98 suburban electronic transfer case won't shift

[Chuck D. Bones]

I have a transfer case problem.
1998 GMC Suburban 5.7L RPO NP8 - 114Kmi

LEDs do not light on 2Hi, auto 4WD, 4Hi or 4Lo. Sometimes neutral LED blinks. Appears to only run in 2Hi, occasionally "slips". "Service 4WD" does not light up (except during instrument panel self test when starting the vehicle).

This all started while on a trip to Santa Barbara last week. Took it to a tranny shop there, they read out the following error code, reset it and it did not recur for them: "CODE#U1026 loss of class 2 com w/auto/trans..." Either there were no transfer case error codes or they were unable to read them.

I read a similar thread on this forum, a GM bulletin was posted indicating a weakness in the stock design. Per the bulletin, I plan on replacing the detent plunger with the teflon coated part. Where exactly is the detent screw located? Is it on the bottom of the transfer case next to the motor/encoder? Would like to know if the motor/encoder is bad before replacing it. Or should I just replace it based on the age of the vehicle?

So far, I have removed the transfer case motor/encoder. Is there a way to test this off of the vehicle? Will the motor run on DC? If so, how much current is required? Alternatively, is it adviseable to hook the cables back up and run it on the vehicle with the motor not driving the transfer case shaft? Are there any other diagnostics I should perform?

Thanks,

-- Chuck

[777stickman]

That code (U1026) indicates a loss of communication on the Class 2 Serial Data system from the ATCM (Automatic Transfer case Control Module). This can be caused by loose connections, bad pins, shorted wires, etc. It can also be caused by a loss of power to the ATCM.

3 fuses supply power to the ATCM. All are in the I/P fuse block.

1. Gauges--Fuse #4 10a (hot in run)
2. 4WD--#24 20a (hot in run)
3. T Case--#2 20a (hot at all times)

REPLACE these fuses, even though they may look good, and post back.

PS: Should have said replace one fuse at a time starting with the T Case then the 4WD and last (though I doubt it) the Guages fuse.

[Chuck D. Bones]

Stickman,
Replacing the transfer case fuse #2 fixed the problem. The old fuse looks good, I'll test it on a milliohmmeter on Wed. I only wish I had replaced the fuse before I removed the shift motor and front driveshaft. Many thanks for shedding light on this problem. I owe you one. If you're ever in the south bay area of Los Angeles, please contact me by replying to this thread, I'll buy you a pitcher or two.

-- Chuck

[777stickman]

Hey Bones: Glad I could help. Replacing that T Case fuse may have just reset the ATCM and cleared the code. Keep an eye on it and let us know if anything else happens.

And thanks for posting back for us all to gather more info.

[v8_ranch]

Any other ideas. Mine is doing the same thing and unfortunately the fuse replacement did not solve it for me. What should I do next? When I turn the key on, my 2wd hi light comes on for about 3-4 secs and then turns off. No other lights at all.

Thanks

['97ventureowner]

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