Service 4WD light on when AUTO 4WD pressed but works?

[Leifmb]

Hello, I have a 2004 Blazer with the 4-Button transfer case control (NP8). I am getting a Service 4WD light comming on when I press the Auto 4WD button however the system seems to engage and the little light on the button stays illuminated (transfer case does not go back to 2WD). If I cycle the key off and back on the Service 4WD light on the dash will go off unless I press another Trasfer case gear selection. If I press any selection, the transfer case will be heard shifting and the flashing light on the selector button will become solid. This indicates to me that the front axle has at least engaged. Then, a couple times I tried pressing the Auto 4WD button while driving. Again the Service 4WD light came on. I pressed the 2WD button and it remained lit. However, the vehicle power was suddenly neutraled. The Transfer case selection 2WD light was lit (not the red neutral) and the transmission was in "D" but the drivetrain was neutral somewhere (prob the transfer case between gears). So, I figured it must be the encoder motor-Nope! I changed the encoder motor with the same results. Of course I now think it's the TC Module but I wanted to make sure to see if anyone else was experiencing a similar situation before I drop the $$ for a new module. Futhermore I tested the two relays IN the module and they both work. I verified operation of the front vacuum actuator and axle position sensor. Wiring looks good everywhere-car's pretty new. Any suggestions from people on this would be appreciated-THANKS!!!

[Leifmb]

Oh, when the car went into neutral while driving I pressed the 4WD HI button in desperation (I was in the middle of a bridge coasting along...). After pressing the 4WD Hi button the trasfercase "clunked" and power was restored. I then selected 2Hi and the light on the button blinked a couple times and then remained lit. After cycling the ignition off and back on, the Service 4WD light always goes off. AFTER this I tried replacing the encoder motor with no change.

Any help would be much appreciated-THANKS!!

[MT-2500]

You will need a engine/transfer case capable scanner to test it or get codes.
Post back any codes.
Shifting problems can be caused by speed sensors also.
Will it shift good when sitting still?

Why did you try to shift on a bridge.
It is best to not use 4W on a dry payment.
How many miles on it or Has the transfer case ever been service with the gm blue fluid?

[Leifmb]

The TC will shift into gear (at least the dash button seady lights indicate as so) while stopped.
I just happened to be on a long flat bridge over water. I occasionally would shift into 4HI and back to 2HI for a few seconds (no real harm) on dry pavement. Excessive wear would occur over longer periods than my 3-5 seconds of being in 4HI or Auto 4WD. The vehicle has 52K miles on it and the transfercase has the proper blue fluid filled to the filler hole.

Thanks,
Leif

[MT-2500]

If you think there is a problem get it on a good transfer case capable scanner and check for codes and test shifting operation.
Also check the service schedule for the flue fluid .
It mat be time to change it.

[Leifmb]

Just went for a drive and pressed the 4HI button, waited a few seconds after the button light stopped flashing, then pressed 2HI. I suddenly found myself in neutral again. I then slowed down to 2-3mph and pressed 4HI and then 2HI shortly after and then I had power again-Strange. The Service 4WD light came on the first time I pressed the 4HI button but after cycling the ignition switch the light went off.

[MT-2500]

What does the owners manual say about shifting?
To shift into 4W high or back to 2W you may need to stop and shift.
Will it shift ino 4W-auto and back into 2-HI on the road.
If not.
It may be Transfer case scanner time.
Post back any codes.
MT

[Leifmb]

I can shift while driving into any mode except 4LO. The transfer case appears to shift into 4HI or AUTO 4WD and back while driving. Shifting to 4LO as per the manual requires low speed or to be stopped to engage.

[MT-2500]

Which mode and at what speed is it giving you problems on shifting out of and back into 2W?
Are you tires matched up good?

[Leifmb]

The car looks new, all tires are original and have 60% of tread left. All tire pressures are at 44psi as recomended. The main problem I'm trying to diagnose (without a scan tool expense) is the immediate lumination of the Service 4WD light when any shift mode button is pressed. This can be at ANY speed or stopped-no difference. Sometimes the vehicle's drive train will remain in a neutral state (at the transfer case) when I select either Auto 4WD OR 4HI and THEN select 2HI again but is intermittent. Regardless, the Service 4WD light goes out upon cycling the Ignition. Furthermore, upon raising the vehicle (all 4 wheels off the ground) I have verified seemingly proper operation of the 4WD system.

[MT-2500]

The only way to test or check or diagnose it is with the proper scan tool.
If there is a problem it will set a code.
Also a scan tool have several shifting test and live data in it.
Without a scan tool all you can do is throw parts at it.
But if that is your only problem you have on shifting I would just slow down or stop before making shifts.
MT

[Leifmb]

Well thanks but as I said, speed is irrelevant to my issue.