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Wont shift to 4 low but will 4 hi

Alexh71

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Jun 9, 2021
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I have an '04 z71 trim and it wont shift into 4 lo, with service 4wd message popping up everytime i start the truck n start moving. And yes, im having it in neutral when trying. It'll shift into 4 high no problem. The light on the selector switch will just blink for the 4 low and go back to 2wd or 4hi. I know, "take the truck in to have it scanned." But maybe i can get suggestions on what to check at home to diagnose the issue. I want to try to avoid paying the shop's scanning/diagnosis fees if i can, if yall can relate lol. Ive tried searching as well but couldnt find anything relating to my issue.
 

BainMan

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The codes really are your best way to start, find a friend with a Tech2 or Snapon Solus or some other mechanic level code scanner and bring them a case of beer. I believe the off-the-shelf scanners will only do P-codes and not what you're after.

But if you just want to take a stab at it, check transfer case fluid, check pinouts on the wiring to transfer case, test for voltage and ground and also continuity from the harness to wherever the wiring ends up.
 

BainMan

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Just thought of something, when you shift gears (PRNDL) on the column, does the N light up when in neutral? Curious if your neutral safety switch is working. It won't go into 4 low without it being operational.
 

Alexh71

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For some giggles before i left work this morning and before i moved the truck, i shifted down into N and hit the 4lo button. Low and behold, it almost immediately popped into it like it should. Then to 4hi, done like it should, and then back to 2wd. Left work and the S4wd came back up like it has been. Stopped at the store and tried it again to no avail. So it SEEMS it only has the issue after the truck's been moving, and thats only when the S4wd pops up. This was the only time ive tried that right after start up and before moving. If that bit of info helps any at where to look specifically.
 

Vaeagleav

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Are you trying to use 4WD on dry pavement or in dirt or snow etc? As your owners manual states don't try to use 4WD on dry pavement or damage could result (especially if turning).
 

ca2kjet

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Is there a way i can do any sort of diagnosis of them with say hand tools, volt meter, test light?

I hate to say it but the best way is to read the code. If the internals of the encoder aren't aligning correctly the sensor thinks it's not where it should be and fails to switch. That may be why it's occasionally working as it hasn't totally failed yet. Sometimes it can be the switch in the truck failing but I can't picture it causing this particular issue. When you try going into 4low does it bounce all the way to 2wd?
 

Alexh71

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I hate to say it but the best way is to read the code. If the internals of the encoder aren't aligning correctly the sensor thinks it's not where it should be and fails to switch. That may be why it's occasionally working as it hasn't totally failed yet. Sometimes it can be the switch in the truck failing but I can't picture it causing this particular issue. When you try going into 4low does it bounce all the way to 2wd?
I havent tried to go from 4hi to 4lo to know if it bounces back to 2wd from there, just from 2 to 4lo but yes. But it does kick to 4hi whenever i select it. But like you said, it may not be picking up the correct spots for 4lo after the truck moves. Im not thinking its the selector switch on the dash either, cause it moves freely all the time from 2 to 4hi and to 4lo after start up before the truck moves. Im guess im just gonna have to suck it up and pay a shop to read the codes since i dont have a buddy with the proper scanner. I was hoping there was a way to test the sensor or encoder motor myself.
 

ca2kjet

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I havent tried to go from 4hi to 4lo to know if it bounces back to 2wd from there, just from 2 to 4lo but yes. But it does kick to 4hi whenever i select it. But like you said, it may not be picking up the correct spots for 4lo after the truck moves. Im not thinking its the selector switch on the dash either, cause it moves freely all the time from 2 to 4hi and to 4lo after start up before the truck moves. Im guess im just gonna have to suck it up and pay a shop to read the codes since i dont have a buddy with the proper scanner. I was hoping there was a way to test the sensor or encoder motor myself.
Only way I know of to truly test it is to remove the encoder motor from the transfer case and watch what the spline does as the selections are made. But that may not tell the whole story since the spline could position itself correctly and the sensor inside doesn't read it. When you try shifting into 4low does the transfer case motor make any noise like it's attempting to shift? Do you get any clunk?

Here's a video on the replacement process of the whole motor unit;


And a video of only the sensor being replaced;

 

Alexh71

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Only way I know of to truly test it is to remove the encoder motor from the transfer case and watch what the spline does as the selections are made. But that may not tell the whole story since the spline could position itself correctly and the sensor inside doesn't read it. When you try shifting into 4low does the transfer case motor make any noise like it's attempting to shift? Do you get any clunk?

Here's a video on the replacement process of the whole motor unit;


And a video of only the sensor being replaced;

After the truck moves, there is nothing that happens. No chirp, bang, thud, or jerk. All i get is the blinking light and it gives up. But again, straight from start up and before movement, it does as it should. Havent been able to move it, since i've been on asphalt at that moment.
 

2004Slickside

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Try engaging/disengaging 4LO when rolling slowly in reverse with trans shifter lever in neutral. I believe this used to be the recommended way for a NP206 transfer case. Definitely not an on-the-fly operation.

who needs 4Lo?
 
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Alexh71

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Try engaging/disengaging 4LO when rolling slowly in reverse with trans shifter lever in neutral. I believe this used to be the recommended way for a NP206 transfer case. Definitely not an on-the-fly operation.

who needs 4Lo?
I have the np8 (?) Coded. But the recommended way is from neutral or a very low speed like 3 mph while in N. But ive tried that with no success. About one time ive needed it so far cause of my dummy self sinking the front in deep mud. Note this problem was occuring since i got it. And i would like to play around a little bit where 4lo would benefit more than 2wd or 4hi.
 

2004Slickside

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RPO NP8 = NP246 (see attachment - sorry I remembered wrong number, but it was the same transfer case I was thinking of). I have one in my 2001 Blazer and one in my 2004 Avalanche Z71.

Maybe attachment will help in some way. Best wishes. Bainman gave good advice. Very difficult to diagnose transfer case without a scanner that will access the TCCM. It can be done (somewhat), but you really need to understand schematics and how the system works. Still you never know what the TCCM is thinking without the proper scanner.

If you want to take a stab without the scanner, check the big red and black wire terminals at the connector going into the TCCM for corrosion. Clean if corroded. If OK, just go ahead and replace the encoder motor on the transfer case. Do not try run the encoder motor when it is off of the TC or you will ruin it. It has a brake circuit that needs activated to release, and it's very easy to go past it's limit when taken off. Also the internal position sensor does not work when the encoder motor is off the transfer case.
 

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