Ls swap 4lo issues

Discussion in 'Transfer Cases' started by sect8dr, Jan 13, 2020 at 12:26 PM.

  1. sect8dr

    sect8dr Newbie

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    Lm7 swapped into a full-size 94 blazer. Running and driving perfect but goes into a sort of limp mode when I put it in low range. Didn't find much in a Google search so I thought I would ask the experts here. Appreciate any info

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  2. Mark Gilbert

    Mark Gilbert OBS Enthusiast

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    How much of the LM7 drivetrain did you swap over. The PCM will need a signal to know it is in low range because the output shaft speed will be much lower than expected in 4lo. To the PCM this would look like slippage and trigger limp mode.

    You didn't mention if you have a 2002 and older or 2003 and newer. I don't know the exact years the LM7was made but according to wikipedia it crossed over.

    It doesn't really matter much. If you have the 2002 and older the C2 connector on the PCM will be red and if the 2003 and later it will be green, but for either it will be pin 16 that should receive a signal to inform the PCM that it is in low range.
     
  3. sect8dr

    sect8dr Newbie

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    Sorry about that, it's an 02. 4l60e is 94 and the 241 is a drivers drop with a slip yoke and a 2 wire vss. Thanks for the info.

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  4. Mark Gilbert

    Mark Gilbert OBS Enthusiast

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  5. RDF1

    RDF1 I'm Awesome

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    The PCM needs to have the correct tune in it also.
    Do you have tuning software?
     
  6. sect8dr

    sect8dr Newbie

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    Had a friend tune it. Runs perfect in high range all the way to 6k. Wiring up a switch to ground that pin tomorrow. Thanks for the help

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  7. Supercharged111

    Supercharged111 I'm Awesome

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    If the PCM came out of a 2wd grounding the pin isn't the answer.
     
  8. sect8dr

    sect8dr Newbie

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    Pretty sure it's a 4x4 0411 and tune. Not positive though. Will be calling him today. If it is a 2wd pcm, what would need to be done at that point? The trans shifts fine in high range 2wd and 4 high so I am assuming the pcm is getting a signal from the vss but I could be mistaken.

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  9. RDF1

    RDF1 I'm Awesome

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    So on a 2wd tune the 4LO tables aren't in the tune so wiring up a switch does nothing.

    But what you can try is copy all the normal shifting parameters over to the Performance section and then divide by 2.73 or whatever the Xcase low range ratio is. In theory that should work but might have to play with the settings some since its going to rev really fast.
    Then set the Performance setting (Tow/haul) to one switch.

    Ive always wanted to try this but haven't..

    The D0963, D0964, D0965 and all other 4lo parameters arent in a 2wd tune.

    th.JPG

    ths.JPG
     
  10. Mark Gilbert

    Mark Gilbert OBS Enthusiast

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    Also keep in mind the PCM isn't any different from 2wd to 4wd. The programming is.

    I'm not sure what is different in the tune specifically, but in a worst case scenario you can write an entire tune from a 4wd, and in a best case scenario you could change the options in the system segment, or perform a segment swap. I am not sure which and would have to go through the tunes on my other computer to find out.

    Depending on the software your buddy is using it should be pretty easy to find a tune to copy from.
     

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