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Re: <E34> M5 OBC misreadings

Hi gang,
i guess it's my ever-lasting 'OBC demon' that  [email protected]
means when he (she?) writes:

>Sorry I forgot who posted, but someone with a 90 model e34 has had
>mpg and range problems with the obc.
>Check the gas gauge, does it show readings between full and 1/2 or
>go to half very quickly?
>Cars produced 6/90 to 2/91 had a non linear gas gauge that must be
>corrected by a software download from the dealer. US service bulletin
>62 02 91 (3237). If you think this might be the problem, let me know
>and i will share more details.

No, i don't think that's my problem. My M5 is produced 12/90, and the
fuel gauge gives linear and fairly correct readings.
My problem is, that the indicated fuel consumption is about twice the
actual consumption. In fact, after running a couple of tanks, i have
found that the numbers are off by almost exactly a factor 2x.
In addition to the 'official' MPG (or l/100km that i use) reading, the
OBC has a 'hidden' function (no 3) that shows 'Current consumption in
l/hour'(see FAQ). The reading is 2x what i expect (verified by driving at
steady 100 km/h. Shows about 20 l/h, while the actual consumption should
be 10 l/h, assuming that the consumption is 10l/100km at that speed).
The only possible source of this data is the DME/Motronic unit,
as it is controlling the amount of injected fuel. Either the signal
from the DME is wrong (clock-doubled like my old Intel 486???), or there
is something wrong with how my OBC interprets the signal.
The mpg and range readings are most likely calculated from the l/hour
numbers, that are wrong (2x), the speed and the time (that are correct).
With that assumption, the calculation is correct but the result is wrong.
As all other readings from my OBC are 'correct', i tend to suspect the
DME unit. In a desperate hope that it is the OBC anyway, i am going to
try out the OBC from a friends M5 before doing anything more drastical...
BTW, does anyone know how to remove the OBC from the dash? I guess it's
some kind of snap-in mounting, but haven't been able to figure it out
yet....  Need to know if i'm going to swap OBC's with friends M5.
Any words of wisdom out there...??
Thanks for listening,

Gunnar Elmgren,
Stockholm, Sweden
BMW Club Schweden no 1436

'71 1600-2 Vollcabrio (soon to hibernate for the winter)
'75 2002 tii (needs some attention, i hate this rust....)
'90 M5 (perfect, if it wasn't for the OBC demon)

Gunnar Elmgren                            /??????\
AS/400 Software Support, IBM Sweden      (oo=OO=oo)
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