Testing a pulse width modulated circuit.August 9, 2018
The ability to control a mechanical device electrically has done more to advance both 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 farm engines and equipment. An electronic fuel injector (gas or diesel) is the most common use of a PWM circuit but is not the only one. In most diesel applications the EGR (exhaust gas recirculation) valve is PWM controlled also. But it does not end there.
The Case/IH Patriot line of field sprayers when equipped with their AIM Command system employs a PWM system to operate each nozzle on the boom and provide finite control.
Since a PWM circuit does not know what it is controlling, the way it works and the diagnostics are the same.
Shared theory of operation
The component being controlled needs to have a solenoid attached (either integral or divorced). It is an electro-magnet that works with a metal rod, plunger or disc. 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 percent from zero (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) (1/1000th of a 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.
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, either a test light or a noid light can be employed. The test light can be attached in series (between the two terminals) and the circuit evoked. The light should pulse. If not, either the wiring back to the ECU is compromised or the ECU is not sending a duty-cycle command. A shorted solenoid can often damage the driver in the ECU.