P38a ODOMETER PROBLEM

Is there anyone in this group that can tell me what is wrong with my
odometer (mileage recorder)?
I have a 1999 (T reg) 2.5 BMW Engine DSE Range Rover. The problem is that it
is recording more miles than I'm actually doing. The trip recorder and the
computer recorded miles are not effected by what is happening.
Let me explain in an example.........I switch of the engine at the end of
the day with the odometer reading 114232.7 miles, when I turn the key to
switch on in the morning, the mileage will mysteriously jump to 114300.0.
Always it will jump miles to the exact whole number. Sometimes it will jump
to the next nearest ten miles (say 23.4 to 30.0) and sometimes to the next
one hundred. The mileage display is digital, so it disappears when you
switch off and can't tell when it happens.
Are the three mileage recordings getting their figures from the same (I
presume) PCB board? Could it be a power surge when I turn the key?
I hope someone can give me some idea or clue as to what is going on. My 4x4
service specialist has no idea except changing the board or dash and says it
could be expensive.

Thanks in advance

Steve

>>>>> "spm" == spm <[email protected]> writes:

spm> the morning, the mileage will mysteriously jump to 114300.0.
spm> Always it will jump miles to the exact whole
spm> number. Sometimes it will jump to the next nearest ten miles
spm> (say 23.4 to 30.0) and sometimes to the next one hundred. The

Wierd. I can't tell you for sure, but...

The mileage is stored in two places on the vehicle: the BeCM and the
instrument binnacle. At each engine start, the two compare notes
("I've got 114324.3" "I've got 76543.1") and then both store the
HIGHEST value.

Your BeCM is obviously dropping bits somewhere and artificially
increasing the values.

The BeCM certainly only reports to the nearest 100 miles
diagnostically, but I don't know how much more detail it stores
internally.

The figure itself is derived from the ABS sensors - the average of all
4 wheels is converted into a frequency-modulated signal of 8,000
pulses per mile, which goes from the ABS ECU to the BeCM, and then to
other ECUs which depend on this information. I don't know, from
memory, how this gets to the odo. display.

Now, there's a couple of things that could be happening

1) Something is causing the voltage on the ABS output to fluctuate
which the BeCM is seeing as pulses as incrementing the counter
overnight. If this were the case, I'd expect other symptoms like
excessive battery drain because the computer is being kept awake.

2) The BeCM is faulty and the number being stored is getting
corrupted.

3) There's a dodgy wire or connector on the link between the two which
is causing the communication between them to get garbled.

Not a conclusive answer, but at least a plausible explanation. I can
look up some specifics on the wiring diagram if you want - drop me an
e-mail if you do, as I might miss a reply here (replace "spam" with
"andy" to reply).

Andy





--
Andy Cunningham -- www.vehicle-diagnostics.co.uk
`Bother,' said Pooh, `Eeyore, ready two photon torpedoes and lock
phasers on the Heffalump; Piglet, meet me in transporter room three.'
--Robert Billing