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Thread: diff ratio change

  1. #11
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    Quote Originally Posted by Slunnie View Post
    The SLABS generates and sends a speed signal that is taken from the 4 ABS sensors. When you change the diff ratio, the driveline speed stops matching what it expects to see at the axle because of the diff change. If you use a speedo correction unit on the speed output signal at the SLABS computer it will correct this. BUT when you are doing this, you're not correcting the speed of the car, you are correcting what it should see because of the diff ratio change - this is important. So, to gain cruise control back again, correct from 3.54 diffs to 4.12 diffs. If you're running 285/75-16 (33's) then your speedo will be accurate. To do this you will need to use a pretty specific speedo correction device that needs to pass through the baseline diagnostic frequency, which most do not do. See this article I wrote about 17 years ago for fitting and a SCU that works.

    Truspeed speedo correction installation - www.slunnie.com
    This was an interesting read just to clarify as it says you had an auto did you have to do this to get everything working like crues control ect? or was it just to correct speedo?

    many thanks

  2. #12
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    It didn't work with 4.12 and 33s the. When I changed to standard ratio it worked again the cruise control
    Quote Originally Posted by shack View Post
    It's quite possible there was an issue with the actual cruise control itself, bigger tyres cannot make any difference to the speed the abs sensors pick up.
    03 D2a Manual Td5 Modified, Ashcroft lockers F&R, Ashcroft R380,Atb lt230, Ashcroft Rear axles, AS manual IC, AS exhuast manifold, AS Coolant bottle, 3inch exhaust, alive GTB550 VGT, billies 7100s, 3inch lift, 4inch staino snorkel, 35s beadlocked

  3. #13
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    Quote Originally Posted by Slunnie View Post
    The SLABS generates and sends a speed signal that is taken from the 4 ABS sensors. When you change the diff ratio, the driveline speed stops matching what it expects to see at the axle because of the diff change. If you use a speedo correction unit on the speed output signal at the SLABS computer it will correct this. BUT when you are doing this, you're not correcting the speed of the car, you are correcting what it should see because of the diff ratio change - this is important. So, to gain cruise control back again, correct from 3.54 diffs to 4.12 diffs. If you're running 285/75-16 (33's) then your speedo will be accurate. To do this you will need to use a pretty specific speedo correction device that needs to pass through the baseline diagnostic frequency, which most do not do. See this article I wrote about 17 years ago for fitting and a SCU that works.

    Truspeed speedo correction installation - www.slunnie.com
    I think the HunterDi one is the only speedo corrector easily available now that actually works on the D2 as it allows that base frequency through to the BCU

    Relatively pricey but worth it. The only thing that has slightly altered is the lockup speed in 4th, it's now 85km/h instead of closer to an indicated 80 with stock rubber, both with a stock map.
    3rd is as low as 60 now in the right situations, and HDC is spot on at 7km/h

  4. #14
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    Quote Originally Posted by dieselcarb View Post
    This was an interesting read just to clarify as it says you had an auto did you have to do this to get everything working like crues control ect? or was it just to correct speedo?

    many thanks
    The auto doesn’t have the CC problem, you can do whatever you like and everything will continue to work, although as Rick130 says, the torque converter lockup points may shift - that was my experience also and I believe this to be correct, although others think there is no electrical connection between the auto controller and the speed output from the SLABS. So, on the auto, its really about correcting the speedo to the tyre size, eg 29’s to 33’s.

    The manual, if you want the CC to work, you need to correct the speedo to suit the driveline speedo. Eg 3.54 to 4.12 and/or 1.2 to 1.4 Transfer case high range - then everything will work.

    In the manual, if you’re running stock driveline ratios and everything works like the CC but you want to correct the speedo still for bigger tyres and not lose the CC, I understand you can pick up the speedo signal where it goes into the dash and so only the dash is altered. I have not done this and can not personally verify, but I understand this is ok to do - I would verify this with someone who has done it and make sure the dash odometer doesn’t become conflicted with the BCU odometer and start flashing because it thinks it has been tampered with.
    Cheers
    Slunnie


    ~ Discovery II Td5 ~ Discovery 3dr V8 ~ Series IIa 6cyl ute ~ Series II V8 ute ~

  5. #15
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    So I bought the speed kit now from hunter so I just follow your online tutorial and then it's all good? Is it adjustable or just install and leave
    Quote Originally Posted by Slunnie View Post
    The auto doesn’t have the CC problem, you can do whatever you like and everything will continue to work, although as Rick130 says, the torque converter lockup points may shift - that was my experience also and I believe this to be correct, although others think there is no electrical connection between the auto controller and the speed output from the SLABS. So, on the auto, its really about correcting the speedo to the tyre size, eg 29’s to 33’s.

    The manual, if you want the CC to work, you need to correct the speedo to suit the driveline speedo. Eg 3.54 to 4.12 and/or 1.2 to 1.4 Transfer case high range - then everything will work.

    In the manual, if you’re running stock driveline ratios and everything works like the CC but you want to correct the speedo still for bigger tyres and not lose the CC, I understand you can pick up the speedo signal where it goes into the dash and so only the dash is altered. I have not done this and can not personally verify, but I understand this is ok to do - I would verify this with someone who has done it and make sure the dash odometer doesn’t become conflicted with the BCU odometer and start flashing because it thinks it has been tampered with.
    03 D2a Manual Td5 Modified, Ashcroft lockers F&R, Ashcroft R380,Atb lt230, Ashcroft Rear axles, AS manual IC, AS exhuast manifold, AS Coolant bottle, 3inch exhaust, alive GTB550 VGT, billies 7100s, 3inch lift, 4inch staino snorkel, 35s beadlocked

  6. #16
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    Quote Originally Posted by Kaaaiju View Post
    So I bought the speed kit now from hunter so I just follow your online tutorial and then it's all good? Is it adjustable or just install and leave
    I havent installed a Hunter DI unit, but it should be exactly the same and you will need to calibrate it - just use a GPS unit and I've calibrated at 100km/h which reads slightly fast at 60km/h.
    Cheers
    Slunnie


    ~ Discovery II Td5 ~ Discovery 3dr V8 ~ Series IIa 6cyl ute ~ Series II V8 ute ~

  7. #17
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    Quote Originally Posted by Kaaaiju View Post
    So I bought the speed kit now from hunter so I just follow your online tutorial and then it's all good? Is it adjustable or just install and leave
    Quote Originally Posted by Slunnie View Post
    I havent installed a Hunter DI unit, but it should be exactly the same and you will need to calibrate it - just use a GPS unit and I've calibrated at 100km/h which reads slightly fast at 60km/h.
    What Slunnie said.
    It comes with a comprehensive ratio chart to get the correct output ratio for your current tyres.
    I just used the GPS on my phone.

  8. #18
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    Quote Originally Posted by Kaaaiju View Post
    It didn't work with 4.12 and 33s the. When I changed to standard ratio it worked again the cruise control
    Just to clarify, with standard diff ratios it will always work fine irrespective of the wheel size, so 33s are fine, but of course in your case the issue was the 4.12 gearing. That would need correcting through the engine mapping.

  9. #19
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    Yeah it was 4.12 but how can it be mapped out? Some here are saying use the speed correct
    Quote Originally Posted by shack View Post
    Just to clarify, with standard diff ratios it will always work fine irrespective of the wheel size, so 33s are fine, but of course in your case the issue was the 4.12 gearing. That would need correcting through the engine mapping.
    03 D2a Manual Td5 Modified, Ashcroft lockers F&R, Ashcroft R380,Atb lt230, Ashcroft Rear axles, AS manual IC, AS exhuast manifold, AS Coolant bottle, 3inch exhaust, alive GTB550 VGT, billies 7100s, 3inch lift, 4inch staino snorkel, 35s beadlocked

  10. #20
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    Quote Originally Posted by Slunnie View Post
    The auto doesn’t have the CC problem, you can do whatever you like and everything will continue to work, although as Rick130 says, the torque converter lockup points may shift - that was my experience also and I believe this to be correct, although others think there is no electrical connection between the auto controller and the speed output from the SLABS. So, on the auto, its really about correcting the speedo to the tyre size, eg 29’s to 33’s.

    The manual, if you want the CC to work, you need to correct the speedo to suit the driveline speedo. Eg 3.54 to 4.12 and/or 1.2 to 1.4 Transfer case high range - then everything will work.

    In the manual, if you’re running stock driveline ratios and everything works like the CC but you want to correct the speedo still for bigger tyres and not lose the CC, I understand you can pick up the speedo signal where it goes into the dash and so only the dash is altered. I have not done this and can not personally verify, but I understand this is ok to do - I would verify this with someone who has done it and make sure the dash odometer doesn’t become conflicted with the BCU odometer and start flashing because it thinks it has been tampered with.
    thanks slunnie thats perfect top bannana

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