obd sensor calibration

[mike_d]

hi guys, i've posted this on a couple of the 4runner boards, so don't get mad for repeats, i just wanted to reach a broader audience for this one:

i'm in the middle of writing my own obd software. i decided that rather than go the sound card route (which i talked about in a couple of other forums), i'm going to use a real adc and take input from the parallel port on my laptop. this way i can monitor more sensors and such.

my question is, does anyone know where i can get the calibration curves for the sensors? i.e. the afm will output a voltage proportional to the amount of air coming in to the engine, but how does the voltage relate to the actual mass of air.

remember, that i do not have obd-ii on my old trucks, so rather than plugging into the obd diagnostic ports, i'm going to be plugging in directly to the ecu inputs and outputs, so i need to do all the voltage -> real number calculations directly.

for those of you who care: once i get this done (or atleast to a point that it's starts to work) i'll be GPLing it and anyone who wants it can have it.

 

[rando]

National Instruments makes a whole product line of measurement and automation instrumentation appropriate for PC use. That would be the easiest (though probably not the cheapest) way to go.

I can't help you with interpreting the 'meaning' of the various sensor outputs. Perhaps you can track down the OEM for the sensors and get a data sheet?

Good luck, sounds like a fun project.

 

[mike_d]

National Instruments makes a whole product line of measurement and automation instrumentation appropriate for PC use. That would be the easiest (though probably not the cheapest) way to go.

the problem national instruments is that you need an expansion board and drivers and such. i'm trying to keep this as simple (and as cheap as i can). i have an old laptop lying around that i have linux on, so if i can just build a simple adc -> parallel port converter (i did this years ago when i was in high school, so i bet i can do it again) then all i need to do is write a fairly simple program to read the data off the parallel port and i'm done. i'm betting i can put everything together for under $10 of electronics.

i found a bunch of obd info at www.autoshop101.com, and it's great all the techincal docs are from toyota, but they didn't have actual calibrations there. i was hoping that obd was standard enough that the calibrations would be easy to find.

 

[rando]

Gotcha, I know you can get and A/D converter for fairly cheap. It's definitely not a huge deal. I just wanted to point out the alternatives, in case you weren't aware. No doubt rolling your own circuit and code will be cheaper though a nice generic DAQ board from NI would be a nice thing to have lying around.

I haven't looked at the OBD sensor outputs that closely. I always assumed that the on board ECU performed translation on the sensor outputs prior to delivering the information to the OBD interface. For instance, it might give you a temperature reading instead of the raw voltage, current, resistence (whatever) from the actual temp sensor. If that's true, then it will probably be fairly difficult to find/discover the mappings. Even if it's not true, the OEM shop manuals still may only give output levels for known test scenarios, if anything. This may or may not be enough to help you.

Again, your best bet will probably be to determine the actual oem of the various sensors and see if you can get your hands on a data sheet. Hopefully the parts in question are not custom made for Toyota such that the data sheets are proprietary and not available for public consumption.