RPM - coil/stator measurements at last....

[meadowfield 2,570]

well after a few PM's with Chuck (SOI) and a lot of threads, I finally hooked up the scope to the coil and B+. Here are the results:-

Top waveform is coil, bottom stator

At tickover

At full revs (I don't like the knocks from my engine at 3600rpm so the governer is set low)

The new dash addition

and the numbers behind the scope traces.

So as we already knew, the coil runs at 2:1 - for every two revolution of the crank we get one fire (4 stroke). So if we get almost 7 a second we can double that to 14 and multiply by 60 for the RPM figure.

As also discussed with SOI, the number of magnets and coils on the stator give us another multiplier as for every turn of the crank we get 18 cycles with comes to a frequency of 116Hz. The scope struggles to measure both simultaneously but gets close.

What does this mean? well it means you could get a more accurate and easier to probe wire for a tacho/rev counter. The 28v pk/pk waveform is quite safe and it is much quieter than the coil - just look at those spike on the top trace.

mark

[puddlejumper 67]

To steal a quote from John Wayne...".You might be right"   When he had no idea what the chinaman said.

[Save Old Iron 1,566]

Come on all you automotive guys, this is classic stuff. Think primary ignition circuit voltage versus crankshaft sensor - it's all there in multicolor digital splendor.

 

If you can't understand this, then please ease up on the automotive techs (and electronic techs) who can and charge you $200 to diagnose an intermittent electrical problem in your new car. This is now the world diagnosticians live in to support USB, CAN, Bluetooth, etc in your new car.

 

Love the new digital dash Mark. Thanks. I could have never provided that level of clarity in display with my older Tek SCOPE. I have to be a good boy this year so Santa brings me a digital scope.

 

Do you want to explain the traces and their relevance to one another? I can certainly help in that regard. Makes perfect sense to me. The way you captured the data was brilliant and makes it easy to explain. Looks like you may have had attenuators on the probes?  And the ignition coil trace - was that collected off the negative terminal of the coil using chassis as a ground ref? 

[puddlejumper 67]

Please do, I might learn something! I normally do from these type threads.

[meadowfield 2,570]

Righto...

 

here goes, I've tried to cover it in the picture

 

 

Top trace is the coil waveform starting with in firing - there are 2 revolutions between these (4 stroke) the cycles are indicated along the top.

 

Bottom trace is the stator waveform, there are 9 coils so we see 9 cycles in one revolution of the crank ( or one cycle every 40 degrees)

 

Although the scope works it out, you can get close by looking at the number of squares in the first revolution. 

 

Just over 3.5 squares at 20ms a square, so lets call it 3.6x0.02 = 0.072

 

1/0.072 =13.89 revs per second

 

13.89 x 60 = 833 rpm.

 

Close enough!!!

 

 

hope this makes things little clearer

[neil 2,409]

By jove Mark  , I think you might be right, Thats what i made it too .

[meadowfield 2,570]

I think the main point here is you could use the B+ connection for a tachometer - as long as you divide the output by 9...