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Torque » Torque OBD ECU Scanner » Torque Discussion / Ideas » SSM reporting different A/F ratios than OBD2

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Author Topic: SSM reporting different A/F ratios than OBD2
danci1973
Member
Posts: 7
Post SSM reporting different A/F ratios than OBD2
on: October 12, 2016 (GMT)

I have an OBDKey 140 BT adapter, that supports SSM (Subaru Select Monitor) protocol as well as several other OBD2 protocols. With my car (2006 Subaru Legacy 2.0R – Hitachi / Unisia JESC ECU) SSM provides much faster refresh rate, so I prefer using that…

Unfortunately, in SSM mode Torque Pro doesn’t display all the stuff I’m interested in – it doesn’t show fuel trims, for example, which I use to check if my LPG system is behaving.

That means that I switch between SSM and OBD2 and a few days ago I noticed that SSM reported A/F ratios in range 14.2-14.9, while OBD2 was reporting higher ratios (15.3-15.9). I tried to switch back and forth several times making sure conditions were always the same (either standing still at idle or just maintaining speed on a straight flat road with cruise control), but that difference persisted.

Since fuel trims are fine (hovering around 0) I guess that real A/F ratios are fine, but what could be causing that difference? Could it be a bug in Torque Pro?
danci1973
Member
Posts: 7
Post Re: SSM reporting different A/F ratios than OBD2
on: October 13, 2016 (GMT)

Today I tried some other OBD2 software on my phone (DashCommand in evaluation mode) and it was showing the same AFR values as Torque Pro with SSM (i.e. ‘correct’ ones).
piemmm
Administrator
Posts: 6629
Post Re: SSM reporting different A/F ratios than OBD2
on: October 13, 2016 (GMT)

Hi

What fuel type do you have your vehicle profile set to and I’ll look into it (though there may be several reasons why it’s slightly different)
piemmm
Administrator
Posts: 6629
Post Re: SSM reporting different A/F ratios than OBD2
on: October 14, 2016 (GMT)

Hi!

Having checked this through, the SSM value is off (as it does not take into account the stoch ratio of the fuel and is fixed at 14.7 per the documentation for it)

I’ll get that sorted so it works of the correct stochiometric ratios for the fuel set in the vehicle profile. (Different fuels have different stoch ratios). As long as you have the fuel set in your vehicle profile correctly then the correct ratio will be used and the value will be accurate

Normal gasoline has a stoch of 14.7, however blended fuels have different ratios (like, say E20) have a stoch of 13.79, and LPG 15.5. (CNG being as high as 17.2!)) and these have to be used in the conversion

I’ve corrected the SSM protocol to use the fuel stoch when converting from lambda to the afr, so as long as the vehicle profile is set correctly it should now display an accrate reading when using SSM (and not OBD)

OBD reads the lambda value so this is subject to correct conversion using the fuel stoch to get the right afr. Some useful reading on this is at: http://www.hotrod.com/articles/wideband-oxygen-sensor/ It would appear that the SSM protocol also requires this

As a side note, I’ll try to find some more sensors to add for the SSM protocol (if you could list the ones you’re interested in then this will help me target those particular sensors)
danci1973
Member
Posts: 7
Post Re: SSM reporting different A/F ratios than OBD2
on: October 14, 2016 (GMT)

Spot on!

I just figured that out yesterday – I had ‘LPG’ set in both profiles, but when I changed fuel to ‘petrol’, AFR readings changed only in OBD2 profile… :)
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