troubleshooting 1996 4wd

Discussion in 'Chevy C/K Truck Forum' started by rwdelling, Jun 27, 2013.

  1. rwdelling

    rwdelling New Member

    Hi all, wanting to get some ideas with this issue. I have a huge advantage in that I have two identical trucks: Two 1996 K1500's (one GMC and One Chevy), both have electric shift 4wd. On the Chevy, the 4wd works perfectly. On the GMC, the indicator switch lamps simply flash erratically and the transfer case motor does not click over. Usually, the 4hi lamp simply flashes continually and sometimes, the 2hi lamp stays on as well. Other times, both lamps flip out and flash back and forth. Regardless of the lamp indicators, the truck stays in 2wd.

    Also, when jumping pin 13 to ground, no DTC codes flash.

    Here is what I know for sure: It is not the TCCM because both TCCM modules work fine in the Chevy. It is not the dash buttons because I swapped those out and it does not change a thing.

    My best guess is that it seems to be some kind of wiring issue or a bad transfer case encoder motor at this point but I am wanting ideas from any of you.

    I also have the complete 2 volume 1996 factory repair manual set for these trucks, but when following the step by step diagnostics, it simply brings me to the "replace TCCM" outcome which is obviously not the issue.



  2. Conlan Rose

    Conlan Rose Epic Member 5+ Years 1000 Posts

    Did you try the Chevy TCCM in the GMC? Also likely the front diff is the problem because if the front does not engage then the TCCM will override the 4wd selection and go to 2wd. Also codes in these trucks only can be pulled up using an OBDII scanner because the old method for the OBDI doesn't work and may damage the ECU.
  3. rwdelling

    rwdelling New Member

    Yes, I tried the Chevy TCCM in the GMC and the same issue continued. Are you saying that grounding pin 13 does not work for diagnostics?
  4. Conlan Rose

    Conlan Rose Epic Member 5+ Years 1000 Posts

    Correct grounding the pin doesn't flash the codes like in the older trucks. Scanners are cheap you can get a Bluetooth adapter for like $12.
  5. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    Wiring and encoder motors are the two most common problems with that system.
    Id start by reading the wires to ensure no bad connections or broken wires are hiding in the harness.
    Then look at the encoder motor, some have been able to clean up the encoder motor and fix bad connections to get things runnings again.
  6. rwdelling

    rwdelling New Member

    Thanks, this confirms what I am suspecting. Last night I was moving the I/P harness and TCCM harness around and the 20A Transfer Case fuse suddenly blew. Certainly seems like something is shorting out in the harness. I will begin the painstaking task of tracing out each circuit beginning with the main power feed circuit from the I/P fuse block to the TCM and the main ground circuit from the TCCM to the I/P grounding bar. According to the schematic there are about 8 factory splices in the harness for these two circuits, so I'll start there. Let me know if I'm on the right track.
  7. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    Yea it sounds like you have the right idea, it certainly isnt an easy task but other than tracing out each individual wire/circuit I dont know of any other way to find the problem connection/connections.
    A friend of mine rewired his system last year and did away with some of the redundant circuitry, Ill have to see if he still has the wire schematic he designed, it still incorporates the system indicator lights but cut the wiring in half and most importantly replaced 17 yr old wire and connections with new stuff.
  8. rwdelling

    rwdelling New Member

    Update: spent the evening troubleshooting and found two issues:

    1. One of the 2 wires that connects the motor feed from the TCCM to the transfer case encoder motor did not have continuity. We ran a temporary long jumper directly from the pin (D16) on the TCCM to the encoder motor and took care of this problem.

    2. Once this electrical problem was fixed, we found another problem: the encoder motor actuates (whirring sound for approx 1 sec), but there is no "clunk". On my chevy (the good truck), there is a whirr and then a clunk when the T case shifts over. More importantly, after the whirr sound and no clunk, the 20amp T Case fuse blows. It does this every time.

    My thought is that the next step is to replace the encoder motor assembly because a bad actuator (motor) would most likely draw higher amps and blow the fuse. I am hoping that it is not an issue with the T case itself being locked up or something. Any thoughts appreciated.
  9. rwdelling

    rwdelling New Member

    For anyone interested, the final problem was the encoder motor was bad. swapped out for a new one and everything works like a charm.

    FINAL SUMMARY: bad encoder motor and/or shorts in the wiring can make the TCCM freak out/flash the indicator lights/etc. without shifting into or out of 4wd.

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