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Measuring RPM

Last Updated: Sep 25, 2018 04:20PM NDT

Important Safety Information

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Before getting started on measuring engine RPM with your BlueDriver Automotive Multimeter there are some important things you should consider to ensure your safety:
  • When working with the electrical system always assume anything you touch could be potentially energized
  • Disconnect the battery and check for voltage using your multimeter before you start any work
  • Be aware of the position of your battery and its terminals - an accidental short between the positive and negative terminals could cause damage or injury (especially if you short it with a limb)
  • When working in the engine bay with the motor running, watch out for potential pinch/crush points, stay clear of:
    • belts & pulleys
    • radiator fans

      important: rotating equipment may start/stop without warning
       
  • Stay clear of coolant hoses, the exhaust manifold, and any other potential sources of heat
 

Background Information

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To measure RPM you'll need to know a few things about your motor, including:
  • What type of ignition system you have
  • What type of motor it is (four vs two stroke)
  • How many times the spark plug fires per rotation of the crank shaft
 

Ignition Systems

 

Conventional

This is the most common ignition system in use today - if your vehicle was sold any time between the 90's and a few years ago you've probably got one of these. With a conventional ignition system high voltage is generated in the ignition coil then travels through the spark plug wires, to the spark plugs where it creates the spark which triggers the combustion.

Your vehicle may have a dedicated ignition coil for each cylinder or a combination 'coilpack' where all coils are contained in one housing. It's important to know which your vehicle has as it may influence which RPM setting you use on your multimeter.
 

Dedicated Ignition Coils

If you start at a spark plug on your motor and trace the cable back it will lead to a small module attached to the head or engine with several wires running to it. If there's a separate module for each spark plug this means you have an ignition coil for each cylinder which will produce high voltage to fire the spark plug once near the end of the compression stroke.
 

Coil Pack

If you trace the spark plug wires back and more than one leads to the same module this is a coilpack and controls the spark for more than one cylinder. This is important because on some vehicles a coilpack might fire the spark plugs for two cylinders at the same time, which will require a different setting on your multimeter.
For example, consider a motor where two cylinders are on separate strokes of the combustion cycle but connected to the same ignition coil. When cylinder A is at the top of its compression stroke and cylinder B is on the exhaust stroke spark plugs A & B will both fire at the same time - this may be referred to as 'wasted spark' since plug B will not have any effect on engine operation. This is important for RPM measurement as the multimeter will see an ignition event for each rotation of the crankshaft and if it is on the wrong setting it will display double the actual RPM.
 

Coil On Plug

Coil on plug setups are similar in operation to conventional ignition systems with the exception that there are no spark plug wires - the ignition coil is it direct contact with the spark plug, sitting on top of it. Similar to conventional systems, you may have a dedicated ignition coil on the top of each plug, or a coilpack 'rail' sitting across a bank of plugs.

Distributor/Ignition Coil

Vehicles predating the 1990's may use a mechanical system for controlling spark timing. A device called a distributor is driven by the engine using a shaft and gear - the rotor inside the distributor is connected to an ignition coil, as it turns contacts are momentarily closed which transfers current through the plug wire and to the spark plug for each cylinder in turn.

 

Small Engine/Magneto

Small engines will typically forego automotive ignition systems in exchange for a much simpler system. A magneto works similarly to an electric motor (only backwards) - as a flywheel attached to the engine rotates, a magnet passes through an armature which creates a momentary electrical pulse. This high voltage pulse is used to fire the spark plugs - since there is one pulse for every rotation of the crankshaft you may see double the expected RPM if the multimeter is on the wrong setting.

 

Diesels

Diesels do not use sparkplugs to ignite the air/fuel mixture - as a result engine RPM can not be measured with the use of a multimeter.

 

Motor Type

 

Four Stroke

Four stroke motors are the most common type in automotive applications. A four stroke motor goes through four distinct stages for each cylinder during one operation cycle, which involves two full rotations of the crank shaft:
 
  1. Intake
    The piston starts at the top of the cylinder and draws down as the crank shaft rotates 180 degrees. As the pistons moves down the intake valves open and air/fuel are drawn in.

     
  2. Compression
    As the crankshaft continues to rotate the intake valves close and the piston travels back up the cylinder - compressing the air/fuel mixture

     
  3. Power/Combustion
    As the piston reaches the top of the cylinder the spark plug fires and ignites the air/fuel mixture. As the mixture ignites it expands and pushes the piston back down - rotating the crank shaft 180 degrees again.

     
  4. Exhaust
    As the crankshaft finishes its second rotation the piston travels upwards again and the exhaust valves open - allowing the combustion gasses to be pushed out of cylinder and leading back to step one.


The important thing to know is that (except in the wasted spark example) a cylinder's spark plug will fire once for every two rotations of the crank shaft.
 

Two Stroke

Typically two stroke motors are found in smaller applications - chainsaws, grass trimmers, some dirtbikes, etc. A two stroke can be identified by the need to mix 2-cycle oil in with the gas. As you might guess from the name, there are two stages in one cycle of a two stroke motor:
 
  1. The piston travels downward following combustion of the air/fuel mixture by the spark plug. As it travels downward exhaust gasses are driven out of the cylinder and it also starts to draw in fresh air/fuel as the crankshaft approaches 180 degrees.
     
  2. As the piston travels up on the second half of the crank rotation, it continues to drive out exhaust and draw in air/fuel at the same time. As the piston approaches the top of the cylinder all ports are sealed and it compresses the new air/fuel mixture in preparation for combustion.


The important distinction is that because a two-stroke executes a power cycle for every one rotation of the crankshaft, the spark plug will fire once per rotation as well.
 

Measuring RPM


Note: Ignition systems can vary by application so the proper "Range" setting for your vehicle may be different from those shown below. If your measured RPM is either half or twice the expected value you may need to press "Range" to change the number of pulses per rotation.
 

Before you start:

  • The black lead is connected to the COM port
  • The red lead is connected to the RPM/Resistance/Voltage port (third from the left)
  • The Selection dial should be on "RPM" unless you will be measuring a value greater than 10,000 RPM - in which case it should be set to "x10RPM"
  • The black lead should be connected to the negative terminal of the battery, or any other good ground. If you can't reach/access the battery you can use most bare metal surfaces or look for a wire that is bolted to the body/frame somewhere - this is likely a dedicated ground point.
 

Conventional & Coil On Plug

Red Lead
  • Connected to the negative wire leading to an ignition coil or the coilpack.
  • You can either strip the insulation/splice a wire in, or use a paperclip or other thin piece of metal to contact the wire inside the connector by pushing it through the seal.

    Important: Exercise caution when stripping the wire or tapping in to the harness/connector - damaging the insulation, wiring, or seals may lead to issues with the ignition system, misfires, etc


     
  • If you are not able to access the negative ignition coil wire and are working on a vehicle with dedicated ignition coils you can unplug one of the coils and connect the lead directly to the negative pin on the plug.

    Note: If you disconnect an ignition coil and start the motor it will run poorly and will illuminate the check engine light/throw codes for misfires.


Range Setting
  • When measuring RPM from a dedicated ignition coil Range should be set to (4)
  • If the motor has a coilpack you may need to set range to either (2) or (4) depending on whether multiple cylinders share one coil. If the measured RPM value seems either 2x higher, or half of what it should be then try the other setting by pressing "Range"
  • For two stroke motors Range should be set to (2)
 

Distributor

Red Lead
  • Connected to either the negative wire leading to the ignition coil, or the "Tach" port if present on your distributor
Range Setting
  • If all spark plugs fire once per rotation Range should be set to (2) - if the spark plugs on your vehicle fire once per power cycle (every two rotations) then range should be set to (4)
  • For two stroke motors Range should be set to (2)
     
A note on measuring RPM from the distributor: If the tach port produces one pulse for every ignition event you may need to divide the displayed RPM by the number of cylinders to get crankshaft RPM. For example if a V8 shows 6000 RPM at idle you divide by 8 to get an engine RPM of 750.
 

Small Engine/Magneto

Red Lead
  • Connected to the lead on the ignition coil or the kill switch wire.
    An important note on kill switches: On some small engines connecting the multimeter to the ignition coil may 'ground' (despite the internal resistance) it out and act as a kill switch, causing the engine to shut down.
Range Setting
  • Often the ignition coil will fire on every rotation of the flywheel, in which case "Range" should be set to (2)
  • For two stroke motors Range should be set to (2)

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