4WD acting odd - intermittent service light

Discussion in 'Engine & Drivetrain' started by cstern71, Jan 6, 2017.

  1. cstern71

    cstern71 Well-Known Member

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    Hard to summarize my issues in an easy title. 2003 Trailblazer LT.

    I've been having a few issues with my 4wd this winter. When it snows, I'll put it in 4hi to leave my neighborhood. I go back to 2hi when I'm on dry pavement (highways etc.). When I get off the highway, I often have to go back to 4hi because of unplowed roads. This is when I get a service 4wd light, but it will go into 4hi. It will switch back to 2hi, but the service 4wd light stays on. After I shut the truck off and turn it back on, the light is gone.

    Yesterday, I switched to 4hi going up a hill near my house heading home. I got the service 4wd light and I kept slipping. It finally engaged and I got traction, only it seemed very sluggish. Turns out it went into 4lo with the switch on 4hi. The switch was physically on hi but light was over lo. Fortunately I only had about a block to drive in snow until my driveway. I tried putting it back in 2hi, but I didn't know it needed to be in neutral. I ended up leaving the switch on 2hi and putting it in park shutting it off and going in to research the issue. I went back out and turned my key to on, the truck lurched and 2hi lit up. This morning, I put it in 4hi, it went, and I left the driveway. It switched to 2hi when I got to dry pavement with no service light. I did have a prior instance where it was stuck in 4hi and I shut it off with it set to 2hi. Corrected itself when I turned it back on later.

    Sorry, for the long story, but I can't find anything similar to my problem. I'm leaning toward encoder motor or tccm. I suppose it could be the switch as well. Just odd it went into 4lo without being commanded. When I was trying to switch it out, the flashing light followed the selector, so it would seem the contacts are ok. I didn't think it could go into 4lo without being in neutral either. I would also conclude the sector light is not tied to the switch position. It must be getting input from somewhere else. My thought is encoder motor is going out.

    Any ideas guys? Thanks
     
  2. Sparky

    Sparky Moderator

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    If it changes to a mode not selected, that's usually an indication of a switch issue.
     
  3. cstern71

    cstern71 Well-Known Member

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    It is the cheapest option to start with. I wasn't sure about the other issues where it would still work but give me the service 4wd light.
     
  4. Sparky

    Sparky Moderator

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    How many miles are on the truck?

    How long since the transfer case fluid was changed? Should be done every 50k with GM Autotrak II fluid.

    Another thing to check is for slop where the passenger side CV axles goes into the disconnect on the side of the oil pan. If there is more than a slight wiggle, the disconnect is worn. If there is delayed engagement of anything in the system, it could possibly throw a light because it didn't activate within the time spec. Just another thought.
     
  5. cstern71

    cstern71 Well-Known Member

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    161,000 miles. AutoTrak II fluid changed at 127,000 when I took possession of it. I replaced the 4wd disconnect and actuator last year with one of the "updated" ones on ebay while I was chasing a rattle noise around. That wasn't what was causing it, but the original did have slop in it. Fairly certain the disconnect is working because the 4wd system is functioning when it engages.
     
  6. Sparky

    Sparky Moderator

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    Sounds good then, just covering bases.

    The mis-selection of the switch makes me suspect it a bit. It also wouldn't hurt to check your transfer case fluid level. Mine developed a leak around the slip yoke when I had a U joint go bad which destroyed the seal, and it drained my tcase. Thankfully the tcase was fine.
     
  7. Mooseman

    Mooseman Moderator

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    Here's a write up on testing the 4wd system:
    4WD not working? Step inside.

    Even though yours seems to sometimes work, maybe going through the steps might help pinpoint the problem.
     
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  8. djthumper

    djthumper Administrator

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    Admin Post
    You can go to our sister site ORTB when it comes back up. We have a full troubleshooting guide on it there.
     
  9. Chickenhawk

    Chickenhawk Well-Known Member

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    I suspect you have two problems at the same time. Bad switch and bad encoder motor. The bad switch is usually the cause of uncommanded modes. The bad encoder is probably because it is slow in shifting modes. If it takes too long to shift, it illuminates the service light, but in some cases it does shift eventually, but too slow and this is why you get the service light even when it does shift eventually.
     
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  10. Wooluf1952

    Wooluf1952 Well-Known Member

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    IIRC, the early selector switches would default to 2LO if there was a problem. Not a great system, but, it was changed in later years. Did you try flipping the switch back and forth a few dozen times (with the key in the OFF position) to clean the contacts?
     
    smt 59 likes this.
  11. smt 59

    smt 59 Well-Known Member

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    For sure the switch, my experience!
     
  12. Chickenhawk

    Chickenhawk Well-Known Member

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    Yes, switch first. Inexpensive and installs in seconds.

    Don't get fancy and start pulling the dash. It can be gently pried out from the front with a screwdriver if you're careful. I went to NAPA and they showed me the two switches they carry. I chose the slightly more expensive one, because it seemed obviously better quality. Never had a problem since.
     
  13. cstern71

    cstern71 Well-Known Member

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    Thanks for the help guys. I stopped receiving emails when you guys replied so i thought my thread went dormant.

    I was thinking along the same lines. Try the switch first for ~$30. That does make sense about the encoder motor though and the service light.

    I think I'll just replace the switch through the front of the dash as you suggested. Might just order an encoder motor too. I have very few dollars of repairs in this thing after 3 years so it won't offend me any to do it.
     
  14. jimbrown87

    jimbrown87 Member

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    I was having the same troubles and as stated,I would start with the switch...I did as a matter of fact. It was the tccm (transfer case control module that fixed it. New dash switch,no luck. Transfer case fluid no luck. Used tccm, 4wd anytime I want/need it. I hope this helps in some way, good luck! It sucks to have a 4x4 and not be able to use it reliably lol.

    That is the write up that helped me fix mine recently!

    Mod edit: Posts merged. Please do not double post.
     
    Last edited by a moderator: Mar 24, 2017 at 7:03 PM

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