checking rpm with multimeter?
Thread Starter
Instructor
Joined: Apr 2009
Posts: 186
Likes: 0
From: Pensacola Florida
checking rpm with multimeter?
Wondering how to do this. I have a pretty decent multimeter but can find the instruction manual that came with it. Iwant to set my base idle and try to tune my carb that I just had rebuilt but my rpm gauge does not work. As I understand it, i need to start with the warm idle correct before I touch the mixture screws. Any info on the subject would be great. I just can't in good faith start probing around until I stumble on it.
Thanks
Frank
Thanks
Frank
Le Mans Master
Joined: Jan 2006
Posts: 9,192
Likes: 27
From: Boca Raton Florida
Why not? The tach is driven by pulses to ground on the negative side of the coil, so its basically an AC signal. DVM might not work to well, but an analog meter should, you would need to do some calibration.
Still be easier to just use a digital timing light, mine has a tack on it.
Still be easier to just use a digital timing light, mine has a tack on it.
Racer
Joined: Aug 2005
Posts: 308
Likes: 0
From: Salem MA
you need an osciloscope.
This is mission impossible
to even try to convert the equivelent RMS A/C voltage to an RPM you would need to know how your multimeter is calculating RMS, the RMS calculation would have to use a very short time window, and you would need a very accurate understanding of the dwell.
so, as everyone else said, this cannot be done.
A sunpro tachometer is like $30 and you could wire it for temporary use.
This is mission impossible
to even try to convert the equivelent RMS A/C voltage to an RPM you would need to know how your multimeter is calculating RMS, the RMS calculation would have to use a very short time window, and you would need a very accurate understanding of the dwell.
so, as everyone else said, this cannot be done.
A sunpro tachometer is like $30 and you could wire it for temporary use.
Pro
Joined: Sep 2002
Posts: 624
Likes: 1
From: Melbourne, Fla. 6 months- New Middletown, Ohio 6 months
I guess after asking myself "why would you want to go to all the trouble" I
think "Sheldon!"
Some of you will know what I mean.
think "Sheldon!"
Some of you will know what I mean.
Last edited by Red 71; Aug 4, 2009 at 09:47 AM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
There's nothing wrong with asking the question....
Your multimeter would be able to display [a relative measure of] RPM, if you first built a circuit to convert the distributor's 'pulses/time' to a voltage signal. The meter in an RPM/dwell meter is probably a millivolt meter...but it has the proper signal conditioner built into it. Making such a device would cost you more than picking up a cheap RPM/dwell meter at a pawn shop.
Your multimeter would be able to display [a relative measure of] RPM, if you first built a circuit to convert the distributor's 'pulses/time' to a voltage signal. The meter in an RPM/dwell meter is probably a millivolt meter...but it has the proper signal conditioner built into it. Making such a device would cost you more than picking up a cheap RPM/dwell meter at a pawn shop.
Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
It is possible if the multimeter can read frequencies ( Hz ).
V8 engine - 4 stroke : 6000 RPM = 400 Hz
BUT : If you've got a stock ignition with points, the coil terminals can generate very high voltage peaks ( a few hundreds volts ) and damage your device. You will probably have to use resistors.
If you have an aftermarket capacitive discharge ignition box, like MSD-6, there is a terminal for tachometers on the box that you can use directly.
To convert Hz into RPM, multiply the number by 15.
V8 engine - 4 stroke : 6000 RPM = 400 Hz
BUT : If you've got a stock ignition with points, the coil terminals can generate very high voltage peaks ( a few hundreds volts ) and damage your device. You will probably have to use resistors.
If you have an aftermarket capacitive discharge ignition box, like MSD-6, there is a terminal for tachometers on the box that you can use directly.
To convert Hz into RPM, multiply the number by 15.
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Burning Brakes
Joined: Jan 2006
Posts: 1,223
Likes: 174
From: Fort Wayne IN
Right but the AC part of a standard multimeter is expecting to measure the RMS of a constant sine wave. Your tach pulse is going to be a funky square wave that will vary in frequency with your RPM - the peak to peak voltage won't change. I'm sure the multimeter will show something if you set it in AC mode, but it won't be anything useful.
