4wd

jrSS

Original poster
Member
Dec 4, 2011
3,950
Alright here's the situation, driving in 2wd and swith to 4wd "on the fly" and its obvious that I am not in 4wd my ass end is everywhere. So I stop turn ignition off swith into 4wd and turn truck on and its working. All 4 wheels spin. Wth? Is it the switch going bad??. 1 month ago changed the front transfer case fluid not the rear (will do that in the spring) any thoughts.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Changing on the fly isn't recommended, but it hardly ever fails if things are working well.

1) There is only ONE transfer case, and it's on the rear of the transmission. Its fluid needs to be changed every 50K miles. Are you current on that?

2) There are two DIFFERENTIALS, to be changed usually every 100K. Old fluid wouldn't cause your symptoms.

3) The switch probably isn't bad, but the way to diagnose that is if your desired mode is lit up on the switch above the selector. The switch is bad if the mode light DISAGREES with the switch position.

4) VERY IMPORTANT to troubleshoot this is to observe the lights as you change the mode switch. Only change it ONE mode at a time, so your first change should be (you can do this while stopped with the engine off even, but the ignition in RUN) from 2HI->A4WD mode. As you change the switch, the light over 2HI should be steady, and then the light over A4WD should blink for a few seconds then go steady. Does it? As that's happening, you should hear TWO motor noises from underneath the vehicle. Memorize these. One is a high-pitched whine that goes for a second or just a bit longer. That's the front axle actuator. The other is lower pitched and shorter duration. That's the transfer case encoder motor.

5) Then change modes from A4WD->4HI. The front axle actuator is already engaged, so you won't hear that noise again. You should ONLY hear the transfer case encoder motor, and the mode switch light will flash over 4HI and then go steady when the TCCM gets the expected position feedback signal.

5) What usually fails is the front axle splined disconnect on the passenger side of the oil pan, assuming you don't get any flashing mode lights on your switch, or the "Service 4WD" light on your dash. Here's links to more reading on troubleshooting or rebuilding that unit:

Offroadtb.com Diagnosing the 4WD System

Offroadtb.com Front Axle 4WD Disconnect

Transfer case theory of operation:

Offroadtb.com Transfer Case Operation: NVG 226 (NP8)
 

jrSS

Original poster
Member
Dec 4, 2011
3,950
Thanks roadie, and everything seems to be working now flawlessly. I have 85000 miles and no haven't changed t-case fluid. Only front diff. It goes into part time 4wd (heard the 2 diff noises) changed it to 4wd (heard the quick noise) put it into 2 lo (while in neutral) and it went in. And visa-versa back to 2wd. I just won't switch "on the fly" anyore. And will definately get the transfer case fluid done. What fluid needs to go in t-case?
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
Auto-Trak II fluid (you'll need 2 liters) from the dealer is only about $9/liter.

Also, that would be 4LO, not 2LO, on the transfer case switch.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
ONLY GM AutotracII fluid. Check the owner's manual and various threads that discuss it here or on the OS.

What's the behavior of the lights on the switch when it fails to engage on the fly? Sounds like problems inside the splined disconnect. Available for around $400 as a full assembly from ATS on Amazon. Less money if you take it apart and order just the broken bits or a weak spring from GM. Or junkyards. May not be worth it to you to fix if your workaround is reliable. It's better off for the spinning parts to not be asked to engage on the fly anyway.
 

jrSS

Original poster
Member
Dec 4, 2011
3,950
Lights say 4wd is in. BUT when at a stop I tried spinning all 4 a ticking noise would happen and just the back 2 wheels would spin.
 

jrSS

Original poster
Member
Dec 4, 2011
3,950
But now when stopped and when I engage 4wd all 4 spin. :biggrin:
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Sometimes, the splined disconnect needs slight vehicle movement and a shallow turn to allow the splines to line up and the collar to engage. So the ticking is the splines hitting the valleys on the collar and not engaging. Could be congealed grease or a weak spring inside the disconnect assembly. A purely mechanical issue.
 

Forum Statistics

Threads
23,442
Posts
639,383
Members
18,664
Latest member
SooMenn22

Members Online