The dreaded 5 MPH derate in a Volvo truck is a frustrating experience for any driver. This significantly reduced speed often stems from issues within the Aftertreatment Control Module (ACM) and related emissions systems. Understanding the common causes, fault codes, and potential solutions can help you get back on the road quickly.
What is a Derate and Why Does it Happen?
A derate is a protective measure initiated by the engine’s computer, limiting speed or power to prevent damage to critical components like the engine or Diesel Particulate Filter (DPF). While various factors can trigger a derate, a 5 MPH restriction specifically points to problems within the emissions or aftertreatment system. This system, implemented in Volvo trucks from 2011 onwards, aims to reduce harmful exhaust emissions.
Components Involved in the 5 MPH Derate
Several components within the emissions system can contribute to a 5 MPH derate:
- DEF Fluid: Issues with DEF (Diesel Exhaust Fluid) quality or low fluid levels, detected by sensors in the tank, can trigger a derate.
- DEF Doser Valve: This valve regulates DEF injection into the SCR (Selective Catalytic Reduction) catalyst. Malfunctions can disrupt the emissions process.
- DEF Pump Assembly: Responsible for circulating DEF throughout the system, a pump failure leads to immediate derate.
- NOx Sensors: Inlet and outlet NOx sensors monitor nitrogen oxide levels in the exhaust. High levels signal the SCR system to inject more urea for emissions reduction. Sensor failures can disrupt this process.
- ACM (Aftertreatment Control Module): The ACM orchestrates the entire emissions system, communicating with the ECM (Engine Control Module). It controls the DPF and associated sensors. ACM faults are a common cause of derates.
Identifying the ACM Fault Codes
Specific fault codes indicate an “SCR Inducement” condition, leading to the 5 MPH derate:
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2011-2013 EPA10 Trucks: SPN 5246 FMI 0 – SCR Inducement Final Level.
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2014-2019 GHG14, GHG17 Trucks: DEF SCR Inducement Faults including codes P208E, P207F, P103B, and P103C.
Temporary Override (Use with Caution)
In some Volvo models, temporarily disconnecting the transmission speed sensor might allow a temporary override. This sensor, located on the transmission tail shaft or on the right side for I-Shift transmissions, disables speed display and limits RPM to approximately 1400. However, this is not a recommended solution and may not work on all models. It also requires manual gear shifting.
Addressing the Root Cause of the Derate
Fixing the 5 MPH derate requires diagnosing the underlying cause of the “SCR Inducement Code.” Common culprits include:
- DEF Doser Valve Failure (SPN 5394)
- Low DEF Fluid Level (SPN 5392, SPN 1761)
- NOx Sensor Failure (SPN 4094)
- EGR System Malfunction
- DEF Pump Failure (SPN 4334, SPN 4339)
- ACM Failure (Harness failure, power issues, or software requiring updates)
Software Updates for the Emission System
Software updates for the ACM and ECM can often resolve underlying bugs and improve system performance. Checking for and installing available updates is a recommended practice, though it might not solve all issues.
Conclusion
A 5 MPH derate in a Volvo truck, often caused by ACM or related emissions system faults, necessitates a thorough diagnostic approach. Identifying the specific fault codes and understanding the interconnected components allows for effective troubleshooting and repair. Addressing the root cause, whether it’s a faulty sensor, DEF issue, or ACM malfunction, is crucial to restoring normal vehicle operation. Consult a qualified Volvo technician for accurate diagnosis and repair.
