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Discussion Starter · #1 ·
i'm fighting the last part of my tune -- the speedometer scan tool scalar. i've calibrated with tunercat's built in speed calibration

it's an M6. 3.42 gears (confirmed by marking/turning driveshaft/wheels). tires are about a half inch taller than stock now, but i calibrated with stock tire sizes. i dont care about small speedo innacuracies at all.

my speedo reads correctly, or close enough, judging by usual traffic speeds im sticking with.

pcm speed is grossly innacurate, though.

for example, i did a datalog on the way to work today. cruising @ 50kph in the city, datamaster reads 14-16kph. smoking the tires on a back road, where the speedo touched 100kph, it reads 32kph.

now, my speedo scalar is @ 45.258 divisor. some rough calculations based on the formula in the manual (tire rev per mile x rear end ratio / 60) yeild a value pretty damn close to that.

so why is it so damn far out?

i know i can just screw with the divisor until it's closer.. but that's a huge pain in the ass

one other side question; i thought for sure there was a constant in the bin somewhere, where you can disable fan turn-on over a certain speed. now i can't find it..
 

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Never really understood all the fan controls.. Look at Tunercat - constants - system parameters - "Fan Threshold High Speed MPH" stock is set at 45 Mph There are also a couple of Hi speed and low speed thresholds that are confusing. Never really played with them, usually by the time you hit 45 MPH the air dam is doing the cooling and the fans s/b going off anyway.

As far as speedometer readings. The dash gauge is run from a single wire from the PCM that outputs 4K pulses per mile , the gauge converts the 4K pulse signal to the analog dial reading. However the scan speed data is a 8 bit field, actually byte 58 in a 60 byte block (module 0, word 58) in a data stream sent from the PCM to the data log requestor. It's the actual speed reading from the PCM, one input to calculation is the speedometer scalar scan tool field. The formula is (tire revs per mile) X (VSS pulses per mile) / 60 . You number in the low 40's s/b OK . So I am confused on why that output is incorrect. Either the PCM calculated it wrong or your scan tool is wrong, if using Datamaster then it's most likely the PCM calculation. When you use TC I assume all data is inputted in SAE (US) format not metric so it should all read out in US format and then is converted to metric by the software. Datamaster displays in MPH or KPH and I assume that's what you are using. I'm about your PCM code , are you running '95 M6 code or A4 code that was tuned for a M6, was the '97 body A4 or M6 before the conversion . Grasping at straws here..as the solution is not that obvious. Just for drill if you change the scalar does to scan speed reading change. In the end you just may do it that way. It may be a PITA to keep tract of every time you tune , however I'm thinking a true M6 code load may be in the future elimination process.
 

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Discussion Starter · #3 ·
dont worry im used to metric/imperial and i go back and forth in my head pretty easily (at least enough to know if im using the wrong units..)

im getting strange behavior from fan vs vehicle speed and iac vs vehicle speed functions, my rpm is wacky on decel.. i'm pretty sure the pcm using that 'scan tool' value..

on that note i'm considering just zeroing the throttle follower iac offset vs. mph table.. has anyone done that? it seems fairly useless to raise idle while moving, it's not like any other standard i've driven with a carb ever stalled on me..
 
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