Reward! Unknown Territory - Manual transfer case causing havoc

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Hummer H2 Running Duramax LBZ / Allison 6spd stock. In an attempt to retain the H2's AWD functionality after conversion, I mounted a built AMG NP 242HD behind the Allison. The 242 is 5 position manual transfer case. 2WD, 4PT, 4FT, N, 4LO. The transfer case runs perfect as it should in position 1 - 2H.

Problem:
in position 3 - 4FT. (4FT is open transfer case going to open front and rear differential and is intended to run on dry pavement) very hard shifts in each gear and eventfully throws a code in efilive P0776 - PC solenoid B stuck off and p2723 in Tech2. When DTC is lit what ever gear its in will not change. I have to reset code to get back to gear 1. With each hard shift it seems to rev rpms a bit prior to each shift. If I stay off the gas or going down hill I can create a soft shift but under power it's exceptionally hard.

The ECM and TCM do not know electronically the difference between 2H and 4FT. I have a jumper on the gear indicator switch and can force signal either 2wd or 4wd and the results are the same in any mode. 2WD works great 4FT hard shifts.

I recorded a log in 4FT (attached) Selected 50+ PID, I thought were important but this is not my wheelhouse. I can also record a log on the same course in 2WD for comparison. For this log file, the ECM thinks it has 30" tires when in actuality it has 37's. I will make calibration and record again.

Driving the front wheels via the 242 transfer case is causing the issue- extra torque, windup in the front end, gear ratio problems, ??

At one point, after running in 4FT, I cleared the tap shifts and did a rapid relearn using Tech2. After, I was initially getting hard shifts in low gear in 2wd. (maybe normal with relearn in any situation)

$100 reward for solving and fixing the problem. But fixing can't hurt my other drive modes. 2WD, 4PT, 4LO

[ScarabEpic22]

Can you post the tunes up? Are they 4WD Duramax tunes?

To summarize:
2HI - shifts properly
4PT - does not shift properly
4FT - does not shift properly
N - N/A
4LO - does not shift properly. Engages 2.72:1 low ratio. This will likely require the Allison TCM and/or ECM to know there is a transfer case in there.


Getting the speedo reading properly will help, but I think this is a TQ and/or gear ratio issue. The ECM/TCM need to know there is a transfer case back there so it can adjust shifts accordingly.

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Thanks for reply - keep them coming please


Can you post the tunes up? Are they 4WD Duramax tunes?
To summarize:
2HI - shifts properly
4PT - does not shift properly
4FT - does not shift properly
N - N/A
4LO - does not shift properly. Engages 2.72:1 low ratio. This will likely require the Allison TCM and/or ECM to know there is a transfer case in there.
Getting the speedo reading properly will help, but I think this is a TQ and/or gear ratio issue. The ECM/TCM need to know there is a transfer case back there so it can adjust shifts accordingly.

stock tunes - Donor truck was 4x4 with the NP263 Automatic transfer case. I recently updated to a 3500 LBZ with NP261 manual option. The issue is the same on either ECM tune. From my understanding, I only gained the cluster 4wd indicator light with the 3500 stock tune and it accomplished that. I've attached both.

I'm not aware of a different A40 tune - 263 and 261 would behave exactly the same with regards to torque with the only difference being Push button or manual shift. But maybe your on to something.

2HI - shifts properly
4PT - I need to test on dirt - Will report tomorrow.
4FT - Shifts as described above
N - N/A
4LO - works as should but never had RPM's high enough to shift


I spoke to a transfer case rebuilder, to help identify if mechanical or programing issue. He did not know for certain but did say that it's possible the Allison is sensing something different with regard to torque between 4PT and 4FT. 4PT sends same and equal and locked power to both front and back drive shafts while 4FT is open t-case that can fluctuate from front to back.

The Allison only bolts to the 261 and 263 and as far as I know, has never been mated in an AWD setup. If this were the case then it definitely would have to be handled in the program. A big clue is going to be comparing shift in 4PT.

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Speedo is within 1mph up to 60mph - not sure there after - I didn't look at the 3500 manual tune yet but maybe tire size was already changed.

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Update:
Shifts normal in 4PT on a gravel road. The issue is only in the open AWD 4FT mode. Now what? Can this be solved with a different Tune? If so, how will that affect the other modes? I can generate a signal for any mode I'm in. Can the Allison be programed to function differently on a trigger?

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Problem has been identified. After my dirt road 4PT test, I concluded the issue was mechanical and found the front diff of the donor truck had 373 gears even though the rpo code stated 410. (gears must have been swapped at some point) H2 is 410 which is why is only created an issue on dry pavement when both front and rear were engaged.

[ScarabEpic22]

Thanks for reporting back, glad you got it sorted out!

[THEFERMANATOR]

IF you ever run into a situation where a tire spins in the 4 wheel drive full time position, you will encounter problems again. The TCM needs to see a direct speed signal that is accurate to the the output shaft of the transmission. The AWD cases that GM used were different from the 242 case. GM used the 246 which does not have a differential inside of it, but instead a viscous clutch that engages the front axle. It keeps the output of the transfer case locked directly to the output shaft of the transmission at ALL times except when it is shifted into nuetral. This is IMPORTANT for the system that GM uses of monitoring the speed sensor in the output of the transfer case for the speed signal so it can calculate the gear ratio and know when to shift. The 242 case is different in that it uses a differential in it when in full time 4 wheel drive. Take the front driveshaft out of it in full time, and you go nowhere as the front output of the transfer case will free spin.

What this means is the output of the transfer case is not locked to the output of the transmission, and the speed sensor signal is not accurate for calculating the gear ratio. So if you ever run it in 4 whell full time off roading, you WILL encounter this problem again and again because when one tire spins, the transfer case diff will allow the spinning tire to turn faster, and increase the speed of the driveshaft to that tire throwing the gear ratio reading off and tripping the codes again. You need to either install the VSS before the transfer case and do away with the 4 low signal to the ECM/TCM, and deal with it reading incorrectly in 4 low. Go with a GM style AWD transfer case and TCCM that engages the front axle as needed and doesn't have a diff in it. Or go with a non full time transfer case if you don't want to have future problems.