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Reason:Fan Control Circuit – Voltage Below Normal or Shorted to Low Source. Low signal voltage detected at the fan control circuit when commanded ON. Effect:The fan can stay on continuously or not run at all. Circuit Description:The fan control circuit is a device used by the engine to control the fan operation. When the engine control module (ECM) energizes the fan control circuit, the engine fan is engaged. The fan control circuit utilizes a pulse width modulated (PWM) signal. A PWM signal is a pulsed voltage signal between 0-VDC and system voltage. The frequency of the pulsed voltage signal is...

Reason:Engine Crankshaft Speed/Position – Data Valid But Above Normal Operating Range – Moderately Severe Level. The engine speed has exceeded a critical limit. Effect:Engine will be shut down. Circuit DescriptionThe air shutoff valve is an optional component used to shutdown the engine in the event of an engine overspeed condition. It can be closed via a manual shutdown switch or via an ECM command. Component Location:The engine air shutoff valve location can vary depending on the OEM. Refer to the OEM service manual. Conditions for Running the Diagnostics:This diagnostic runs continuously when the keyswitch is in the ON position or...

Reason:Engine Crankshaft Speed/Position – Data Valid But Above Normal Operating Range – Moderately Severe Level. The engine speed has exceeded a critical limit. Effect:Engine will be shut down. Circuit DescriptionThe air shutoff valve is an optional component used to shutdown the engine in the event of an engine overspeed condition. It can be closed via a manual shutdown switch or via an ECM command. Component Location:The engine air shutoff valve location can vary depending on the OEM. Refer to the OEM service manual. Conditions for Running the Diagnostics:This diagnostic runs continuously when the keyswitch is in the ON position or...

Reason:Exhaust Gas Pressure – Data Erratic, Intermittent or Incorrect. The exhaust gas pressure sensor is reading an erratic value. Effect:Possible reduced engine performance Circuit Description:The exhaust gas pressure sensor is used to measure exhaust gas pressure in the exhaust manifold. This information is used by the engine control module (ECM) to control emissions and exhaust gas recirculation (EGR) valve operation. The ECM provides a 5 volt supply to the exhaust gas pressure sensor on the sensor supply circuit. The ECM also provides a ground on the sensor return circuit. The exhaust gas pressure sensor provides a signal to the ECM...

Reason:Auxiliary PWM Driver 1 Circuit – Voltage Above Normal or Shorted to High Source. High signal voltage detected at the pulse width modulation (PWM) auxiliary circuit. Effect:Possible reduced engine performance. Circuit Description:The transmission shift modulator uses this signal from the electronic control module (ECM) to determine when to shift the transmission. The return circuit is dependent on original equipment manufacturer (OEM) wiring. It may be wired back to the ECM on some vehicles or wired to chassis or block ground on others. Consult the OEM wiring diagram for return circuit details. Component Location:Refer to the OEM wiring diagram for the...