Question about a project I'm working on
 

I need to know exactly how to get an accurate rev count, is it just a voltage from the OBD, do I need to tap into the signal to the tach. TIA

well from my experience the most accurate way is to fit an autometer unit to the coilpack like this one.
http://autometer.carshopinc.com/prod...s_id/9144/9117

The MINI shift light kit connects to one of the many wires on the OBD. The install instructions are posted somewhere.
John

Yeah I have that, its actually what got me started on this project idea. With the JCW shift light install it appears that there is a simple signal (voltage) that is sent from the ECU, I tried to get a reading with this from a digital voltmeter and had a steady mV of .2 so I was thinking perhaps it was something a little more complex, I didn't know if it was something that may have been mentioned in the Bently Manual (anxiously awaiting an 05 version) beause I had heard there is an abundance of electrical information in there. My current assesment is that I should try to get some analog readings w/ ethier anothervoltmeter or go ahead with the project making it slighty less precise than I would have liked with an intial prototype to be refined later. Anyway off to do more research.

What wire did you get that reading from? .2 mv is probably a ground.

white with red stripe in the obd socket (my guess as to where the RPM signal is sent for diagnostics based on the JCW install instructions) I've been doing some reading over on MINI2 a few years back there was some hype about this, starting to think this will be more difficult than originally thought.

checked the JCW instructions, was indeed carrying the signal for thr revs, in what form is yet to be known, but alas more research to be done. From what I understand now each cylinder sends a pulse every 2 cylcles.

[quote=Profpatpending]well from my experience the most accurate way is to fit an autometer unit to the coilpack like this one.
http://autometer.carshopinc.com/prod...s_id/9144/9117 [/quote0 ] What exactly does it do? How do I install it? Not sure exactly where coilpack is, also looking to see if I can find an alternate way to fabricate something like this so I could have a compact and less expensive unit.

did you rev up the engine to see how the voltage would correspond to the rpm?

Yes unfortunaltely MINI thought that'd be too simple, the voltage stayed the same everywhere under 5k revs. From what I gather on MINI2 and a little more reading here on NAM in order to get a reading I've got to get a special tool to read the revs directly or the previously mentioned autometer converter, which is what I'm doing some more reasearch on now so I can bypass as many connections as possible eleminating points of possible error and being as cheap as possible (hey I'm a young male and I have high insurance premiums now).

Since the shift light has dip switches for the number of cylinders, My guess is that the wire carries pulses, and you might not see a change in voltage level if the "duty cycle" stays the same regardless of rpm.

J D

You want the wire that goes to pin 9 of the OBD II connector. That is the rpm signal and it is frequency based. So you'd need an oscilloscope to get the measurements. Nowdays pretty much all signals are digitized and you definitely need an oscope to see what's going on. But for now I'll save you the time as I did this today myself.

The voltage is about 13-14V, and at 1k the frequency is 33.333Hz.
At 2k 66.666Hz, etc etc. Linear frequency progression with rpm, so 33.333Hz x whatever RPM you want, and you get the frequency.

This signal is basically the same frequency as a signal from the ignition coil, but nicely digitized for easy use :)

Oh and none of the wires going to the tach can be used for the rpm signal. the rpm signal is hidden in a bus line and is for all end user purposes, useless.

thanks for the info