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03 Yukon Service 4WD now sometimes no reverse


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My wife's '03 Yukon (VIN:1gkek13z03j140424) with 180k miles has had the 'Service 4WD' message for about 6 months now. When it originally appeared, I pulled codes CO306, CO327, CO379, and BO790. The issue the truck had then was only that it would be in a random t-case gear after startup and sometimes we couldn't shift out of it. Not usually an issue except when stuck in 4LO. I traced the issue then to a broken ground wire on the frame under the drivers door. I soldered in a new wire, cleaned the frame connections and applied dielectric grease. That solved the problem for about 3 months. Then later last winter, it started getting stuck in 4 and 4LO with what felt like the front end fully locked in. Later it became a clunking sound as if the t-case or front-end wasn't fully engaging. We've kept the truck in 2HI since then and all has been fine. I was planning on digging into it once bug season dies down since I work outside on a gravel driveway lol

 

Then today we started hearing what sounded like an electric motor/fan revving while we drove at 60 mph. The noise had no effect on power to the wheels and didn't seem to be effected by engine speed or wheel speed -- the truck seemed to operate just fine when it was happening. The first time it happened, we thought a motorcycle was passing us -- that's how loud it was. The noise lasted only a minute or so as we drove and then was gone. We pulled over to check things out and visually all looked ok. I then put the truck in R to back out of our parking spot and could hear the loud motor sound. It seemed to be an actuator on the t-case or transmission. Shifting back into P stopped the sound and shifting back into R brought the noise back. The truck also had no reverse like the transmission was in neutral and not able to shift into R. All forward gears worked fine. I pushed the truck out of the parking spot and we drove home without incident. After the truck sat for an hour, all gears including R work just fine with no noise. That seems to tell me the issue is electric and not mechanical but what do I know. Could it be the transfer case was shifting into N when we tried to shift the transmission to R? I'm just thinking a ground issue could cause weird behavior across all these things. And I can't think of what could make that revving sound while we're cruising down the road...

 

So my questions:

- Any thoughts or recommended diagnostic steps on what could cause the revving sound when shifting into R? And why would this noise happen when cruising along at 60mph in D?

- Any thoughts or recommended diagnostic steps on the Service 4WD message and diagnosing the t-case actuator, TCCM or maybe even the front diff actuator? I'd like to pin all of these (t-case actuator, TCCM, and front actuator) out to see if I could have another ground or wiring issue. Anyone have diagrams?

 

I have two of these trucks. The other is an '06 Sierra. They both have had similar t-case shifting issues. The funny thing is that the '06 Sierra only has the issue when temps are below 0F.

 

Thanks in advance for any help

Edited by Marc Ohmann
Fixed pulled codes
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I'll plan to follow these steps to check the signals coming out of the TCCM at the t-case actuator but something similar for the front axel actuator would help as well.

 

Quote

 

Disconnect the electrical connector at the transfer case. Using a digital voltmeter set to the 12-volt scale, with the Key in the RUN position check for the 5-volt reference signal on the light green/black wire. Check the black/white wire for good ground. If both check out, reinstall the electrical connector to the transfer case motor.

Next, backprobe the brown/white signal feedback wire while you operate the 4WD switch. Look for these values:

 

When the switch is set to 2 Hi, you should see 2.7 volts

When the switch is set to AUTO, you should see 3.4 volts

When the switch is set to 4 Hi, you should see 3.9 volts

When the switch is set to 4 LO, you should see 0.72.7 volts

If you don’t see those values, check the entire wiring harness back to the TCCM located under the dash behind the steering column. If the harness checks out, then replace the transfer case motor.

 

 

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ok, found this pinout for the front actuator also. That should be an easy test tomorrow. That'll help with the 'Service 4WD' message but still no idea how we could temporarily lose reverse...

2022-06-20_21-05-41.png

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I got a chance to test the front actuator and it shifted just fine when adding/removing ground from the signal terminal. I also attempted to test the t-case encoder but the directions I shared above don't seem to match up color-wise to my harness. I did get 1ohm to ground as the lowest reading on any pin so I'm assuming that's the ground. It was the second pin to the right when looking at the truck connector with the clip oriented down. In the same orientation, I had a constant 12vdc on the second pin from the left in the top row. So assuming that is the ground and not some short, 1 ohm seems good to me. 

 

Now I need a good way to test the TCCM and I still have no idea how we could temporarily lose reverse the other day. I did have the TREC fuse out to keep the truck from randomly shifting the t-case when we lost R the other day. The fuse had been out for days though and we still didn't have reverse even after I replaced it. Any ideas?

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Still can't find a good pinout for the encoder plug but I pulled some voltages with the key in run position. Following the same orientation with the clip pointed down, I got 5vdc signal on the lower left pin, 1.7v on the top-left pint, 1.3v on the lower-right pin, and the same 12vdc on the second pin from the left in the top row. 

 

I have an OBDII MX+ reader. From what I see, it doesn't look like I can get PID (is that the right term) values from the encoder. Do I need a Tech 2 to test the TCCM or is there someway I can do it with my reader?

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So ordered a new encoder ring. Seems worth a try at $80. Right now I'm guessing the t-case was shifting into neutral and that's how we lost reverse but still no idea if that's realistic or why it would happen. Again all forward gears worked fine and all gears have worked fine for days now.

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Posted (edited)

I got the encoder ring installed today and we also still haven't seen the issue with no reverse appear again. I'm now wondering if it was somehow related to me having the TREC fuse out. The problem persisted even after replacing the fuse but has been gone after every restart since replacing the fuse. So hopefully the computer just needed a shutdown/restart reset with the fuse in and all is good...

 

The encoder ring was a pretty easy job even on the gravel driveway without a lift. It only took two hours and that was with stopping regularly to swat/cuss mosquitoes. The install was a partial success as it got rid of the 'Service 4WD' message, the t-case shifts into any gear we choose, and so far it hasn't randomly chosen a new gear after startup. But we still have a problem with the front end clunking when in 4HI or 4LO under power and when turning either direction. I did test that front encoder and it shifts but it seems for some reason it's maybe not engaging all the way. I'll jack up the front end tomorrow to check the CVs and hubs. Any ideas or other tests I can do?

Edited by Marc Ohmann
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I couldn't wait so I went out and checked the frontend tonight. The clunking sounded too much like front hubs and with almost 200k on the originals, they've probably seen their best days. Anyhow I got just a little play in them when wiggling the wheel so I ordered a pair of Timkens. Hopefully that gets her back to working 4wd.

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  • 2 weeks later...

So replaced the front hubs and that took care of one clunk but now I can hear another clunk from the tcase area. I'm starting a new thread for that issue to be more on topic. 

 

I'm considering this issue resolved. I think pulling the TREC fuse somehow screwed up our ability to shift into reverse. It hasn't happened again since I replaced the fuse and the Service 4wd message was fixed by the new encoder ring. 

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  • 1 month later...

Got a chance to tear into the tcase today. Chain is trashed so that must be the other clunk. Oil pump wore a hole through the back half so she didn't have much chance with no oil in her. If you own one with the NP246, do yourself a favor and tear into it to add a BRNY or similar while the repair is still cheap.

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  • 2 weeks later...

Popular thread I have here I guess lol

 

Anyhow, I got the tcase back in today. The rain delayed things a bit. Overall it went pretty well considering I'm working out in the elements. The planetary was trashed so I'm guessing that was the loud motorcycle like noise we heard while driving and I'm guessing was also the reason for no reverse. Best I can figure, losing all the oil trashed clutch housing bearing which created enough driveline slop to mess with the planetary. It was an expensive rebuild with the new planetary, new pump, and new rear case half but at least I know what parts went into it and how the work was done. I also upgraded to the 6 planet planetary since it added minimally to the build cost. And for anyone keeping score at home, I did buy the J-44295 to set the clutch shim and after 200k miles, the original clutch pack was still in spec with the original shim -- even after being pulled apart and cleaned. Wishing I would have ordered new case bolts but I'm just gonna coat the outside of these with some lanolin. Also wish the rebuild kit came with all new clips. Seems pointless to risk a whole rebuild on one previously stressed clip failing. 

 

I'll be pulling the 246 on my '06 next to put a BRNY in before the rear case gives out. That one still holds oil for now.

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