Thread: Yet Another OBDII VB6 Class

So anyone used it yet?

Originally Posted by carabuser

So anyone used it yet?

Yes!!!

I'm using it! It works - My app isn't finished yet, but it's a re-hash of this:

OBDII / Elm MPG meter. Almost working but not quite!

The OBDII stuff is working very well. I've successfully had it running for hours on end pulling out the data. My MPG calculations seem to be a bit iffy though - I am having trouble finding a comprehensive list of all the basic formulas for temp, mass air flow etc for when I'm converting from the output long to it's 'real' value! I found one a long time ago when I first started on this, before your VB6 class, but can't find it now, so having a little trouble double checking everything.

I haven't tested standby/resume properly yet - my only try resulted in failure but I was thinking about polling for the standby system call and disconnecting, then on resume reconnecting if it causes any issues - my main thing is getting the instant MPG quick and reliable.

But yes... It works!

Originally Posted by knubile

Any progress on this? I'm wondering if MPG calculations are possible using OBD-II.

PHP Code:

Function GetMPG(tMAF As Variant, tSpeed As Variant, tLTFuelTrim As Variant) As Variant

''MAF is maf_air_flow sensor. To convert from what the OBD returns, value=OBDData * 0.01
''tSpeed is vehicle speed in MPH. To convert from what the OBD returns, value=OBDData * 0.621371192
''tLTFuelTrim is long-term fuel trim. To convert from what the OBD returns, value=0.7812 * (OBDData - 128)
    
    If IsNull(tMAF) Or IsNull(tSpeed) Or IsNull(tLTFuelTrim) Then
        GetMPG = Null
        Exit Function
    End If

    Dim tMPG As String

    '' Do MAF over an hour
    tMAF = tMAF * 60
    tMAF = tMAF * 60

    '' Turn into KG/litre equivalent
    tMAF = tMAF / 1000

    '' Get petrol flow based on 14.7:1
    tMAF = tMAF / 14.7

    '' convert litres to gallons
    tMAF = tMAF * 0.219969157

    '' convert long term fuel trim to percentage
        tLTFuelTrim = (tLTFuelTrim / 100) + 1

    '' Calculate MPG
    tMPG = (tSpeed / tMAF) * tLTFuelTrim

    GetMPG = FormatNumber(tMPG, 1, True, False, False)
End Function 


The problem I have with this is the air flow sensor is very optimistic, and you probably run richer than this formula indicates. It says I average around 60-90MPG when cruising on the hwy and up to 200+ MPG when coasting. However, it does change when you have the pedal down, etc. I forget where I stole this code from.

Originally Posted by Erorus

PHP Code:

Function GetMPG(tMAF As Variant, tSpeed As Variant, tLTFuelTrim As Variant) As Variant

''MAF is maf_air_flow sensor. To convert from what the OBD returns, value=OBDData * 0.01
''tSpeed is vehicle speed in MPH. To convert from what the OBD returns, value=OBDData * 0.621371192
''tLTFuelTrim is long-term fuel trim. To convert from what the OBD returns, value=0.7812 * (OBDData - 128)
    
    If IsNull(tMAF) Or IsNull(tSpeed) Or IsNull(tLTFuelTrim) Then
        GetMPG = Null
        Exit Function
    End If

    Dim tMPG As String

    '' Do MAF over an hour
    tMAF = tMAF * 60
    tMAF = tMAF * 60

    '' Turn into KG/litre equivalent
    tMAF = tMAF / 1000

    '' Get petrol flow based on 14.7:1
    tMAF = tMAF / 14.7

    '' convert litres to gallons
    tMAF = tMAF * 0.219969157

    '' convert long term fuel trim to percentage
        tLTFuelTrim = (tLTFuelTrim / 100) + 1

    '' Calculate MPG
    tMPG = (tSpeed / tMAF) * tLTFuelTrim

    GetMPG = FormatNumber(tMPG, 1, True, False, False)
End Function 


The problem I have with this is the air flow sensor is very optimistic, and you probably run richer than this formula indicates. It says I average around 60-90MPG when cruising on the hwy and up to 200+ MPG when coasting. However, it does change when you have the pedal down, etc. I forget where I stole this code from.

It looks like my botched attempt at MPG conversion I think.

I didn't think it was accurate as it could be either, and haven't actually used it in real-life.

Any idea what could be done using the OBD to get a more accurate measure?

Originally Posted by takissd

great! will that work with vb.net??? if yes, how?

You can attempt to convert it into .NET. But its very possible it will get botched up. At least that's been my experience with most of the .NET conversions I've done.