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Correcting speed in a conversion. Or is it even needed?


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Barra/zf6 in a patrol. Seems to drive ok, but what i notice is when i scan with forscan the TCM/OBD speed reads about 20% low, and the PCM speed reads about 10% high. I have tried modifying the diff ratio and rev per mile settings in VID and these seem to change nothing. I played around with the tyre size figure in VID and what i noticed is when i lower it i can make the speedo more incorrect in PCM only, but when i raise it to try and correct it nothing changes. How do i get these values correct?

I know from my LS experience that the 6l80 trans needs to read speed correctly otherwise they dont drive very well, i assume the zf6 would be the same? Or am i wasting my time here on a non problem.

FG2 ecu/tcm that was flashed to FG1 and return to stock file made/flashed

Edited by mothertruckercustoms
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  • 2 weeks later...

Change the axle ratio in the VID block if the VID block is enabled, if its not enabled use the VID block PCM overrides.

image.png.bfff4f073ede64fcab515b93d681ac37.png

If you have a ZF also change the ZF diff ratio in ZF03985 (there are two values in case you had AWD).

image.png.30cb93bc4501e6dcaace7c4d84dae570.png

There is also a correction factor here which should not need to be changed but can be in some circumstances.

ID    Name    Value
ZF00132    correct wheel speed diference    0.150024
ZF04252    Correction factor wheel circumference, ratio of differential    1

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i played around a little more with this, increasing the maximums i was about to get the speed right via pcm

the tcm i had already changed the diff ratio to 3.9, the other two values are not present, namely 

ID    Name    Value
ZF00132    correct wheel speed diference    0.150024
ZF04252    Correction factor wheel circumference, ratio of differential    1

I have pro edition, are these workshop edition only or something?

I can get the ZF reading right speed by fudging the diff ratio until its right, would this be adequate without access to the other values?

Does it even affect how the zf behaves?

 

 

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