OBC reading speed incorrectly

[bravo 35]

The OBC in my 325 seems to be reading the speed incorrectly.

I have "tested" my speedo against Josh (hybrid)'s e46, his e30, and Tony (dirtysix)'s e36 and we all had exactly the same speed reading over a range of speeds form 80km/h up to just ofver the ton driving a flat straight road with even (as we could manage) following distances.

Thus I beleive my speedo works correctly.

However, if I reset my obc whilst maintaining, say an even 100km/h, then the average spped reading on the OBC reads about 7-10km/h slow. The speed limit warning bell sounds at 7-10km/h over the prescribed limit and my fuel consumption is out (when compared to a similar car doing similar speed so not entirely scientific I know) by around 10% at 100k/h which all points to the OBC being out of calibration. Anyone know how to adjust or fix it?

Cheers

[TronSpec 0]

Grayham you have to much time on your hands

[cainchapman 0]

Maybe a stupid question, but have you changed wheel size, diff or gear ratios?

[TronSpec 0]

That's a very good question cain.

[bravo 35]

Shouldn't make a difference if the speedo is reading correctly. The car was on factory wheels when I bought it, i have changed the rims and tyres (to the correct size for the new rims) but did not play with the speedo. Therefore if the speedo is reading correctly (as it appears to be) then as before, it is the obc that is out.

So any ideas how to calibrate it? i found heaps of info on how to do the e36 one, but nothing on the e30.

Edited October 15, 2006 by bravomikewhiskey

[Guest Ken]

Hi - Via bimmerwerkz.com but for E34s, might give you some leads though:

If the OBC appears to be reporting inaccurate fuel consumption figures it can be calibrated to give the correct values. - Fill the fuel tank to the maximum and reset the trip counter and one of the OBC's average fuel consumption monitors. Run the fuel tank nearly empty and fill the fuel tank again to the maximum. The amount of fuel it now takes to fill the tank is the amount used to cover the distance recorded on the trip meter so, from this, work out the actual fuel consumption of the car and then compare this to what the OBC reported as the average fuel consumption. If these two average fuel consumption figures are not exactly the same then you need to calibrate the OBC by entering a new correction factor.

The fuel consumption correction factor is a locked function of the OBC so before you can access it, you must first unlock the OBC - Access the OBC test menu by simultaneously pressing the 1000 and 10 keys so that the display changes to "TEST". With the 10 & 1 keys enter 19 on the display and then press the S/R button. Add the current day and month (as it is known to the OBC) and enter this value and then press the S/R button, the OBC should now be unlocked. To access the fuel consumption calibration factor you once more need to simultaneously press the 1000 and 10 keys so that the display changes to "TEST". With the 10 & 1 keys enter 20 (Be careful not to enter 21 as this will reset the OBC) on the display and then press the S/R button - you should now see something like 1000 CORR, this is your current correction factor. To calculate your new correction factor use the following:

new_correction_factor = old_correction_factor * actual_mpg / obc_mpg

Enter your new correction factor into the OBC using the 1000/100/10/1 keys and then press the S/R button. If you want to relock the OBC - enter function 19 again then press the C/F button to lock.

Also: http://www.strictlyeta.net/technical/obcfunctions.html

HTH

Ken.

[bravo 35]

The test codes are for newer obc's. Don't work with the e30 ones AFAIK. but thatnks anyway.