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Testing a Pulse Width Modulated Circuit

The ability to control a mechanical device electrically has done more to advance gasoline and diesel engine operation than anything else.

One method to accomplish this is called the pulse width modulated (PWM) circuit. It is found in many places on modern engines and equipment. An electronic fuel injector (gas or diesel) is the most common use of a PWM circuit, but it is not the only one. In most diesel applications, the exhaust gas recirculation valve is PWM controlled also.

It does not end there. Case IH’s Patriot line of sprayers, when equipped with the AIM Command system, employs a PWM system to operate each nozzle on the boom, providing finite control of spray application.

Shared theory of operation
Since a PWM circuit does not know what it is controlling, the way it works and the diagnostics are the same.

The component being controlled needs to have a solenoid attached (either integral or divorced). It is an electromagnet that works with a metal rod, plunger, or disk.

When power is applied, the solenoid is energized, and the induced magnetic field works against the metal component usually inducing flow of what is being controlled. When the power is shut off, an internal spring then closes the device and flow stops. The part is turned on and off at a rapid pace. The control comes by altering the on and off time. This is responsible for the ticking sound when operating.

The length of time the solenoid is energized is called a duty cycle and is read in percentage from 0% (completely shut off) to 100% (fully open). This is known as dwell.

In most applications, the duty cycle translates by design to an opening time in milliseconds (ms), which is ¹∕1,000 second. For example, if 100% duty cycle is 10 ms, then 50% duty cycle would be 5 ms.

A control unit is usually identified as an ECU. It is the brain and handles the task of the duty cycle. In most – if not all – applications, the solenoid is supplied with system voltage, and the ground circuit is turned on and off by the ECU. Switching the ground is quieter electrically, which means cleaner. The ECU has drivers that can be considered an electronic switch with no moving parts.

Basic diagnostics
The circuit will always have two wires (power and ground). The solenoid has a specified resistance that can be confirmed by unplugging it and placing an ohm meter across the two terminals.

To confirm if the ECU is commanding control, you need to employ either a test light or a noid light. The test light can be attached in series (between the two terminals) and the circuit evoked.

The light should pulse. If it doesn’t, either the wiring back to the ECU is compromised, or the ECU is not sending a command. A shorted solenoid can often damage the driver in the ECU.

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