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I have a 1999 GMC Jimmy/Envoy/Blazer SUV with the NP8 (all electronic) 4WD transfer case.   After a dash warning to “Service 4WD” came on and the vehicle would not shift out of the default position of 2HI, I attempted my own repairs.

CURRENT STATUS: We are able to shift into 4HI, AWD or 4LO - but can drive only for 4-5 secs. before an audible relay click is heard in TCCM and MIL SVC 4WD light returns - with system freezing in 4WD.

MY DIAGNOSIS: I suspect that I have a failed TCCM (transfer case control module) and that both my original TCCM and a salvage unit I installed are bad.   Would you please send me a quick reply so I can follow with a summary of the steps I have taken to date?   After reading my summary of actions taken, you may be better equipped to confirm my diagnosis or suggest if I should look at something else before proceeding to obtain another TCCM?

Gratefully, Jim
Have you had this vehicle scanned for trouble code regarding this problem?
Customer: replied 9 years ago.
Reply to carhelp4uhere's Post: 1.     Checked fluid level in transfer case and sampled for any metallic dust/debris (no evidence of metallic failure in transfer case- including sound).
2.     Attempted to pull DTCs with a scanner (0 codes).
3.     Pulled Trsfr case contrl module (TCCM), checked contacts, pins and harnesses for any corrosion, damage, misalignment, etc.
4.     Reconnected TCCM and tried to pull code again (0 codes) MIL light still on.
5.     Replaced encoder motor (attached to trsfr case) w/remanufactured unit.
6.     Pulled dash mounted 4-position electronic switch and reconnected.
7.     Obtained used TCCM from salvage yard and switched out with original.
8.     Checked for DTC’s (0 codes)
9.     Did “erase/clear codes” process with scanner.
10.     Powered up vehicle and test drove. STATUS: Service 4WD MIL light gone but attempts to shift to 4WD unsuccessful would not shift into any other position but remained in default (2HI).
11.     Checked for DTC’s again (0 codes).
12.     Switched back to orig. TCCM and attempted to pull DTCs (0 codes).
13.     Test-drove: STATUS: MIL light (SVC 4WD) went away and vehicle shifted properly into all 4WD positions BUT would only run for 4-5secs. Before audible click was heard in TCCM and MIL SVC 4WD light came back on.   AND vehicle remains stuck in current 4WD position until ignition is turned completely off and dash switch is reset immediately after starting engine.   Once back in 2HI, MIL SVC 4WD light disappears. Still no DTCs.
14.      Switched back to used TCCM from original (but this time with vehicle stuck in 4HI) – no MIL SVC 4WD light – dash switch still reporting in 4HI – no DTCs – BUT cannot switch out of 4HI into any other position – even after killing ignition and restart.    
15.     Switched back to orig. TCCM with same results as original condition.
Ok, but my concern is that a problem like this will set a trouble code. The main thing to make certain of would be that the scan tool you are using has the ability to determine body failure codes. This scan tool would be one of the more expensive units. Simple obd2 scan tools cannot access a trouble code of this nature. In my experience this problem you are having would be more likely caused by a faulty encoder motor than the TCCM.
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Customer: replied 9 years ago.
I am using a $50 OBDII PocketScan by Actron and its manual references only PXXXX codes and not BXXXX codes, though it does show some PXXX codes that are transfer case control system related in the reference info. According to a tech svc advisory from, an incommunicado TCCM was the only one of two reasons to replace the TCCM - the other would have been had it given a DTC C0550.   But I have not been able to get any codes. But I also had replaced the encoder motor with a remanufactured one, just to try and hit the two most common points of failure (the TCCM AND the encoder motor).   If you think I can still get codes with a better scanner, I will make that my next step.   Thought it odd that I had a SVC 4WD light but no codes.

Thank you again! Jim
Well honeslty yes I would get the scan done. This could help you properly diagnose this system and help with the parts that are not fixing it. This could even be the dash switch, and that is where the trouble code will further assist you. If and when you get the code, I can further assist you. It to me does sound as though the position switch inside the encoder is faulty.
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