How to Test CDI Box with Multimeter

How to Test CDI Box with Multimeter

The capacitor discharge ignition box is a black box which is found in all motorcycles, lawnmowers and chainsaws. It is an essential part of your motorcycle’s ignition system. It works based on a trigger mechanism.

Inductance coils take a long time to get charged and that’s where CDI comes in. The CDI controls spark plugs and fuel injectors. Therefore, it is essential to make sure the CDI box in your motorbike is working properly. In this article, we are going to talk about how CDI works, its importance and how to test CDI box with multimeter.

What is a CDI Box?

CDI stands for capacitor or capacitive discharge ignition. A CDI box is also known as a brain box, igniter box, pulse pack, and so on. All modern motorcycles, outboard motors, chainsaws, lawnmowers, and some cars have this ignition box. It is a black box that is located under the seat of your motorcycle.

High Performance CDI Box ECM for Yamaha YFZ 450 2004-2009 YFZ450
High Performance CDI Box ECM for Yamaha YFZ 450 2004-2009 YFZ450

In their working procedures, capacitors are quite alike batteries. Like batteries, they can also store energy inside them that is to be used later. However, capacitors are unique in the sense that they can emit all their stored energy almost at once. This characteristic of capacitors makes them perfect to use in an ignition circuit.

Along with capacitors and other circuit elements, the CDI system consists of coils, a box, and a mechanism for the trigger. The working process of a CDI box is eventful. Firstly, the trigger indicates the box when it’s time to fire. The box then selects which coil to fire along with the capacitors that are inside.

Alongside the discharging of capacitors, the CDI box has an impact on your motorcycle’s rev limit. The CDI box also makes sure your engine gets charged very quickly. Furthermore, the CDI box is shunt resistant and has a short transient response.

What does a CDI Box do?

CDI or Capacitor Discharge Ignition box is available in all modern motorcycles, outboard motors, and some cars. The main function of the CDI box is igniting the motor engine. It also makes sure the motor is running properly. The CDI system is fundamental in ensuring your vehicle’s smooth operation.

The CDI box in your motorcycle comes in handy as it helps in your engine getting charged very fast. In addition, The CDI has a very short spark duration and is shunt resistant. Moreover, the CDI has a transient response which is very short.

How Does a CDI Box Work?

A CDI box is fundamental in kick-starting the ignition and combustion processes in your bike. It works through the voltage from the battery of the motorcycle and electric load discharges. The capacitor in the CDI box has the ability to start the ignition without any battery connection.

The in-built capacitor takes a pulse of voltage that passes through the box. Then it uses the voltage to ignite the spark plug.

Architectural Construction of a CDI Box

A capacitor discharge ignition box or CDI box consists of different parts. It is integrated with a bike’s ignition system and is a fundamental element of it. The main parts of a CDI box are discussed below:

Stator and Flywheel

The stator is the plate that holds every electrical coil of the CDI box’s wires. These wires are used in turning on the bike’s lights, ignition coil, and different battery charging circuits. On the other hand, the flywheel is a large permanent magnet. This magnet is rolled into a circle and used power ON the crankshaft of your motorbike.

Charging Coil

The charging coil can be found in the stator. It is used for producing a voltage of 6 volts to charge the capacitor in CDI. The charging coil also makes sure that the maximum spark is produced by supplying single pulse power to the sparking plug

The Hall Sensor

As evident from its name, the Hall Sensor is used to calculate the Hall effect. The hall effect is the exact point when the magnet of the flywheel changes from a north pole to a south pole.

The Timing Mark

The random alignment point that is shared by the engine case and stator plate is called the timing mark. It is a point where the top of the piston’s travel is  identical to the trigger point that is situated on the stator and flywheel

Trigger Circuit

It is a switch that uses a Transistor, SCR, or Thyristor. The trigger circuit gets triggered by a pulse coming from the stator’s Hall sensor.

CDI Ignition Schematic Diagram

In order to demonstrate how CDI circuits are constructed and works, we are going to use Honda C90’s CDI ignition box. The circuit diagram of the CDI box of C-90 is shown below:

CDI Ignition Schematic Diagram


As you can see from the circuit diagram that it has two resistors, R1 and R2. Both these resistors are 5.6 ohms 05 W resistors. There are two capacitors C1 and C2, both 2uF 400V. These capacitors are Mylar capacitors.

There are also 3 1N4007, 1000V 1A diodes(D1, D2, D3). And finally, there’s a  TIC 106D SCR 5A 400V semiconductor-controlled rectifier or silicon-controlled rectifier (SCR1).

After the flywheel starts rotating, the magnetic field cuts the core of the charging coil. This makes sure that AC voltage at the coil flows through the D3 diode. The AC current that flows through D3 will turn into DC to charge in both the capacitors (C1 and C2).

In the meantime, on the other side of the coil, the current will go through the R1 resistor to the D1 and D3 diode. This current will charge the C1 and C2 capacitors. Moreover, the current that flows through R1 will lead K of SCR, and that current will flow through R2 to the full circuit at G lead of semiconductor SCR1.

After that, there is a voltage drop in R2 that triggered the G lead of SCR2.  Then the current will go through both the capacitors and A and K lead of SCR1. Next, it will flow through D3 to the Primary of the high volt coil.

However, when the SCR1 is not running, there is not any current flowing to the high volt coil. But, the magnetic field of the high volt coil collapses to cut with the high volt coil core. This causes the induced current to flow out and the secondary coils spark at the spark socket plug. This happens while the piston gets stroked up to the maximum.

Advantages of a CDI Box

There are various advantages of a CDI box. They are mentioned below:

  • You can fully charge a capacitor in a very short time. Usually, it takes 1ms to fully charge a capacitor. T
  • The CDI’s transient response time is very short. Also, it has a very fast voltage rise of 3 to 10 kV per µs. Whereas inductive systems have a voltage rise of 300-500 V/µs.
  • Much shorter peak duration of 50 to 80 µs.
  • As the voltage rise is very fast, the CDI systems are not affected by shunt resistance.

Disadvantages of a CDI Box

Despite many of its advantages, the CDI box is not completely perfect. The disadvantages of a CDI box are:

The capacitor discharge ignition system generates huge electromagnetic noise and this is the main reason why CDIs are rarely used by automobile manufacturers.

The short spark duration is not good for lighting relatively lean mixtures as used at low power levels. To solve this problem many CDI ignitions release multiple sparks at low engine speeds.

  • The CDI system produces harsh electromagnetic noises.
  • For igniting comparatively lean mixtures used at low powers, the short spark duration of CDI is bad.

Troubleshooting a CDI Box

Troubleshooting a CDI box is tricky. You always have to be on the lookout for bad CDI box symptoms.

Any time your motorcycle is having electrical problems, there’s a high chance that it has got something to do with the CDI box. The longevity of a motorcycle depends on having a fully functional electrical system.

Add to that, the cost of repair is also very high for a CDI box. But it is essential to fix your CDI box whenever there’s a problem. The signs of a bad CDI box are:

  • Backfiring
  • Misfiring
  • Dead cylinders
  • Rough running
  • Problems with engine starting
  • Rough running
  • Engine getting stalled

However, these symptoms can also be associated with other problems. Fuel pump issues, bad sparking plugs, etc can also be the reason. However, these parts are easier to test than a CDI box. Therefore, it is wiser to diagnose them first.

On the other hand, the CDI components are sealed inside a box. These boxes are tricky to deal with. Although, you can take help from your bike’s manual. All bikes provide you point by point guide on how to troubleshoot your CDI box.

However, our recommendation is either resorting to a professional or replacing a suspicious CDI box with a new one. As it is very complicated and tricky to diagnose a CDI box at home, taking a  professional’s help is strongly recommended.

How to Test CDI Box with Multimeter?

There are different ways to test your motorcycle’s CDI box. You can use different testing tools for testing the CDI box. Oscilloscope and multimeter are two of the most effective tools for testing your CDI box. The process that will be discussed here bypasses the CDI box to check for problems in the CDI box.

Fluke 87V MAX True-rms Digital Multimeter

To begin with, with oscilloscopes, you can carry out more complex tests. However, oscilloscopes are not that easily accessible for many people. Therefore, multimeters are the most available testing tool for your bike’s CDI box.

To test your bike’s CDI box with multimeters, first, you need to find a connector under the seat of your motorcycle. This is a connector that comes with a white and blue wire. The connector comes directly from the stator and goes straight to the ignition box or CDI box of your motorbike.

After finding the connector, set your multimeter at a resistance of 2k Ohms. Then, take a measurement of the resistance between the white wire to the blue wire.

After that, measure the resistance between the ground to the white wire. The ideal resistance of the white wire to the blue wire should range from 77 to 85 and 387 to 487 should be the resistance for the white wire to the ground.

However, if the results are not in the specified range, then there might be an issue with the stator. In that case, you need to make an assessment of your stator. On the other hand, if you have got results within the specified range, it’s time to reconnect the white and blue wires.

Moreover, you will also see black, yellow, and white wire with white-stripe cables that are going to your bike’s front from your CDI box.

Now, you need to make sure that your multimeter is reading in DC volts and the range is set within 250. Then you need to clamp your multimeter’s black probe to the ground. After clamping your multimeter you have to test yellow and pink wires in turn. You have to press the probe that is at the cable’s end.

While testing and probing at the end of the cable, it is recommended to try cranking your bike for a few seconds. The multimeter reading you get will not be a steady reading.

However, if your CDI box is working properly, then you should get a flicker of 150v DC on your multimeter.

Afterward, when you are sure both sides are working properly, it’s time for testing the wire that has white and black stripes. You have to also make sure that the killswitch is properly running. Now, after testing if your results are not the same as before, there’s a chance that your killswitch is getting shorted to the ground.

Finally, you have to press the killswitch and observe if your bike is failing to stop. If it is failing to stop while pressing the killswitch, when your bike is still running it is necessary to disconnect the white and black stripe wire and ground it.

Now, if your bike is not stopping immediately, then there’s a high chance that your CDI box is faltering. This whole process of testing your CDI box with a multimeter is also called “ignition coil resistance test” or “bench test” or “two stroke ignition coil test”.


Diagnosing and troubleshooting CDI boxes is very essential for your motorbike’s engine and ignition system. A faulty CDI box can harm your motorcycle’s batteries too.

There are various ways to troubleshoot and test your CDI box. But the most convenient one is testing it with the multimeter as it’s easily available and the testing process is simpler too. So, every now and then. If you face trouble with starting your bike or have issues with charging your batteries, you should conduct the ignition coil resistance test or two stroke ignition coil test with a multimeter to check for problems.

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