How to Measure Engine RPM with a Multimeter


Keeping track of your engine’s rpm will help you determine its functionality, prevent future damages from occurring, assist you to know the right time to change the gear, and enable you to boost your car’s mileage. As you accelerate your car’s speed, the rpm increases, and more fuel is used. The rpm decreases as you slow down your vehicle.

To use a multimeter to measure your engine’s rpm, start by connecting the multimeter to the hanging wire, then bringing it closer to the spark plug wire after starting the engine. Record the frequency displayed and convert the frequency to rpm by multiplying the Hz by 60.

However, a multimeter may not the best tool to measure the rpm of an engine; the best tool to use is a tachometer which converts ignition pulses to voltage and displays it in meters. Read along to know how to measure engine RPM with a multimeter.

How to Measure Engine RPM with a Multimeter (Digital/Analog Multimeter)

A multimeter is a device that measures flowing current, voltage, and resistance. There are a variety of multimeters manufactured for various functions and benefits. However, the best one to use for our case is a digital multimeter.

The steps below will assist you in measuring the engine rpm using a digital multimeter.

  • Begin by disconnecting the black and red lead from your multimeter and placing them aside.
  • Take a wire and at one end detach it by removing the rubber insulation to leave it bare.
  • Bend the detached side to form a u-shape. Place the u-shaped wire in the red probe socket that is visible on your device.
  • Set your multimeter to measure in Hz which is the frequency of the rpm. Start your engine and after a few minutes move your multimeter closer to the spark plug wire which is along the protruding visible wire. Make sure the two are not meeting.
  • Record the frequency readings which you will use to calculate the rpm.

The frequency shows the number of pulses per second for your engine. Therefore, to get the revolution per minute, you will have to multiply by 60. With that simple calculation, you will have your rpm measurements.

What is a Low RPM and How Does it Cause Surging?

When the engine runs fast, the rpm tends to go up but when it slows down, the rpm declines. A normal car has an rpm of between 800-1000. If the rpm reduces beyond 600 it is said to be below. However, when the engine rpm keeps increasing and decreasing constantly, the car undergoes surging. Surging mostly occurs at low rpm or when the car is at an idle state. It is noted when the car vibrates, produces some weird sounds or shakes a lot.

Common Symptoms of a Bad RPM or a Low RPM

Faulty engine sensors

If your car has a malfunctioned temperature sensor, it will always mislead the computer to have an incorrect engine combination. The air sensor is responsible for detecting the amount of air in the engine; therefore, it prompts the computer to prepare a proper mixture with the air. If it becomes faulty, you will end up having low rpm, consuming more fuel, and surging due to a bad mixture.

Clogging of Fuel filters

Fuel filters are used to sieve fuel for possible contamination before mixing it. These filters sometimes get clogged and end up blocking the proper flow of fuel. This may result in surging and backfire engine problems.

Damaged fuel injectors

Fuel injectors are used to place fuel into piston cylinder after air is proportionally mixed with fuel.  Faulty injectors may fail to inject the mixture on time, causing rpm to fall and lead to surging.

Clogging of fuel pump

The pump sucks fuel from the tanker and places it in the mixing chamber. Over time the pump might clog resulting to wear and tear, hence stopping working. A faulty pump prevents a smooth mixing of fuel causing low rpm.

Clogging of carbon on the spark plug

Carbon accumulation on spark plug may cause a lousy spark or lack of spark at all. The spark is essential to bur fuel for the generation of maximum power.

Conclusion

All drivers enjoy a smooth and problem-free ride experience. To have a flawless performance, you will have to regularly take your vehicle for check-ups and servicing. Since modern motors are computerized, carrying out a computer scan once in a while will help detect any issue with your car and its components. A multimeter will help you detect a low rpm on time and this will give you time to deal with the issue.

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