Exploring the (2000) 528 OBC
From: Lon Leader

Just for reference here - if you have a 540 or, I believe,
a 528 with NAV, with the 'high' or 8 function computer you
can learn about it at Peter Glaskowsky's web site:


It makes interesting reading because the displays appear
with short text discriptions that are not available with 
the 'low' OBC. Please note however that while there are
simularities between the two computers there are also
significant differences in some of the displays.

Back to the 528 ('low' or 4-function) computer. There are 
21 'tests' or 'displays' in total, and some of them have 
sub-displays. Tests 1 (car data) and 2 (instrument test)
are accessable without 'unlocking' the OBC. There is NO 
chance you will screw anything up (except MAYBE reset the 
trip meter) if you don't 'unlock' the computer (which 
requires you to do something specific). Even with the 
computer unlocked there are only two tests (20 and 21) 
that allow you to change anything and those will be 
explained later. (One of them, 20, is the fuel correction 
factor that I suppose is the reason you're reading this.)

Access is a little more complicated on the 'low' OBC because
there is only one control that you use - the trip reset
button on the dash. There are only three things you can do 
with one button - a 'short press' (less than 1 second), a 
'longer press' (greater than 1 second), and a 'don't press'
(ie: wait a second or two without pressing the button). 

OK, with the key in the accessory position (no need to have 
the car running) to get started you press and hold the trip 
button for several (more than 5) seconds. As you first 
press the button in, the trip meter will appear to reset 
to zero - don't worry, if you keep the button pressed until
you're into test mode when you exit test mode the trip 
meter will be restored to what it was.

After 5 seconds or so, the mileage/trip display will 
change to:

_teSt_ __1._

VERY QUICKLY release the button (if you continue to hold 
the button, it will pop you back out of test mode)

One thing to note here is the word 'teSt'. When that is 
on the display you are in what I call 'index' mode - it 
is at this point you can step from one test to another. 

What happens next depends on whether you quickly press the 
button again ('short press') or do nothing ('don't press').

If you quickly press the button again the word 'teSt' will 
remain and the __1._ will change to __2._, and if you 
repeatedly 'short press' again and again, it will step up 
through the index of tests to _teSt_ _21._ and then start 
over again at __1._. These are the 21 available test 
displays and only __1._ and __2._ (and _19._) are available
to access without 'unlocking' the computer. (_19._ is the 
'unlock' panel). 

OK, back at _teSt_ __1._ - if you pause there ('don't 
press' the button for a second or two) you will 'drop into' 
(or access) test 1's first display (it has 8 sub-displays).
You should see something like:

_xxxxx __1.0 (the first digits, censored here, will 
be your VIN number)

If you then 'short press' the button repeatedly you will 
step through all the 8 displays of test 1 (car info) 
- here are what mine say:

_xxxxx __1.0 (VIN number) 
__4812 __1.1 (body number)
838119 5_1.2 (part number for cluster)
063309 __1.3 (coding / diagnosis / bus index) 
__2999 __1.4 (week/year cluster manufactured)
07_150 0_1.5 (hardware / software versions) 
095_06 3_1.6 (engine data)
03_28_ __1.7 (ROM status)

Continued short presses will just cycle you through this 
list again and again. The brief descriptions listed
above were provided by Jim Cash. More info might be 
extrapolated from the 540 OBC described at the web page 
mentioned above.

To get out of this display and back to the index just do 
a 'long press' until the display changes to _teSt_ __1._
again. If you 'short press' at this point you will step 
to _teSt_ __2._. If you continue your 'long press' 
(greater than 2 seconds) it will drop you right out of 
test mode entirely and restore your trip meter reading. 

The fun display is _teSt_ __2._. Go to it and pause 
there so you 'drop into' _teSt_ __2.0. (This is the 
only time the word 'teSt' appears when you're not in
index mode) Do a 'short press' here and watch the dash 
displays (don't worry, you won't miss it!) To get out 
of this wait until the gauges stop moving and do a 'long 
press' to back out of test 2 and into the index. You
can't get this display to work when the car is running.

If you attempt to 'drop into' tests other than 1 and 2 
(without unlocking the computer) you will be immediately 
booted out of test mode entirely. The only exception is 
test 19 - the unlock panel. If you accidentally drop into 
it, and don't want to unlock, do a 'long press' and you
will get back out. 

OK, now to unlocking. NOTE: from now on, you're on your
own. If you venture past this point understand that I
bear no responsibility for anything that may happen. I've
been there and done that, and my car still runs fine and
no magic smoke escaped. As far as I know, tests 3 through
18 are displays only. Test 19 is unlock, 20 is the fuel
calibration (and I've changed mine several times as I 
zero in on getting the mileage right), and test 21 is
RESET. I suggest you don't do a RESET, but only because 
I can't tell you what it does. While learning how all 
this works I inadvertently did the RESET. The only thing 
that I noticed happen was the clock in the MID zeroed out 
and had to be reset. What DID NOT happen: the oil change
lights didn't reset. The vehicle mileage didn't change.
The stereo presets and settings didn't change (I have the
DSP). The heater/air cond/fan settings didn't change.
I suspect you would have more grief if you disconnected 
the battery. But I don't KNOW if anything else happened,
so I recommend you don't do it. If you accidentally 
pause at test 21 and 'drop into' it, the display will 
say 'RESET'. Nothing has happened yet and won't unless 
you do a 'short press'. Simply do a 'long press' (press 
and hold) and you will be popped out of test 21 without 

Enough waffling. Here's how you unlock. You have to be
paying attention because it can be a bit tricky. Step
up through the index to _teSt_ 19._ as described above.
Pause there (don't press) and allow it to 'drop into' 
test 19. You will see a display that consists of a 
large 'L' followed by the words 'on' and 'off' 
flip-flopping back and forth. Watch it for few seconds 
and when the word 'off' pops up press the button and 
watch for the display to change. Should require just a 
short press. Be ready to do another short press right 
away, because usually the display will change to 
_teSt_ _0._. You don't want to pause here and 'drop 
into' this test 0 because if you let that happen you 
will be booted right out of test mode entirely. So you 
jump in with however many short presses you need to 
move through the index to get to the test you want. 
You can stop at any one, even 1 and 2 (which won't be 
any different in unlock mode). You can back-out of any
test you are in (and get to the index) with a 'long 
press'. While in the index another long press will take
you right out of test mode entirely. If this happens
you become 'locked' again and have to do the 'unlock'
thing all over again.

I'll skip the other tests for now and get right to the
only one (besides 21-RESET and 19-UNLOCK) that allows 
you to change anything: test 20 - fuel factor. While 
unlocked step up to _teSt_ _20._ as described above 
and allow it to 'drop into' _20.0_ You will see
your current fuel correction factor displayed there as 
a four digit number - probably '1000'; I think that is 
the default. You can sit there and look at it, it doesn't 
start changing by itself. If it is not 1000 you should 
write down what it is for reference. If you want to back 
out without changing it just press and hold the button
('long press') until you pop out of test 20.

Changing this number works like this: If you 'short press'
once while the number is displayed you change from panel
20.0 to 20.1 and the '1's digit starts scrolling. When
it gets to what you want it to be quickly do a 'short
press' to advance to panel 20.2 - where the '10's digit
will start scrolling after a second. When that gets to
what you want quickly do a 'short press' and move to
panel 20.3 which will scroll the '100's digit(s). When
that gets to what you want another 'short press' puts
you back to panel 20.0 where you can admire your efforts.
If you're happy with what you have set, do a 'long press'
at this point and pop out of test 20. A furthur long press
will get you right out of test mode if you're done.
Before getting out of test mode entirely I usually 'drop
into' test 20 again just to verify my changes are still
there. Don't be surprised if this takes a little practice.
You can go to whichever digit you want to change without
changing others by quickly stepping over them with quick
presses of the button. The number has to be between
750 and 1250 (according to Peter Glaskowsky's 540 OBC
web page).

What should you change the number to? Well, a starting
point would be to do a percentage. If you figure your
OBC is giving you an mpg figure too high by 5%, increase
the factor by 5% from 1000 to 1050. It will take a while
to get it right because you should change it right when 
you do a gas fill-up and do a compare at your next fill-up.
My OBC was high (optimistic) by approx. 1 mpg so I changed
it from 1000 to 1050. Next two tankfulls the OBC was low 
by .5 mpg so I changed it to 1032. It's still off a 
little so I'm going to set it to 1035. You will probably
have to check it over several fill-ups to average out
inconsistencies in your fill procedure. 

Back to the other tests (3 through 18). While unlocked
you can check out these other displays by 'droping into'
them. For each one you should try a few 'short presses'
to see if there are sub-panels in each one before doing
a 'long press' to back out and select the next test.
To get the most out of some of them the engine has to be
running or the car moving. You can start the engine or
shut it off and stay in the computer as long as you don't
turn the key right off. 

The brief descriptions for each test listed below were 
found in a post in the bimmer.org E39 forum by 'slc1230'
on October 21,1998. Some of these may be wrong or changed 
since that was back in 1998. (stuff in brackets are my 

- test 3 (3.0-3.1)is european inspection data 
- test 4 (4.0)is momentary fuel consumption
- test 4 (4.1)is fuel consumption per hour
- test 5 (5.0-5.1)is distance consumption 
- test 6 (6.0-6.2)is fuel level inputs
- test 7 (7.0) is coolant temp 
- test 7 (7.1) is outside temp 
- test 7 (7.2) is engine speed (digital tach!)
- test 7 (7.3) is vehicle speed (digital speedo!) (kph)
- test 7 (7.4) speed again, matches optimistic speedo (new-2001 only?, 530 only?) 
- test 8 (8.0-8.3)is hex codes
- test 9 is battery voltage
- test 10 is country code
- test 11 is cluster code
- test 12 is not used
- test 13 is gong test
- test 14 (14.0-14.6) is error memories
- test 15 through 18 not used 

Compare the above with the info on the 540 OBC at the
web site I noted at the beginning of this essay.