EngineLink for OBDII in iOS

[Noah Harbinger]

I finally spent some time figuring out how to read signed-integer values in the EngineLink ODB-II reader app for the iPhone/iPad/iOS.

 

The values that come over the bus are 2s-compliment signed integers, but EngineLink uses the (open source but unmaintained) GCMathParser library for its formula evaluations. It doesn't have any concept for treating values as signed, and EngineLink doesn't seem to have the chops to modify it or to pre-process any (signed) evaluations. 

 

Someone (who? I can't find the post anymore!) somewhat recently posted a link to http://i-fix.us/Torque-Pro-Settings-for-Ford-Fusion-and-C-MAX-Energi.php, which provided a bunch of interesting PIDs I'd never seen before (specifically, the gen/motor torque and speed PIDs). That finally gave me the motivation to look at this some more. 

 

I played with different ways to use the mod function, which seemed the most relevant function they provide. 

 

Suppose u(x) is the equation for the unsigned value of the 16-bit 2s-compement signed x. Then, u(x)%2^15 - x yields a function whose value is -(2^15) where X < 0, and 0 where x > 0. With that, you can subtract that offset function from the unsigned value, and the result is equal to the signed value. 

 

That simplifies to Value = 2*(Unsigned%MaxSigned) - Unsigned. 

 

To clarify visually, in the chart below, red indicates the unsigned value of the signed integer X; blue the offset you add twice to X, and green the desired value. 

 

 

 

That let me convert the formulas that use signed values to work in Enginelink, including the long-sought HV Battery Current. I have loaded these into EngineLink and taken some test drives, and the values all seem reasonable and seem to match my understanding of how the powersplit transmission works. (The min/max values seem to be off, though - I'll have to update those based on data I collect). 

ModeAndPID,Name,Min,Max,Unit,Formula,Header
22480B,Hv Current,-110,110,amps,(2*((A*256+B)%32768)-(A*256+B))/50,0
221e2c,Motor Speed,-10000,10000,RPM,(2*((A*256+B)%32768)-(A*256+B)),0
22481b,Motor Torque Command,-500,500,Nm,(2*((A*256+B)%32768)-(A*256+B))/10,0
22481c,Motor Torque from AC Source,-500,500,Nm,(2*((A*256+B)%32768)-(A*256+B))/10,0
224821,Generator Speed,-10000,10000,RPM,(2*((A*256+B)%32768)-(A*256+B)),0
224819,Generator Torque Command,-500,500,Nm,(2*((A*256+B)%32768)-(A*256+B))/10,0
22481a,Generator Torque from AC Source,-500,500,Nm,(2*((A*256+B)%32768)-(A*256+B))/10,0

If there's anyone else out there using EngineLink, I hope that's useful!

Edited April 25, 2015 by Noah Harbinger

[plus 3 golfer]

I provided the link to the PIDs above.  I have EngineLink but since my old IPad cannot be upgraded past iOS 5.1.1, I cannot upgrade my old version of EngineLink to the current version which I believe requires iOS 6.  Hence, I don't use EngineLink much anymore.