Decoding OBDII Comms

[HenrikOlsson]

Hi,
It looks like you have a simple scale of ~20 counts per ampere.
$005F is 90 in the decimal, 90/4.6A=19.5 counts per Amp
$0116 is 278 in decimal, 278/13.6A=20.4 counts per Amp
$0161 is 353 in decimal, 353/17.1A=20.6 counts per Amp

$FFA6 = 65546 which in two complements is -89, 89/4.5=19.8 counts per Amp

Average that out a bit and I'd call it 20 counts per ampere.

/Henrik.

[retepsnikrep]

Thanks Henrik that fits in with some other tinkering i have been doing. More data to follow.

[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?