NefMoto

Hey guys,

So I am trying to figure out how to log my correct speed or actually I am trying to figure out if this can be fixed with software modifications or just my instrument cluster is inaccurate.
For testing I fired up my GPS module and let it sit for 15 minutes so that it can find enough satellites to give me proper information. Then I configured my climate control to show the vehicle speed on the small display and it turns out that my speed reported by the GPS and the climate control are basically the same (1-2 km/h) difference, but my instrument cluster goes way out while speed increases, which means that my
instrument cluster shows 5km/h less up to 120 km/h and then 10km/h less at 200 km/h (I get 200km/h on GPS and climate control, but 190 km/h on the instrument cluster).
So I wonder if the climate control and the instrument cluster is using a different sensor or it is just the instrument cluster being old and acting up?
If they use different sensors, then it may be possible to fix this with AIMVM (0x12A56)?

Thanks for the input

Hi,

ECU and instrument cluster uses the same sensor, but calculate the vehicle speed separately.
The instrument cluster gives an very inaccurate analog result.
If you calculates AIMVM correctly, the ECU will calculates the vehicle speed with great precision.

The Wheel speed sensor gives 8 impulses every full revolution of the wheel.

To calculate AIMVM exactly, you must divide 8 through the right wheel circumference.

As example:

AIMVM = 8 : circumference [m]

Rolling circumference of 255/35_18 = 1,99711m  => AIMVM = 4,00579
Rolling circumference of 235/35_19 = 2,032925m => AIMVM = 3,93522

***

An other way is to calibrate the instrument cluster.
This is not testet by me yet, but I think it is possible to recalculate the MAP of the analog speedometer.
 
Read out the  93c86 EEPROM of the instrument cluster and you will see some interesting MAPs.

I think there are MAPs for the instruments between address 1AB and 247, and between 6AC and 6E7.

No idea whether there are checksums, but in the area of stored odometer value at 042 - 051 there is no checksum activ.
This part is testet by me.

In the dump you will also find the VIN and the IMO number, which will be compared with the VIN and IMO data in the EEPROM of the ECU.

Here a hexdump of my RS4 VDO instrument cluster EEPROM.

(http://www7.pic-upload.de/08.06.12/yoss38cbrrsf.jpg) (http://www.pic-upload.de/view-14566229/EEPROM-part-1.jpg.html)

(http://www7.pic-upload.de/08.06.12/c5nsaguwgy1u.jpg) (http://www.pic-upload.de/view-14566230/EEPROM-part-2.jpg.html)

If anybody read the EEPROM from a S4 VDO cluster instrument,
we can identify the MAP of the speedometer because it must be unequal.

(S4=260Km/h; RS4=310Km/h)

Here is my RS4 EEPROM.bin  

These calculations aren't correct.  I am not saying I know how to do them, but using this logic please follow:

audi oem, width, aspect, size, circ in m, theo, aimvm, approx error
b5 s4     225       45            17   1.9927   4.0146   4.131     .12
b5 rs4     255       35            18   1.9971   4.0058   4.077     .07

I think my calculation is correct.

RS4 255/35_18
((18Zoll*25,4mm) + (35% from 255mm) + (35% from 255mm)) * 3,1415927 = 1997,11047939mm
AIMVM = 8 : 1,99711047939m = 4,005787402629588

S4 225/45_17
((17Zoll*25,4mm) + (45% from 225mm) + (45% from 225mm)) * 3,1415927 = 1992,71224961mm
AIMVM = 8 : 1,99271224961m = 4,014628806324498

Car with 235/35_19
((19Zoll*25,4mm) + (35% from 235mm) + (35% from 235mm)) * 3,1415927 = 2032,92463617mm
AIMVM = 8 : 2,03292463617m = 3,935217202675984

yes our numbers agree.  however they do not agree with the stock ecu constants.  i suspect we are missing a fudge factor of some kind.

b5s4:
theoretical value for 225/45/17:               4.015
tirerack's RE040 diameter (the stock tire):  4.043
actual m-box value set:                          4.131

I've written a small tool to calculate AIMVM...

http://nefariousmotorsports.com/forum/index.php?topic=2087.0title=

Oops it's been a long time that I've checked this thread, but this is the last annoying thing I have on my tune. So you guys say that the stock AIMVM value is not correctly set for the OEM wheel setup. Maybe the difference is there so that the stock value works reasonably well with bigger/smaller tire setups and only gives a small amount of difference in speed readings?