OBDuino RPM output?

[Gargamel]

Hello! I am quite new to Arduino and its programming. I have an electronics background and enjoy all things automotive. In my spare time, I do a lot of automotive wiring projects, mostly the ones nobody wants to tackle. Past projects include an '05 wrx running gear in a sand rail, '92 acura legend v6 in a sand rail, and an '01 cummins with manual transmission in a '49 international panel van.
My latest project is another '01 cummins engine coupled with an automatic in a '49 international pickup. The builder is using classic hot rod style gauges, all analog, and has requested a tachometer. In a chrysler vehicle from around '01, the ECM, PCM, gauge cluster, and overhead console all communicate via chrysler's CCD bus. (a proprietary top secret language) Luckily for me, there is an OBDII port in the mix, providing me (with the help of OBDuino) all the live engine data I could ever want. Short of cutting up the original gauge cluster and incorporating it into the dash somehow, (yuck!) or using the hillbilly magnet glued to the crank pulley for RPM measurement, the OBDuino is a blessing. I have constructed mine using a bare bones board from modern devices as well as the entire elm327 circuit so i can test it in my service vehicle I drive every day, then transfer it to the project. I am happy to report that it works flawlessly and many thanks go out to those who have worked so hard on this project!
My question is: how must I modify the code to output a low pulse on an output pin to simulate the negative coil pulses of a gasoline powered engine. Being that the cummins is a 6 cylinder engine, this would equate to 3 pulses per RPM. Any insight would be greatly appreciated!

John

[dcb]

I would look up digitalwrite in the arduino.cc playground, there is probably an example in
your arduino menu (that is how blink works), but you'll need to use a simple transistor circit to make a 12v signal out of it.

fyi, there may already be a factory installed "hillbilly magnet" if your ecu has rpm data. that could also make a 12v rpm signal with the addition of a transistor (no arduino or $30 elm required) and that wouldn't have any delays associated with layers of computer processing.

It is always nice to know what is trying to be accomplished too, especially if it relates to efficiency.

Welcome to ecomodder BTW

[Gargamel]

Thanks for your reply! I will be sure to check out the playground for the digitalwrite function.
I was pondering using the existing magnetic pickup on the engine, but I enjoy the other features offered by the OBDuino, and was hoping to learn more about the arduino.
The ELM ic was a bit of an un-needed expense, but it allows me to test on multiple vehicles. In the end, I can cut costs dramatically seeing as the dodge computers use the ISO protocol which only requires use of the Freescale MC33290 ($1.39 at Mouser) instead of the ELM.

Thanks Again!