Form of tach data?

[MacGyverS2000]

Is the form of tachometer data fairly standardized in most modern cars these days (or even in earlier cars)? Is it generally a PWM duty cycle that determines rpms, or maybe a simple pulse per second count, etc.?

I would like to make measurements of rpm, but the unit should be as generic as possible (i.e., I can use it on as many vehicles as possible). If more than one common method existed, I could either build more than one unit or attempt to combine all methods into a single package with a simple, switchable universal interface.

Thanks!

 

[76GMC1500]

They use pulses, and are for the most part are standard. A 4-stroke v8 pulses 4 times per revolution, a 4-stroke 4 cylinder pusles 2 times per revolution, a 2-stroke 4 cylinder pulses 4 times per revolution. I think you can figure out what other engine configurations do. Coil on plug setups in a 4-stroke would fire once every other revolution, unless, they shared a common ground wire. Then, they would fire the same as any other similar engine.

 

[enginesrus]

PWM is what the old dwell meters would measure.
Old tachs would see the make break of the points, meaning just on and off pulses not a pulse width. If it was an 8 cylinder engine, it makes 4 pulses per revolution.
So if you have an 8 cyl engine running at 1000 rpms, you have 4000 ppm or 66.666 Hz. So what you need to measure is the frequency. Just like the dwell meter, the pulse width will stay pretty much the same no matter what the rpms are. Opps looks like 76GMC covered some of that too.
As far as newer cars, it is still a frequency deal. ie measuring pulses. Pulse width is only important for something that is controlled by on and off duration, like the fuel injector.

 

[MacGyverS2000]

Great info, thanks guys! Looks like I'll have to allow some sort of divider for cylinder count, be it a set of jumpers or software programmable.

Thanks again...

 

[76GMC1500]

Almost forgot to mention waste spark engines, they are common in single cylinder applications and many of today's modern applications. They fire once every revolution per cylinder.