I'm realizing that this isn't quite a comprehensive DIY article yet, I'll have to clean it up when I get something working.
Status, instant mpg isn't nearly as good as a short term average, so I'm shying away from the analog/tach dwell approach.
I was going to persue obdII/ISO after assembling a real trivial circuit. BUT the scangauge stopped working on the saturn (AGAIN?!?) so I need a non obd solution for that car as the scangauge is back in the metro.
So currently I'm trying to load up Visual C++ 6.0 on an old win 98 laptop that has a line-in jack and will try and modify some simple windows based oscilloscope source I found to keep track of how long the injectors have been open and how many clicks we've travelled. It may miss a few beats under windows, but it should be close enough (and provide a reasonable platform for keeping other useful apps on, i.e. some maps and whatnot)
A more hardcore embedded person would do it with a handful of counters and a PIC (i.e. basic stamp) and have a standalone device. But the problem becomes the fact that they have to put more complicated circuts together and purchase a special programmer to load the BASIC (or C++ or assembler or whatever) program onto the chip and learn how to operate the programmer, or someone has to sell chips to the "Do It yourselfers".
I see a lot of utility in having a full fledged computer handy though, play mp3 files, talk to your lan from your driveway, run maps/whatever. All my portable computers can run on 12 volts directly (no accident). I've even got a ramline 510 touchscreen (used to be in a cop car) that needs to find a second life which would be ideal for this. But the ramline is a PITA to develop on (as are embedded chips), which may be my real motivation for this approach
So back to an OBD solution then, instead of tapping the injector's duty cycle directly?
Too bad for my '93 OBD-1...
I'm NOT currently planning on doing OBD, it won't help me as my saturn is not happy with OBD MPG attempts, for whatever reason, and I'd like to get something out of my efforts.
I'm planning on the audio approach on a laptop for ease of implementation and more universal application. Any any car with electronic FI should work. One just need a laptop with a Line-In to run the mpg program on and a simple circuit to limit the voltage going in.
44100/8 bit/mono recording using a diode clamp interface and a 10k resistor on a 133mhz laptop with integral sound card via line in left channel . Car in neutral in driveway. Probe connected to yellow wire between fuel injector resistor and fuel injector.
3 samples, 1 low rpm, 1 accalerating to higher rpm, 1 high rpm. Images stretched horizontally for easier duty cycle visualization/comparison.
for the above images. It was a raw wav, so I could pipe it through a program for development, but it is here converted to mpeg 3 compression for space considerations. Recorded with windows sound recorder and subsequently compressed with sound recorder.
if you run it in windows media player, set the visualizations to "scope" by:
views->visualizations->bars and waves->scope
OOPS, this is the recording of the wrong channel!! The other side records a bit cleaner. I sure hope there isn't this much crosstalk when the other channel is plugged into the VSS.