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<E34> M5 OBC misreadings - revisited

Fellow digesters,
last week i posted a question about how to calibrate the OBC in my
new (to me) '90 M5, to get correct 'average fuel consumption' numbers.
I would like to express my thanks to Russ, Pat and Tony for providing
the calibration info (i eventually found the FAQ at
http://cbsgi1.bu.ed/bmw/obc.html ) and valuable tips.
However, the reading is so much off, that the correction factor range
is insufficient. From my 'manual' calculation (fill tank - run empty -
fill again and calculate) it appears like the MPG figures are about
half of what they should be. The OBC says 10.5 MPG (27l/100km), and
the 'real' consumption is in the 20-21 MPG range (13-14l/100km), which
is better than i expected.....
I would need a correction factor of 500 (default is 1000) to get the
correct readings, but the smallest number my OBC accepts is 750.....
All other readings of my OBC are perfectly OK, except of course
'range' that is also half of what i expect.
I went thru all the 'hidden' functions of my OBC, everything makes
sense except the AVG and RANGE. The fixed OBC data is as follows:
(14) Date of SW/Mask of OBC:   27.08 1988
(15) Production/Diagnostics??  01C1  DIAG
(16) Procuction/Diagnostics??  01AC  PORT
(17) Vehicle Specific          00EC  PROM

I think that the OBC itself might origin from another vehicle, as
the car in its earlier life has been the victim of theft, and had
the interior removed....  The interior is from a wrecked M5, but
i cannot say for sure where the OBC comes from.
Any ideas? My suspicions are, that some signal from the DME/Motronic
unit is missing every other pulse (alternatively doubling the pulse
rate). OR that the i have an OBC that expects something different than
the output from an M5 DME unit.......
Any tips/suggestions/theories welcomed, posted to digest or as
private E-mail. I promise to publish my findings to the digest, once
the mystery is solved.
TIA,


Gunnar Elmgren
Stockholm, Sweden
BMW Club Schweden no 1436
- ---------
'71 1600-2 Vollcabrio
'75 2002 tii
'90 M5
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Gunnar Elmgren
AS/400 Software Support, IBM Sweden