Strange 4/Low No-Shift Problem

[kpers]

I am new to the forum and I'm hoping somebody can help me out with an issue but first some history. My son is in the service stationed out of state and through me he purchased a 2000 Silverado with 89k miles for delivery to him at a later date. I did an extensive test drive before the purchase which included testing all the 4x4 functions which operated as designed. Before I delivered the truck to him I had a few things done, one of which was having tranny flushed and fluid changed in the t-case by a very knowledgable tranny shop that I've used for over 20 years. While they had it they replaced the problematic rear main seal and housing on the engine. They are good friends and I trust them immensely.

Fast forward a week and I find myself delivering the truck to my son and while I'm explaining the 4x4 functions we discover 4 low isn't working. 4 high seems to work - indicator light illuminates and the front axles are engaging. Auto 4 light illuminates but we didn't have a good surface to test it on. 4 low flashes until it times out and returns to 2 high. We tried the 1-3 mph roll in neutral, stopped, backing up.... nothing worked. Then we noticed that when the truck was placed into neutral the shift indicator would indicate neutral then in the blink of an eye, and as fast as it lit up, the indicator rolled from neutral to reverse to park and then none were lit up - literally all within a blink of an eye. - all while the trans was in neutral. The indicator works in all other positions except neutral.

I'm guessing this is the source of the problem but not sure where to look. Possibly a neutral safety switch and the computer doesn't think the trans is in neutral so it won't shift to 4 low? Could the tranny shop have damaged the switch or a connector causing that? I haven't been able to find any info on this exact symptom and I'm hoping somebody here has seen this before.

Thanks.

 

[dietz4ibanez]

It's been a long time since I've had a push button 4X4 and it may be that you have to put the truck in neutral to get 4 low to work. For giggles, I would also try putting it in drive from 4 HI and have the truck moving at 3-5 mph. I would hop that it would walk itself right into 4 Low.

If you are in neutral and you try 4 low like you described while also hearing a clunking sound with the indicator moving back to 4 HI. I'd say it may be the Transfer Case Motor encoder ring. I think most people would just replace the transfer case if this was your problem... It could also be a relay of some sort.

 

[iowaredneck]

You have to be in nuetral to shift into 4 low

 

[kpers]

Thanks for the replies.

It is in neutral when attempting to shift to 4Low, the backing up I described was simply rolling backwards in neutral. The truck is not making any mechanical attempt to shift to 4 low. It's acting as though it's still in drive and the 4 low indicator continues to flash until it times out.

Here are my thoughts, and I may be completely off base here...
Since neutral is required for a 4 low shift from 2 high or 4high and the neutral light on the shift indicator isn't working properly I'm thinking it has something to do with the computer thinking the truck is not in neutral.

My confusion is not so much that the neutral indicator isn't working but the way in which the shift indicator sequenced from neutral through reverse and through park while the trans was still in neutral. Something's goofy in that circuit and I was hoping somebody had seen a shift indicator do that in neutral, maybe that would get me going in the right direction.

 

[Draenor]

Possibility of the Neutral safety switch. Problem is, the truck should not start either if it is bad. As Brian stated, the encoder ring in the T-Case may be bad. Any messages on the DIC?

To shift to the Four-Wheel-Drive Low position, the
ignition must be in RUN and the vehicle must be
stopped or moving less than 3 mph (5 km/h) with the
transmission in NEUTRAL (N). The preferred method
for shifting into Four-Wheel-Drive Low is to have
your vehicle moving 1 to 2 mph (1.6 to 3.2 km/h).
Push the Four-Wheel-Drive Low button.
You must wait for the Four-Wheel-Drive Low indicator
light to stop flashing and remain on before shifting
your transmission in gear.
Notice: Shifting the transmission into gear before
the requested mode indicator light has stopped
flashing could damage the transfer case. To help
avoid damaging your vehicle, always wait for the
mode indicator lights to stop flashing before shifting
the transmission into gear.
It is typical for your vehicle to exhibit significant
engagement noise and bump when shifting between
Four-Wheel-Drive Low and Four-Wheel-Drive High
ranges or from NEUTRAL with the engine running.

Here is a TSB too that may help.

Document ID# 1656808
2005 GMC Truck GMC K Sierra - 4wd

Subject: Electronic Transfer Case Unable to Complete Shift
keywords 226 236 246 263 246EAU 263 EAU 4482 4484 blink flash inoperative light module NP1 NP8 NR4 switch TCCM #PIP3424 - (05/11/2005)

Models:

1999-2000 Cadillac Escalade
2002-2005 Chevrolet Avalanche, Trailblazer
2004-2005 Chevrolet Colorado
1999-2004 Chevrolet Blazer
1999-2005 Chevrolet Silverado, Suburban, Tahoe
2004-2005 GMC Canyon
1999-2005 GMC Envoy, Sierra, Yukon, Yukon Denali, Yukon XL
1999-2004 GMC Jimmy
1999 GMC Suburban
2003-2005 Hummer H2
with RPOs NR4, NP1, or NP8 Transfer case

The following diagnosis might be helpful if the vehicle exhibits the symptoms(s) described in the PI.

Condition/concern:

Electronic transfer case may stop shifting if you repeatedly try to shift the transfer case in a short period of time.

Recommendation/Instructions:

The software in the transfer case control module has a "shift abuse lock out" mode to protect the module and encoder motor. This concern will occur if the module senses repeated shift requests in a short period of time. If the driver continuously tries to shift in and out of different modes, the transfer case control module will stop trying to shift for several seconds to prevent abuse damage to occur. Cycling the ignition off for 30 seconds or more should allow the transfer case to resume normal operation.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

 

[JETMECH]

Any resolution to your problem... My 2002 Yukon XL is doing the same thing... I replaced the neutral safety switch with no help.

 

[kpers]

No, the issue still persists.

 

[JETMECH]

have you tried replacing the encoder motor already?

 

[kpers]

I don't even know what that is. What is it, where is it and what does it do?

 

[JETMECH]

The encoder motor assembly ($110) is what actually shifts the transfer case and tells the TCCM (transfer case control module ($150)) what position it is in. Based on the symptoms you and I are having, I still think it has something to do with not sensing the transmission is in neutral rather than an actual transfer case issue.