Decoding OBDII Comms

[retepsnikrep]

Here is a different current sensor

This one gives the following data and values, seems to work on a different formula?

$7F $9E = 2A
$7C $9C = 8A
$78 $9B = 16A
$83 $9F = -6A

Looks like anything over $7F in first byte is a negative value


And one more this is reporting voltage.

$98 $B1 = 158V
$9B $99 = 162V
$9C $80 = 163V

Seems to be just the first byte involved + 6?

[HenrikOlsson]

Hi,
Well I can only guess but here goes.

Voltage:
$98B1=39089=158V : 39089/158=247
$9B99=39833=162V : 39899/162=246
$9C80=40064=163V : 40064/163=246
So, it looks like there's ~246 "counts" to a volt but again, that's just an educated guess.

Or perhaps there's an offset to it making values above 32768 positive and values below negative. In that case it might look like this:
(39089-32768)/158=40
(39833-32768)/162=43
(40064-32768)/163=45
Nah, that's probably not it.

The current is a little more tricky, here's my initial thought.
$7F9E=32670=2A : (32768-32670) / 2 = 49
$7C9C=31900=8A : (32768-31900) / 8 = 108
$789B=30875=16A : (32768-30875) / 16 = 118
$839F=33695=-6A : (32768-33695) / 6 = 154 (-154)

Clearly the above doesn't add up at all so there's something else involved.

So it's all about sitting down with pen, paper and a calculator and try to reverse engineer the numbers, try to find something that matches all the readings you have. Or try to find the specification of the protocol...

/Henrik.

[ivanrosales]

For the current, it seems that the amps are 128 reading units apart...
33695 = -6
33567 = -5
33439 = -4
33311 = -3
33183 = -2
33055 = -1
32927 = 0
32799 = 1
32671 = 2
32543 = 3
32415 = 4
32287 = 5
32159 = 6
32031 = 7
31903 = 8
31775 = 9
31647 = 10
31519 = 11
31391 = 12
31263 = 13
31135 = 14
31007 = 15
30879 = 16

Pretty close...

[retepsnikrep]

Thanks for the ideas so far.

Here is some more data for a few functions.

'MPI_MDL Temp C
'$9C $97 = 21C
'$88 $77 = 29C
'$83 $80 = 31C
'$6B $76 = 40C
'$67 $73 = 42C

'DC-DC Converter Temp C
'$97 $22 = 22C
'$7F $41 = 33C
'$77 $41 = 36C
'$76 $6F = 37C
'$72 $6F = 39C

'BAT_MDL Volts
'$91 $10 = 152V
'$95 $00 = 155V
'$98 $0C = 158V
'$9A $05 = 160V
'$9C $00 = 162V
'$9D $00 = 163V
'$9E $00 = 164V

'MPI_MDL Volts
'$97 $7F = 157V
'$98 $B2 = 158V
'$9B $99 = 162V
'$9C $80 = 163V

'MPI_MDL Amps
'$7F $9E = 2A
'$7C $9C = 8A
'$78 $9B = 16A
'$83 $9F = -6A
'$83 $9A = -6 to -8A

TORQUE NM
'$2D $5A = 0 NM
'$3A $5A = 8.4NM
'$41 $5A = 28 to 30 NM
'$2A $5A = -8.4 NM
'$29 $5A = -11.2 NM

Any further help?

[Demon]

The guys above probably already know this stuff. But for those that follow this thread with interest and want to catch up, here's an interesting read:

http://www.obd2crazy.com/techstd.html

(unfortunately the links they posted do not seem to work)

Robert

[retepsnikrep]

Thanks for the ideas so far.

Here is some more refined data for a few functions.

'MPI_MDL Temp C
'$9C = 21C
'$88 = 29C
'$83 = 31C
'$6B = 40C
'$67 = 42C

'DC-DC Converter Temp C
'$97 = 22C
'$7F = 33C
'$77 = 36C
'$76 = 37C
'$72 = 39C

Any ideas on these two?

[retepsnikrep]

Some new data to fox you from an Air Fuel ratio sensor.

The data from the car (left number) converts to the AF (Right number).

255=7.31
224=8.36
192=9.71
160=11.65
128=14.56
96=19.41
64=29.12
32=58.24
16=116.48
8=232.96
4=465.92
2=931.85
1=1863.70

It looks like a typical wideband 02 sensor response with a very steep rise at the end.

I need to establish a formula (if possible) from this data so I can convert the data into the AF.

Now I could capture every value from 0-255 and get exact numbers for each value but that will take a lot of time, and then I would have to do a lookup table for the normal operating range. I could just gather 128 data points from 64 to 192, that encompases the AF range 29.12 - 9.71 which i'm sure covers my cars operating area.

How can I do it without using a look up table? Formula?