E38 - Speedo Calibration

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ben_att
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E38 - Speedo Calibration

Post by ben_att »

Hi all,

This may be a silly question, however I was wondering if it's possible to modify the no of pulses that the E38 sends out on X1-57?
I have fitted a VF ECU to a VZ the ECU details are
Main OS 12656930
System 92271525
Speedometer 92271528

Now that I have set the diff and tyre ratio correctly in the ECU, the readout from the ECU seems spot on when measuring to a GPS device or driving along side another car comparing speeds but the cluster is saying 50km when doing 60km.
I have set the PPK in the cluster as low as it allows which is 3000, I did it with the Tech II and also tried out of curiosity to force a lower number into the eeprom directly but it doesn't seem to make a difference.

One thing I've noticed in HP Tuners is that on the VZ OS, there is an extra variable on the speedometer page that is not there on the VF OS called 'PCM Output Calibration' (photo attached).
I was trying to avoid introducing a pulse adjuster between the ECU and dash, but looks like it may be my only option.

Thanks
Attachments
VZ OS 12609099.jpg
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Tre-Cool
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Re: E38 - Speedo Calibration

Post by Tre-Cool »

your using a 6l80 too yeah?

if you read the trans/ecu out togethor it might not be showing u all the options for just the ecu.
ben_att
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Re: E38 - Speedo Calibration

Post by ben_att »

Sorry, that was silly of me to leave that out, yes running the 6l80.

Do you mean take a read of the E38 on its own without the TCM?
I’ve got a read of the E38 on its own, E38 with TCM and another one with E38,TCM & FPCM.
I’ve just had a look and all of them look the same on the speedometer page in HPT.

Maybe I misunderstood what you meant?
In-Tech
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Re: E38 - Speedo Calibration

Post by In-Tech »

Tre-Cool brings up a good point.
I always make the ECM and TCM match on speedo correction because I can control that much. Maybe it doesn't matter except to the ECM or other modules as long as the TCM is correct. It would be nice to know. Also, the ECM has a switch to only listen to the ECM for speed control as well as ignoring the other modules. I dunno, all I know is what has worked for me.
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Tre-Cool
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Re: E38 - Speedo Calibration

Post by Tre-Cool »

yeah i meant seperate read from the controllers. hpt does some wierd shit were options disappear/get added when they are a combined read.

tends to happen in the speedo area since both the trans & ecu have similar table settings.

My experience with auto combo's is get the speedo working correctly/accurate in the auto, then you should be free to play/fudge the output pulse from the ecu. in efilive for instance i always log the vss pids for ecu/tcm. u can tweak the internal pcm calculations by adjusting tire size or ecm pulses per km.

basically 6l80 uses the diff ratio/tyre size-revs per km to work out the speed. ecu then picks this info up, then can manipulate it for internal calculations & or the output pulse.

just gotta play with it.

easy peasy on a dyno :-)
ben_att
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Re: E38 - Speedo Calibration

Post by ben_att »

Tre-Cool wrote: Thu Nov 16, 2023 9:54 pm yeah i meant seperate read from the controllers. hpt does some wierd shit were options disappear/get added when they are a combined read.

tends to happen in the speedo area since both the trans & ecu have similar table settings.

My experience with auto combo's is get the speedo working correctly/accurate in the auto, then you should be free to play/fudge the output pulse from the ecu. in efilive for instance i always log the vss pids for ecu/tcm. u can tweak the internal pcm calculations by adjusting tire size or ecm pulses per km.

basically 6l80 uses the diff ratio/tyre size-revs per km to work out the speed. ecu then picks this info up, then can manipulate it for internal calculations & or the output pulse.

just gotta play with it.

easy peasy on a dyno :-)
I finally got a chance to play with this today. So my starting point today is as I had it working nicely and the VCM speedo was correct auto shifts properly but the dash cluster speedo was reading slow.
So I did a read of ECU only without T43 and changed the wheel/tire rev and the ratio and wrote it in, speedo on cluster now good and shifts are still good, drives well but the VCM Speedo is now 25km over. Does that matter for any other calculations in the ECU? Or have I not understood what you meant and done that wrong? Not sure if related or not but after doing this, my cruise control didn’t work, when I tried to activate I got a message on the cluster I’ve never seen before that said something like ‘press the brake pedal before activating cruise’. Would the cruise be comparing auto/ECU speed and be unhappy? The last time I drove this car the cruise was working, I could also see on VCM scanner that the brake pedal state was changing, so I’ll put the original values back in and check to see if it’s that or if something else has happened while it’s been sitting there for the last 5 or 6 months.
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Tre-Cool
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Re: E38 - Speedo Calibration

Post by Tre-Cool »

ideally you want the tcm values to read correct since the ecu gets that as the incoming speed.

what os & cal is in the tcm
ben_att
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Re: E38 - Speedo Calibration

Post by ben_att »

So, I've been playing around with this. One thing I'm not sure about is the factory tune info doesn't seem to match any of the tyre calculators I've tried. All the tyre dimension sites I've looked at seem to all give the same data, but none of them match what I have in any of the ECU's with factory parameters.
For eg.
VF SS Ute (LS3/6L80) - Tyre 245/40R19 - diameter is 678mm x 𝝅 = 2,129mm circumference / 1000km = 469.7 rev per 1,000km
Factory value is 481 for ECU and TCM? This OS (12658778) actually shows 'Driven Tyre Circumference' to be 2,079mm which also doesn't match the tyre calculators.

VF Calais (LS3/6l80) - Tyre 245/40R19 - diameter is 678mm x 𝝅 = 2,129mm circumference / 1000km = 469.7 rev per 1,000km
Factory value is 481 for ECU and TCM? This OS (12673958) doesn't show the circumference like above

VZ Crewman (L98/4l60) - Tyre 225/55R17 - diameter is 679mm x 𝝅 = 2,133 / 1000km = 468 rev per 1000km
Factory value for ECU is 502 and TCM 482

Where am I going wrong here?

In any case, I tried playing with the values, I can get the speedo to read correctly but then the auto shifts are terrible.
Now with the values as I calculate them, it's terrible, holds gears even barely touching the pedal it won't shift until 3,000rpm, so looks like I need to guess the values to find where it shifts half decent, there must be something I'm doing wrong here to calculate these values, what am I missing?

By the way, TCM OS is 24267577
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Tre-Cool
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Re: E38 - Speedo Calibration

Post by Tre-Cool »

so did some quick testing on the vz 6 speed conversion car. i keep both engine & trans to 478 pulses per km. in efilive thats H0105 (this is basically worked out diff gear/tyre size)
In scanner both ecu/tcm show same KPH. but dash needed adjusting, which is also done in the ecu, because it's putting out the pulses to the dash.
Again efilive it's H0101 - Vehicle Speed Sensor Pulses Per Km. set to 23152 & it matches close enough to the ecu/trans in scanner.

I'll have a quick look in the code, but i suspect that maybe GM removed the output pulse variable because everything was canbus by then. no real need to have a pulsed output.
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