Use a Multimeter to Read RPM - DIY Tach
What You Need To Make It Happen
1.) A multimeter capable of reading Frequency (Hertz). 2.) A short (3-5 inch/8-12 cm) length of semi-stiff wire. 3.) Access to a spark plug wire. 4.) A calculator (or math whiz).
Explanation & Caveats
The How and the Why
A DMM (digital multimeter) generally detects signals via a direct connection (i.e. over a wire).In this case, we're taking advantage of the unusually high level of EMI (electromagnetic interference) produced by most ignition systems so that a direct connection is unnecessary and we still get a good reading.
IGNITION SYSTEMS (AND SPARK PLUG WIRES) CONTAIN HIGH VOLTAGE - BE CAREFUL!!!
Making It Happen
G'Wan! Do It!
First, remove both red and black leads from your DMM (multimeter). Strip about an inch (2.5cm) of the insulation from the short length of wire, then bend the bare metal stub over on itself so that it will just fit the DMM's positive (red) socket, maintaining a metal-to-metal connection and remaining upright on its own. At this point, you should have a DMM with a sort of antenna sticking out of the positive socket.
Now, set the DMM to read Frequency or 'Hz', indicating cycles per second.
Start the engine in question and situate the 'antenna' of the DMM as close as possible to a spark plug wire, being careful to stay well clear of any moving or hot parts.
Interpreting the Results
Whaddya Mean My Engine Is Doing 25 RPM??!!
Write down the readings in Hertz, or whip out your pocket calculator and convert on the fly!
Keep in mind, your multimeter is reading in pulses per SECOND, so it should be clear what the calculator's for at this point.
Simply multiply your readings by 60 to obtain the RPM reading.
If you want to set the idle RPM on a vehicle, one strategy is to find out beforehand what RPM range is ideal and then do the conversion so you can simply adjust to a certain Hz reading (or in the ball-park, since most DMMs will resolve tiny fluctuations in RPM (tenths and hundredths of Hz) usually at several samples per second; much better on both counts than most cheap aftermarket digital tachs...)
I also suggest charting the typical RPM values you'll be working with on a piece of paper, PDA or laptop. See below:
HZ x 60 = RPM
20.0 Hz = 1200 RPM
21.0 " = 1260 "
21.5 " = 1290 "
22 = 1320
23 = 1380
24 = 1440
25 = 1500
26 = 1560
27 = 1620
28 = 1680
29 = 1740
30 = 1800
If you end up with a ridiculously high reading, keep in mind that it is very possible that your reading may be DOUBLED due to a very common ignition peculiarity known as a 'wasted' spark which occurs on the exhaust stroke of some 4-stroke and 2-stroke engines.
My buddy Steve Hawking made sense of this quirk of quantum RPM theory with the following rather clever pronouncement: "Divide by 2 or make a custom chart. Now stop bothering me." Steve-O is a swell guy.
This provides a more sensitive tachometer (tach) for engine diagnostics and tuning than the $20-and-up meters that are generally available, which have a useable resolution of about 250 RPM and update once per second at most without smoothing. Without smoothing, most cheap meters are just as 'jumpy' as the DMM-tach, but even an entry-level DMM should resolve to tens of RPM.
Some DMMs will lack shielding or otherwise, due to various other factors, will not be suitable for use near sources of EMI. Other factors inevitably come into play, so if something doesn't work as it should, change things around a bit. Try a different length of wire or a thin metal rod, a different DMM, a different engine, etc. With time, inspiration and effort, almost anything is possible.