peterbilt 379 dip switch settings help

Discussion in 'Trucks [ Eighteen Wheelers ]' started by prerunner404, Jun 2, 2016.

  1. prerunner404

    prerunner404 Medium Load Member

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    Aug 3, 2011
    lake havasu city az
    0
    Does any one on here have a 379 with 3.70 rears 22.5 lp tires 10 speed trans and a 12.7 detroit? If so could you get me setting on your speedo. my truck came with tall 24.5 and the previous owner switched the truck to lp 22.5 and now im off 5mph on the speedo. So i have to drive 5mph slower or faster depending on how fast i need to go. Thanks for your help fellas.
     
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  3. shovel98

    shovel98 Light Load Member

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    Jan 15, 2011
    mn
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    what is tire size do you have as in hgt in inches need that number to figure what dip switch settings
     
  4. shovel98

    shovel98 Light Load Member

    251
    241
    Jan 15, 2011
    mn
    0
    I used a 295 tire size which is about 39 inches tall. try setting dip switches 9 and 10 to the on position and rest of the switches to the off position that should get you close. Its a little trial and error to get it exact but I have my speedo dead on.
     
  5. prerunner404

    prerunner404 Medium Load Member

    452
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    Aug 3, 2011
    lake havasu city az
    0
    Ya there 295/22.5 tires. I'll check those out
     
  6. prerunner404

    prerunner404 Medium Load Member

    452
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    Aug 3, 2011
    lake havasu city az
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  7. North Idaho

    North Idaho Bobtail Member

    6
    2
    Feb 18, 2020
    0
    Hello Shovel98,

    Hope you can assist with this issue I have. Took it to a shop to request a speedo adjustment, but not sure if it was executed properly.

    Vehicle -
    1999 Peterbilt 379
    ISX 565
    Eaton 18 Speed
    Tires - Toyo M506Z Off Road, Size - 11R24.5, Inflated Dia 44.3, Rear End Ratio 3.58, 469 Revs per mile

    The mechanic didn't record the original settings when I brought it in. Not sure why, but oh well. Here we are. It was showing on the gauge about 10 mph slow at 55 mph, along with the odometer being off as well.

    These are the switch set points tried by the mechanic, not sure how he got to these setting numbers. I believe he used a formula.
    1st try - 3 6 9 10 speedo was very slow. gauge showed 25 mph, gps was 55 mph
    2nd try - 1 2 4 5 7 8, still way off
    3rd try - 2 4 6 this setting actually placed it very very close. MPH on gauge and gps were almost exact, if not right on. It worked and I was happy with it.

    BUT........., when I left and got on the road, it resorted to being real slow again.

    Any suggestions?

    The mechanic mentioned to check the speed sensor on the trans.

    Any input would be great.

    Thank you.
     
  8. North Idaho

    North Idaho Bobtail Member

    6
    2
    Feb 18, 2020
    0
    Can anyone assist with this post?
    Peterbilt Speedo Dip Switch Settings

    Took it to a shop to request a speedo adjustment, but not sure if it was executed properly.

    Vehicle -
    1999 Peterbilt 379
    ISX 565
    Eaton 18 Speed
    Tires - Toyo M506Z Off Road, Size - 11R24.5, Inflated Dia 44.3, Rear End Ratio 3.58, 469 Revs per mile

    The mechanic didn't record the original settings when I brought it in. Not sure why, but oh well. Here we are. It was showing on the gauge about 10 mph slow at 55 mph, along with the odometer being off as well.

    These are the switch set points tried by the mechanic, not sure how he got to these setting numbers. I believe he used a formula.
    1st try - 3 6 9 10 speedo was very slow. gauge showed 25 mph, gps was 55 mph
    2nd try - 1 2 4 5 7 8, still way off
    3rd try - 2 4 6 this setting actually placed it very very close. MPH on gauge and gps were almost exact, if not right on. It worked and I was happy with it.

    BUT........., when I left and got on the road, it resorted to being real slow again.

    Any suggestions?

    The mechanic mentioned to check the speed sensor on the trans.

    Any input would be great.

    Thank you.
     
  9. 062

    062 Road Train Member

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    blairandgretchen Thanks this.
  10. North Idaho

    North Idaho Bobtail Member

    6
    2
    Feb 18, 2020
    0
    blairandgretchen and 062 Thank this.
  11. blairandgretchen

    blairandgretchen Road Train Member

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    60,362
    Dec 9, 2011
    South west Missouri
    0
    Dip switch settings on a 379

    Go here - page 2 and 3 - the charts listed by @mtoo

    I started about half way on the charts, and left dash open. Drive an exit, check against GPS.

    Move up or down the list about 7 or 8 steps each time, and you’ll get it.

    Took about 7 tries, but I have mine to within a half of a percent error.
     
    SAR, pushbroom, spsauerland and 2 others Thank this.
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