How to Test a CDI Box with a Multimeter

In your automotive vehicle, the CDI is one of the most important components. What is a CDI box and what does a CDI box do?

In a motorcycle, a CDI is a black box underneath the seat that functions as the heart of your ignition system. It is an electronic component that replaces the pre-1980 mechanical ignition processes and one that your bike can’t work without.

Nonetheless, just like every other component in your bike, issues arise with it and making a diagnosis may be tough.

This article presents you with all you need to know about a CDI box. Let’s get right in.

How A CDI Works

Here is the system of components in a CDI:

Source: Usman032

As the key is turned, the rotating magnet induces a voltage of up to 400Vac into the Excitor coil. When this coil becomes positive, the charge is directed to the forward biased diode till the capacitor gets fully charged ( usually after 3 to 4 magnet rotations).

Once the capacitor is charged, the pulse rotor sends a trigger to the SCR which, in turn, initiates a conduction process that discharges the capacitor immediately. This sudden discharge produces a high spike of voltage in the ignition coil.

High current on both contacts of the spark plug is created and this provides power to the engine.

The ignition stop switch grounds all excess voltage.

Symptoms Of A Bad CDI

Of course, before getting into your CDI, you want to be sure that there are problems with it. Here are some of the symptoms your bike may be exhibiting that point you to issues with a CDI.

  • Misfiring Engine
  • Dead Cylinder
  • Unusual  Tachometer Behavior 
  • Ignition problems
  • Engine stalling
  • Backfiring engine

These symptoms represent problems with particular components of the CDI box. For instance, a misfiring engine may be caused by either worn out spark plugs or a worn out ignition coil. A dead cylinder may also be caused by a bad ignition coil or a bad diode.

Identifying exactly where the problem lies helps you to easily make fixes or replacements and get your ignition systems up and running again. 

How do you identify these problems? A multimeter proves useful throughout the whole process and here is how you test your CDI box with it.

Tools Needed To Troubleshoot Your CDI

All you need is your;

Before moving forward, you also need to take safety measures and protect yourself. These measures include wearing cut and water resistant gloves as well as eye safety equipment. 

cdi box and multimeter

How To Test CDI Box With Multimeter

To test your CDI box, you disconnect it from your bike, use the positive and negative leads of your multimeter to check for continuity, and listen for a beeping sound which signals that it is faulty.

There is a whole lot more to this seemingly straightforward process and here is further details on it.
To test your CDI, you engage in both cold testing and hot testing procedures. Cold testing is when you run diagnosis on the CDI box with it disconnected from the stator, while hot testing has it still connected to the stator.

Do the following.

Step 1: Remove The CDI Box From Your Bike

This is for cold testing procedures. The CDI Box is typically located underneath the seat of your bike. As you check, you should see a blue and white wire connecting the stator and black CDI box together through lead and pin connectors.

Once disconnected, you avoid working on your CDI with any equipment for about 30 minutes to an hour. As the internal capacitor discharges during this waiting process, you carry out a visual inspection on your CDI.

Visual inspections may allow you to quickly identify physical deformations on the CDI.

Remove The CDI Box From Your Bike

Step 2: Run A Cold Test On Your CDI

Cold testing involves inspecting the continuity of your CDI box components. What you do is set your multimeter to continuity mode and test the continuity between the ground point and other terminal points within the CDI.

If a problem exists, your multimeter gives off a beeping sound. You know the exact component that has issues and a fix to this component may be the solution.

Continuity issues within a CDI are usually caused by problems with the SCR, Diode, or internal capacitor. If these cold steps prove a little difficult to follow, this video from youtube may help you out.

Step 3: Run A Hot Test On Your CDI

If you don’t want to disconnect the CDI from the bike, you could run a hot test. Tests are made from the stator end of the blue and white wire that connects it to the CDI.
To do this, you set your multimeter to a 2k ohms resistance and measure resistance between these two points; the blue wire to the white wire and the white wire to ground.

For the blue wire to the white wire, you check for a resistance reading between 77 and 85. With the white wire to ground, you use your multimeter to check for a resistance between 360 to 490 ohms. If any of these don’t match, your stator could be faulty and a professional mechanic could prove useful.

If they match, however, then your CDI is a likely culprit. 

FAQ About CDI box

How Do I Know If My CDI Box Is Bad?

You know a CDI box is bad when your bike misfires, has dead cylinders, unusual tach behavior, runs roughly, experiences ignition problems, or its engine stalls.

How Do You Bypass A CDI Box?

To bypass your CDI box, you clear your bench, remove the box, check resistance specifications, measure the primary and secondary oil resistance, and compare readings.

Can A Bad CDI Cause No Spark?

A bad CDI box may not cause a spark at all. However, your bike shows symptoms like ignition problems, dead cylinders, and engine stalling.

Can A Bike Start Without CDI?

A bike will not start without a CDI box as it is the component that controls the ignition system.

Are CDI Boxes Universal?

No. CDI boxes are not universal because ignition systems vary across different vehicle models. They come as either AC or DC.

How Do You Test A Four Wheeler CDI Box?

To test the CDI box for an ATV, you use a multimeter to inspect the fuses, ignition switch, ignition trigger coil, electronic module, and check for loose wires.


The CDI box is an important component of your vehicle’s ignition system and you should take good care of it. No matter how clear these steps may be, hiring the expertise of a professional mechanic seems a better option.