How To - Measure RPM's with a Multimeter

[Save Old Iron 1,566]

If your multimeter has a HZ or FREQ function, with the right amount of fussing and experimentation, you may be able to calculate engine RPM's from the HZ or FREQ readout.

A few wraps of wire around the spark plug cable may act as enough of an antenna to pickup the high voltage pulse emission from the spark wire and allow the multimeter to see the interference / emission as a stream of pulses that can be interpreted as RPM.

View the slides below. Depending on the engine, the spark plug is either fired once per 2 crank revolutions (coil based system) or once every crank rotation (magneto fired off the flywheel.) The difference is important to know as you will need a different mathematical multiplier to calculate engine RPM.

I know, I know. Buy a $20 optical tachometer. I post his because the question was asked and understanding the principle behind this measurement allows for understanding on how all of the new "maintenance meters" function. All these $30 maintenance meters read RPM and calculate hours off detection of the electrical emissions from the spark plug wire.

First, consider the coil based ignition system with a spark produced every other crankshaft revolution.

Note the multiplier used is 120 to calculate RPM.

Next is the magneto system with a spark produced every crankshaft rotation.

Note the multiplier here is 60.

It is also very import to note the orange wire does not make any electrical contact with the spark plug wire.

DO NOT CONNECT THE ORANGE WIRE DIRECTLY TO THE SPARK PLUG

OR THE CONDUCTOR IN THE SPARK PLUG WIRE.

You could receive a high voltage electrical shock and your meter could be destroyed if connected directly to the spark plug or spark plug wire conductors.

[Coadster32 793]

Pretty darn cool. I'll have to see if my meter can do this. Thanks Chuck! :thanks:

[posifour11 723]

That's neat.

Professor, I have a dumb question. Would it be possible to make my own tach for a single cylinder? Is that sort of thing beyond a layman? I used to be good at soldering.

[Save Old Iron 1,566]

Posi,

the parts to build the equivalent of a 1970's era tach exist at your local radio shack. A transistor or two and a few capacitors and resistors are all that is needed. Even the breadboard to build it on and test it are available at Radio Shack.

The fun comes in when you start to make decisions on how to display this info to the outside world.

Mechanical pointer gauge ? LED lights? Digital numerical readout ?

What type of case do you put your new creation in ?

Is this a handheld test equipment type setup or will it be mounted on the tractor?

Where will you get the tach signal from ? points? spark plug wire? flywheel ? optical sensor?

Do you have the ability to create a meter "face" to show the RPM indication? Is this going to be a 0 to 4000 RPM range or higher?

lots of choices that quickly add up beyond the cost of a "tiny tach" http://www.amazon.co..._sim_sbs_auto_1

I for one understand the allure and the educational value of experimentation beyond the "you can buy it cheaper" factor. Soooo...

I would be glad to coach you thru construction of a prototype if you decide to take a shot at it.

The simplest version to construct would be a 70's style mechanical pointer triggered off points.

[Save Old Iron 1,566]

Pretty darn cool. I'll have to see if my meter can do this. Thanks Chuck!

Coadster,

you may need to experiment with the number of antenna wire wraps around the plug wire. Different multimeters may have different sensitivities to picking up interferences (which is what this spark "signal" actually is). Wire gauge is not important but using solid core wire will allow for tighter wraps around the spark wire.

Too few wraps will result in no or erratic readouts. Too many wraps may lead to false triggering of the meters function and result in unbelievably high readouts.

Again, this method is in no way the gold standard of sensing RPM's, but it can be done. A little patience and finesse is needed.

This is so odd. A week or two back we were trying to find a way to filter spark noise from double triggering a strobe light and this week we are trying to measure and use that very same "noise". I love electronics ............

[joel_400 59]

I have an actron timing light available from advance auto for around 100 bucks with a built in tach. Does 2 and 4 stroke and even works good for checking timing! Set the advance on the light and set from the pto end. When the keyways straight youre real close.

Joel

[posifour11 723]

SOI, I'm willinf to give it a shot. I haven't played with electronics in years, so it should be fun. The nearest radio shack is about an hour away, but i go that way a couple times a month.

In short, I'm game if you're willing to lead me through it.

[Save Old Iron 1,566]

You betcha I will.

 

Maybe it is best to start a new thread. If you don't mind folks "looking over your shoulder", start the thread here. If not, start one over on my site at SOI University or we can go offline. I have not been doing too many ambitious projects at SOI U lately due to the upcoming (now for months) switchover from FORUMER to YUKU servers. I just don't want to put too many good posts at risk of being lost in the transfer.

 

One item I will require from you up front -

 

 

 

 

Not too big, not too small.

 

Sorry, had to get that old Monty Python reference in there - a classic.

 

In all seriousness, I need your final vision for the project. When you close your eyes and vision what this will become, what do you see?

A gauge on the tractor? A handheld piece of test equipment? Meter readout? Digital readout?

 

Give a starting point to share your vision and a place to start advising you what parts to accumulate for the build.

[posifour11 723]

SOI, couldn't find a shrubber to help me. I did find a herring though....

[Save Old Iron 1,566]

I find your alternative herring proposal acceptable. You may pass into the halls of SOI University and seek the grail of higher understanding.

[rueger-99 0]

Just an update to this thread. You can do this with a Fluke 117 meter with Volt/Alert (Non-contact) set to read Hz. I'm sure other meters will do the same but this is the one I have.  No need to wrap the plug wire to get a reading, just put the meter near the plug wire.

Edited September 14, 2019 by rueger-99

[adsm08 2,173]

My BluePoint has a feature that does RPM. It just needs a spark tester clamp, which came with mine.