Transfer Case not engaging 4WD - '00 Yukon

[denhajm]

Hello,

Discovered recently during a snowy drive that our '00 Yukon XL is not sending power to the front wheels.

What I have checked/verified.

1. Buttons on dash seem to be operating fine--can shift from 2WD into 4-Auto, 4-Hi, 4-Lo and neutral and button lights do what they are supposed to--go solid on selected mode within 2-3 seconds.

2. 4WD fuses in dash and engine bay are good--get 4WD trouble light only when one of these are removed. If both installed nor trouble light. Never have had 4WD trouble light on dash since we owned the car (about 1 year now).

3. Front axle actuator is stroking--pulled it out and watched it. Put screw driver in the hole where actuator goes and manually cycled the shift fork for the front axle a few times. this does engage the front axles as then I cannot turn the front drive shaft by hand (T-case in 2WD). Otherwise, when front axle fork is not engaged, I can turn the front drive shaft by hand.

4. T-case does shift into low but does not ever engage the drive shaft going to the front.

5. With dash button selecting 2WD, I can rotate by hand the drive shaft coming out of the T-case going to the front axle. When I select 4-hi or 4-lo then I cannot turn this by hand because the front axle is engaging and I cant move the vehicle by turning the drive shaft by hand (vehicle on ground/front ramps). However, when I 'trick' the system by removing the front axle actuator from the axle (still connected to wiring harness) and shift into 4-hi or 4-lo I can still turn the drive shaft coming out of the t-case going to the front by hand. The T-case simply is not shifting into 4wd. The output of the T-case to the front drive shaft never is engaged.

6. I can put my hand on the encoder motor (black cylinder) on the transfer case and hear/feel it doing something when selecting a 4-wd mode on the dash.

So what is happening? No error messages on dash. Button panel lights are all indicating that things are shifting fine--bright/solid within 2-3 seconds of pushing a button. I have 4-low as far as low gear, but not the "4" part.

There must be two shift mechanisms here--one for the low/hi gearing and one for the 4wd engagement. How can I test/check for an electrical component issue vs. a failed mechanical issue in the T-case itself?

The T-case is not making any funny sounds during shifting or normal day to day driving.

Fluid in T-case just changed with new to correct level (old stuff was black and tech figured it was original ~137k miles). After this, he said 4WD was working, but I do not believe this as I do not have power to the front--based on above test and trying to get front tires to spin in gravel--only rears spin--no power to front in any 4wd mode.

Thanks for any troubleshooting ideas.

 

[04tahoels]

yukon

Hi if service 4wd is on your dashboard you need someone with a GM tech II scanner or have Aamco transmission scan your truck to check whats going with The 4wd system i had a similar problems like yours and it was Transfer case ECU and the encoder motor Aamco Transmission fixed them for me and now it works perfect for the last 2 years.

 

[denhajm]

The service 4WD light is not on.

 

[big_b_in_PA]

Does the transfer case actually shift to 4low and drive in the lower gear? Or are you assuming it is by feeling the encoder motor? To me it sounds like the encoder motor drive is shot. The only way you could check this is to take encoder motor off and try to move the shaft going into the transfer case manually.

 

[denhajm]

Yes, the transfer case does actually shift into low gear. So I have low gear when driving in 4-low, but no power is going to the front out of the transfer case. But I can drive around and it is obviously in low gear in the transfer case. That part of the transfer case is working fine.

What is not working is the shifting into 4 wheel drive. Whatever is necessary for the transfer case to shift into 4wd and provide power to the front, is not happening.

 

[denhajm]

Bump. Anyone have ideas?

 

[Shookstein]

Did you ever figure this out? My suburban is doing exactly the same thing.

 

[denhajm]

Yes, In the transfer case are a series of clutch plates, much like in a motorcycle (at least the Honda motorcycles I've seen). For whatever reason these clutch plates were extremely worn out.

It was a local shop that diagnosed and fixed my problem. They put the car on a lift, got the wheels above the ground, started the engine and engaged 4WD. To my astonishment the front wheels were turning. But would stop if they had any load put on them. Based on this I had them pull the transfer case and open it up. The clutches were shot, the fluid was contaminated because of this.

They rebuilt the T/C with new plates, put it all back together and $1350 later I have functional 4WD.

They asked me if I drove in 4-high or 4 auto very much--due to the clutch plate wear. I did not, but had purchased the car only 1 year earlier, so I have no idea how the previous owners drove it.

Hope this helps.

 

[Shookstein]

Thanks for the reply. I'm sure mine is the same issue. Was just hoping it was a cheaper fix. Not sure I'm willing to throw that much money into mine though.