now the selector switch lights just blink and no motor noise. I checked fuse #48 in the rear fuse blick to see if that was the issue not powering the encoder motor. I guess I'm paying for being unaware of the fluid maintenance on the transfer case.

 

[MichEnvoyGuy]

I dont have much to add here except when my sunshell blew in my trans, it thumped and would grind in park. Maybe its not your transfer case after all?

 

[duluthsnow]

My mechanic swapped my encoder motor with another old one he had in his shop and got me rolling again, albeit in 2wd. Looks like the motor I bought was no good. I will buy a new one this time and see what that does. My selecter light is solid now.

 

[RayVoy]

I dont have much to add here except when my sunshell blew in my trans, it thumped and would grind in park. Maybe its not your transfer case after all?

I'm with Mitch, if your grinding going into PARK the problem is inside the transmission.

 

[duluthsnow]

I purchased a new AC Delco Encoder motor and had it installed this morning. Now all indicator lights are dark (except for on startup) and my service 4WD light is lit. I disconnected the pos and neg from the battery, touched the pos-neg together, pushed on the brake pedal, did the hokey-pokey and still same situation when I restarted. It seems to me the TCCM is not waking up......ideas? There is no longer any grinding when putting into park (that only happened when I lost all forward and reverse) Jim

 

[duluthsnow]

Just posting as a follow up. Ended up needing another transfer case. Lots of swings and misses getting to that result. 4WD working properly now.

 

[The_Roadie]

Thx for the followup. That was my first guess, and I you hadn't even posted about the fluid age until post #5. Everybody needs to read this to motivate them to keep up with the 50K TC fluid change interval. Sorry it happened to you.