ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

Use a Multimeter to Read RPM - DIY Tach

Updated on July 30, 2008

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.

That said...


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.


Submit a Comment

No comments yet.


This website uses cookies

As a user in the EEA, your approval is needed on a few things. To provide a better website experience, uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at:

Show Details
HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
LoginThis is necessary to sign in to the HubPages Service.
Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
AkismetThis is used to detect comment spam. (Privacy Policy)
HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the or domains, for performance and efficiency reasons. (Privacy Policy)
Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
MavenThis supports the Maven widget and search functionality. (Privacy Policy)
Google AdSenseThis is an ad network. (Privacy Policy)
Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
Index ExchangeThis is an ad network. (Privacy Policy)
SovrnThis is an ad network. (Privacy Policy)
Facebook AdsThis is an ad network. (Privacy Policy)
Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
AppNexusThis is an ad network. (Privacy Policy)
OpenxThis is an ad network. (Privacy Policy)
Rubicon ProjectThis is an ad network. (Privacy Policy)
TripleLiftThis is an ad network. (Privacy Policy)
Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)
ClickscoThis is a data management platform studying reader behavior (Privacy Policy)