A Freightliner truck experiencing MIL, CEL, and derate issues presented several fault codes related to the accelerator pedal position sensor (APPS) and a 5V supply circuit. Codes included: 1696 (Sensor supply 5 circuit voltage below normal), 1241 and 1239 (Accelerator pedal sensor 2 circuit – below/above normal), and 132 (Accelerator pedal sensor 1 circuit below normal). Additionally, code 3596 pointed to erratic DEF pressure data.
Diagnostics revealed that the DEF pressure supply circuit, APPS2, aftertreatment system, and a splice pack shared the same 5V supply and a common ground. Live data monitoring showed fluctuating voltage in the APPS2 and DEF pressure signal while air pressure built. Visual inspection of the harness didn’t reveal any obvious damage. However, the truck, with 23,000 hours of service, showed signs of neglect and potential previous repairs, including a replaced accelerator pedal and DEF pump.
Further investigation using a picoscope on the accelerator pedal’s 5V supply line revealed significant noise (700-800mV) with intermittent spikes and drops. This noise was minimal with the key on and engine off but increased significantly when the engine started, exceeding 6V. Interestingly, the noise decreased under acceleration until the desired throttle command was reached. This behavior suggests a potential issue with the chassis harness allowing noise to interfere with the 5V supply circuit.
While a faulty chassis harness is suspected, other potential causes of Freightliner Accelerator Pedal Problems could include a failing APPS sensor, damaged wiring within the pedal assembly itself, or issues with the engine control module (ECM). Further diagnostic steps might involve:
- Harness Continuity Test: A thorough check of the chassis harness for continuity and shorts between the 5V supply, APPS, and other components sharing the circuit.
- APPS Sensor Testing: Testing the APPS sensor’s resistance values and output voltage across its range of motion to ensure it’s operating within specifications.
- Power Supply Inspection: Verifying the integrity of the 5V power supply itself, including the voltage regulator and associated components.
- ECM Diagnostics: Checking for any ECM-related fault codes or unusual data readings that might contribute to the issue.
Resolving Freightliner accelerator pedal problems requires a systematic approach to diagnose the root cause. Addressing the noisy 5V supply circuit is crucial, as it can affect multiple systems beyond the accelerator pedal. A thorough inspection and testing of the related components and wiring are essential for a proper repair.
