How to Check RPM with a Multimeter (Step-by-Step Guide)

RPM is the abbreviation for Revolutions Per Minute and is a valuable measurement for determining how hard your engine is working. Most drivers consider rpm fluctuations as a minor inconvenience. However, if the RPM fluctuates strangely when you step on the gas pedal, it’s worth looking into because there might be an underlying problem.

In general, to measure RPMs with a multimeter, set up the device’s dial to the hertz (Hz) value. Connect the multimeter’s black test lead jack to the black ignition coil. Take the reading value after turning the engine for one minute. Multiply the value by 60 (sec) and that is your engine’s rpm.

To further understand this guide, let me show you how to check engine RPMs with a multimeter in detail.

Things You Need

  • Digital Multimeter
  • Calculator
  • Access to the spark plug wires in the engine
  • 4-inch wire piece

How to Check Engine RPM with a Multimeter in 5 Steps

Step 1: Get your digital multimeter and set its rotary switch to the hertz (Hz) value.

mechanic clipping the RPM of Kawasaki motor

Step 2: Take your device’s black lead jack and clip it to the black terminal ignition coil, which is often on the motor’s left/negative side.

Step 3: Start the engine of your automobile or vehicle, and keep it turned on for one minute.

Step 4: After a minute, take note of the value or readings after one revolution, and now we measure RPM

mechanic testing the RPM of a Kawasaki motor with a multimeter at 027v reading

Step 5: This is how you convert it to RPMs. The pulses per second of the engine are displayed in the findings. As a result, to calculate the RPM, multiply by 60 seconds (1 minute), and you’re done.

For example:

During the test, the multimeter reads at 22 Hz.

Multiply that value (22 Hz) by 60 sec. to get the RPM, which is 1,320.

Tip: If the engine switches off for less than a minute, your motor or vehicle’s rpm is lower than recommended. That is when you need to adjust the rpm in an idle state.

Common Symptoms of RPM Fluctuation

When trying to figure out why your car’s RPM is fluctuating, a few factors should be considered. If you check RPM and it fluctuates when accelerating, it’s most likely due to worn-out spark plugs, but there are a few additional possibilities:

Faulty Idle Air Control Valve

When there’s a problem with the idle control valve, and you notice engine RPM fluctuations, it’s typically due to a lack of communication. To correctly manage airflow and maintain constant RPMs, the IAC and ECU must communicate. If there is any interference, the IAC will not function correctly, causing RPM irregularities. Engine stalling and choppy idling are two more classic signs of a faulty IAC. (1)

Filthy Fuel Injectors

Grime and other contaminants in the gasoline cover the fuel injectors. When this happens, acceleration becomes jerky, and attempting to accelerate from a stop or maintain a consistent speed might result in the vehicle losing power. Your vehicle’s performance may deteriorate, and its fuel efficiency may dramatically decrease. (2)

Spark Plugs

mechanic getting out the spark plug of a Kawasaki motor

Check RPM variations because it is a frequent sign of damaged spark plugs, particularly in older automobiles. Worn-out spark plugs might cause slow acceleration. Misfires and jerks may occur when your vehicle accelerates. Remember that if your spark plug wires have gone bad, you may suffer identical symptoms.

Malfunctioning Engine Sensors

A malfunctioning temperature sensor might lead to the computer preparing the incorrect engine combination. Low RPM and surging might result from the incorrect combination. The air sensor will detect the amount of air in the engine, causing the computer to prepare the right air mixture.

If that sensor fails, it will burn more gasoline. A faulty mixture also causes surging and low RPMs.

Take a look at some of our related articles below.





References
(1) lack of communication – https://psychcentral.com/blog/is-lack-of-communication-a-red-flag
(2) gasoline – https://www.sciencedirect.com/topics/chemistry/gasoline

About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gI realized early on carpentry was a huge passion for me and I’ve stayed in the industry for over 20 years now. This gives me a unique ability to really be able to tell you what the best tools and recommendations are. I’m not only a carpenter but I also like machinery and anything to do with electrics. One of my career paths starting off was as an apprentice electrician so I also have a lot of experience with electrical products and anything related.