GM/Detroit 6.5L – Engine Management – FUEL CONTROL – ACCELERATOR PEDAL POSITION (APP) MODULE

Description

The 6.5L EFI PCM uses Accelerator Pedal Position (APP) sensors to control fuel delivery as requested by driver demands at the accelerator pedal. Three APP sensors are located in a module at the base of the accelerator pedal (figure 5-27).

The APP module replaces the throttle linkage found on non-EFI 6.5L diesel engines. While it is new to GM diesel engines, similar “drive by wire” technology has been in use for several years on diesel engines from other manufactur­ers.

APP Module Circuit Operation
The APP sensors are each variable-resistor type. Each sensor receives its own 5 volt reference signal from the PCM. The PCM provides a ground for each sensor to complete the circuit.

As the accelerator pedal is depressed, resistance in each sensor changes. By monitoring the output voltage from the module, the PCM can calculate proper fuel delivery as required by acceleration needs.

The PCM provides each of the three Accelerator Pedal Position (APP) sensors with independent 5 volt reference signals on the following circuits: CKT 997 to APP 1 sensor; CKT 996 to APP 2 sensor; CKT 995 to APP 3 sensor (figure 5-28).The sensors provide voltage signals back to the PCM on the following circuits: CKT 992 from APP 1 sensor; CKT 993 from APP 2 sensor; CKT 994 from APP 3 sensor.

Three sensors are used to ensure reliability in the system. Each is scaled differently. The PCM compares signal voltage of the three to determine proper operation. The PCM only requires information from one sensor; the other two serve as fail safes. Failure modes for the APP system are as follows:
• If only one sensor fails, the vehicle will operate normally. The PCM will store a DTC, but will not
illuminate the “Service Throttle Soon” lamp in the instrument cluster.
• If two sensors fail, the engine will operate at reduced power. The PCM will store a DTC and will
illuminate the “Service Throttle Soon” lamp.
• If all three sensors fail, the PCM will store a DTC and illuminate the “Service Throttle Soon” lamp.
The PCM will also reduce engine power to idle.

APP DTCs
The following Diagnostic Trouble Codes relate to Accelerator Pedal Position (APP) sensor circuit operation.

APP 1 Sensor
• DTC 21 “Accelerator Pedal Position (APP) 1 Circuit High” sets when the voltage signal from APP 1 sensor reads greater than 4.75 volts for 2 seconds
• DTC 22 “Accelerator Pedal Position (APP) 1 Circuit Low” sets when the voltage signal from APP 1 sensor reads less than .25 volts for 2 seconds.
• DTC 23 “Accelerator Pedal Position (APP) 1 Circuit Range Fault” sets when the PCM recognizes a skewed or misscaled sensor. The PCM compares all three sensors to each other. DTC 23 sets when APP 1 sensor voltage differs from APP 2 voltage by 6 percent or more and
APP 1 voltage differs from APP 3 voltage by 10 percent or more. Both of these differences must be present for DTC 23 to set.

APP 2 Sensor
• DTC 25 “Accelerator Pedal Position (APP) 2 Circuit High” sets when the voltage signal from APP 2 sensor reads greater than 4.75 volts for 2 seconds.
• DTC 26 “Accelerator Pedal Position (APP) 2 Circuit Low” sets when the voltage signal from APP 2 sensor reads less than .25 volts for 2 seconds.
• DTC 27 “Accelerator Pedal Position (APP) 2 Circuit Range Fault” sets when the PCM recognizes a skewed or misscaled sensor. The PCM compares all three sensors to each other. DTC 27 sets when APP 2 sensor voltage differs from APP 1 voltage by 6 percent or more and APP 2 voltage differs from
APP 3 voltage by 10 percent or more. Both of these differences must be present for DTC 27 to set.

APP 3 Sensor
•DTC 63 “Accelerator Pedal Position (APP) 3 Circuit High” sets when the voltage signal from APP 3 sensor reads greater than 4.75 volts for 2 seconds.
• DTC 64 “Accelerator Pedal Position (APP) 3 Circuit Low” sets when the voltage signal from APP 3 sensor reads less than .25 volts for 2 seconds.
• DTC 65 “Accelerator Pedal Position (APP) 3 Circuit Range Fault” sets when the PCM recognizes a
skewed or misscaled sensor. The PCM compares all three sensors to each other. DTC 65 sets when
APP 3 sensor voltage differs from APP 1 voltage and APP 2 voltage each by more than 10 percent.

Other App DTCs
• DTC 84 “Accelerator Pedal Position (APP) Circuit Fault” identifies an intermittent APP fault. It sets
if there are no other current APP codes stored. DTC 84 will store as a current and history code, but will not illuminate the “Service Throttle Soon” lamp. The vehicle will have limited power.
• DTC 99 “Accelerator Pedal Position (APP) 2 5-Volt Reference Fault” identifies reference voltage on APP sensor 2 below 4.8 volts for 2 seconds. DTC 99 will illuminate the STS lamp. The vehicle will have limited power.

 

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