How to Check Engine RPMs with an Analog Multimeter
An analog multimeter can be used to test any number of functioning electrical parts on your vehicle or small engine. Everything from your distributor to the variances in your alternator or even the ignition coils. There are all types of analog and digital multimeter's, that give you different readings and measurement tools.
The most common analog multimeter is used for reading current, voltage and resistance. You can find other more advanced multimeters that also allow you to read other things, like vacuum pressure for example. A simple attachment can read the magnetic field pulses coming through a wire or coil, letting you gauge by how many pulses a second the Rpm of a motor is. This attachment is an inductive pickup and can be purchased to go with most analog multimeter's.
Step 1 - Set your Multimeter to the Correct Setting
You will want to set your analog multimeter to normal or DIS setting to read an Rpm. Once you have the inductive pickup attachment set up you will be reading to begin checking the rpm of your motor. This can be any motor that has a spark plug ignite in it, from lawn mowers and chain saws, to cars and trucks. You will then want to clamp your inductive pickup around the spark plug cable of your engine, between the spark plug housing and the distributor.
Step 2 - Clamping your Inductive Pickup Attachment
Clamp the attachment around the spark plug cable, between the spark plug housing and the distributor. On a multipurpose meter, be sure the output plug of your inductive pickup attachment is connected to your analog multimeter, in the first socket selection. These should be located on the bottom left. Plug the socket end in here and watch the results.
More advanced analog multimeter's may have this multiple socket function as other models do not. On standard units this would be the normal read setting sockets, that you use to read the Rpm of the motor. Connecting to the spark plug cable, allows your multimeter to record the pulse from the motor as a charge is set off by the engine. The more charges, the higher the Rpm of the motor.
Step 3 - Reading the Results
Once your analog multimeter is connected and ready to go, turn on your engine. Be cautious as to not get cables near spinning fan blades or other rotary parts, to avoid injury. Read the secondary impulses coming from the spark plugs. The length of charge is picked up by your inductive pickup attachment, as an electromagnetic pulse. This pulse comes from the secondary ignition impulse charges from the spark plug wires as the engine pulls electricity.
The inductive pickup attachment is designed to read these sensitive magnetic pulses and send the data to your analog multimeter. This number can be used to tell you the revolutions for every minute of your engines inner mechanics, through mathematical formulas. Digital readers, can be actually set to convert the pulses into an Rpm format for you, which is one of the many reasons companies are trying to phase out analog multimeter's.