How Do You Bypass a CDI Box?

A capacitor discharge ignition box (CDI) is utilised in motorcycles, turbine-powered aircraft, outboard motors, and certain automobiles. A CDI box can be temporarily bypassed for diagnostic purposes. When your fuel-air mixture is ignited, the box amplifies the little signal generated by the rotating magnet in front of the pickup coil in the stator, immediately before top dead centre (TDC). The unit is unable to produce continuous sparks. Therefore, it cannot store sufficient voltage to keep the engine running. Follow the instructions below to effectively bypass your CDI box by conducting an ignition coil resistance test (also known as a “bench test”).

Empty Your Seat

Because it can ignite the air and fuel mixture that powers your engine, you must ensure that your workbench is devoid of flammable liquids such as gasoline. The battery must be tested to ensure that it is fully charged; otherwise, it must be replaced.

Eliminate the CDI Box

The black container includes two plugs and a bullet connector. After locating it, you must remove it to gain easy access to its electrical terminals. At this stage, your repair manual comes in helpful, as you must follow specific instructions to remove the CDI box. The vehicle must be turned off and allowed to cool. Use an ohmmeter to quantitatively measure the electrical resistance of the CDI box.

Check the Specifications for Resistance

The electrical resistance of the coil varies between automobiles. If the correct resistance levels of the coil do not satisfy these standards, you will know that the coil is defective. Typically, the resistance criteria for your particular vehicle can be found in the service manual. After connecting the wire from the stator to the motor to the ground, detach the main wires from the CDI box.

Determine the primary winding resistance

Three electrical connectors, which may be external or internal, are available on the distributor. Touch the exterior electrical contacts with your ohmmeter and note the resistance reading. This is known as the primary winding resistance of the coil.

Measure the resistance of the secondary coil

Touch one of the ignition coil’s outer contact leads and the inner, central contact. Here is where the distributor’s primary wire joins. Document the reading. This is known as the ignition coil’s secondary winding resistance.

Compare the Literature

The ignition coils of the CDI box are fragile components of the electronic ignition system of a vehicle. If either the primary or secondary windings deviate from the vehicle’s requirements, you may need to replace your CDI box, since the existing one may be defective. A malfunctioning CDI box is characterised by an engine spark, whereas a properly functional box prevents the engine from sparking.

Advantages of a CDI Box

The primary advantage of a CDI box is that its capacitor can be charged in the shortest time feasible. Due to the rapid rise in voltage, shunt resistance has no effect on the CDI box. The CDI box system offers a lower transient response and spark duration than inductive systems.

Consequences of a CID Box

The CDI system generates enormous electromagnetic sounds that automotive manufacturers find objectionable. The short duration of its spark makes it unsuitable for lighting somewhat lean mixes. The emission of multiple sparks at low engine rpm solves this issue.


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