The Suspect Parameter Number (SPN) 929, identified as “Tire Location,” plays a pivotal role in modern vehicular systems by indicating which tire-specific parametric data is being referenced. This parameter is crucial in heavy-duty trucks, buses, and off-road equipment, where accurate tire monitoring is essential for safety and performance. It is widely used in systems from manufacturers such as Cummins, Volvo, and Caterpillar, providing data that assists in tire pressure monitoring systems (TPMS) and other tire condition assessments. Proper identification of tire location helps in diagnosing tire-related issues, ensuring optimal vehicle operation, and preventing potential safety hazards.
Technical Overview
The SPN 929 is engineered to provide a precise identification of tire positions by using a 1-byte data format. This byte is split into two 4-bit segments. The low-order 4 bits represent the tire’s position when counted from left to right, facing the vehicle’s normal forward travel direction. Conversely, the high-order 4 bits denote the position counted from front to back. This encoding method allows for up to 15 distinct positions, accommodating configurations from simple two-axle setups to more complex multi-axle vehicles. The Electronic Control Module (ECM) gathers this information through digital signals, typically integrated within the vehicle’s CAN communication system. Tire location data is sourced from sensors embedded in or near each tire, transmitting signals via the J1939 CAN bus. The normal operating range excludes the hexadecimal value FFh, which indicates data unavailability.
J1939 Network Behavior
On the J1939 CAN bus, the SPN 929 is communicated as part of the “Tire Condition Message 1” Parameter Group (PG). This data is transmitted with a predefined frequency, ensuring that real-time tire status is continuously available to various Electronic Control Units (ECUs) within the network. The typical broadcast rate is set to once per second, although this may vary depending on the manufacturer’s specifications. The source address for this PGN is usually the ECU responsible for tire management functions, such as the Body Controller Module (BCM) or a dedicated Tire Pressure Monitoring System (TPMS) ECU. This configuration allows for seamless integration with other vehicle systems, enabling features like automatic tire inflation systems and advanced driver assistance systems (ADAS) to utilize tire location data effectively.
Diagnostic Importance
Understanding and diagnosing the SPN 929 is critical for maintaining vehicular safety and efficiency. Faulty data or failures in tire location identification can lead to incorrect tire pressure readings, uneven tire wear, and reduced fuel efficiency. In severe cases, it may compromise vehicle stability and safety. When the ECM detects a fault related to SPN 929, it may trigger engine protection strategies such as speed limitation or alert the operator through warning lights on the dashboard. Ignoring these fault codes can lead to more significant issues, including tire blowouts or damage to vehicle suspension components due to improper load distribution.
Common Failure Patterns
Several common failure patterns are associated with SPN 929. Technicians frequently encounter issues such as wiring harness damage due to environmental exposure or mechanical stress, leading to intermittent or lost signals. Sensor degradation over time can also cause erroneous data, often exacerbated by exposure to harsh road conditions or temperature extremes. Contamination from dirt or moisture can interfere with sensor accuracy, while calibration drift might occur if the system is not periodically re-calibrated following tire changes or rotations. Mechanical failures, such as damaged sensor mounts or connectors, also contribute to the incorrect identification of tire locations.
Diagnostic Approach
Diagnosing issues related to SPN 929 requires a systematic approach. Technicians should begin by using a diagnostic scanner capable of interfacing with J1939 networks to retrieve fault codes and real-time data. Visual inspection of the wiring harness and connectors for physical damage is crucial, followed by continuity checks using a multimeter. Reference values for sensor outputs should be compared against manufacturer specifications — for instance, Volvo and PACCAR provide detailed voltage ranges for their TPMS sensors. If wiring and sensors appear functional, recalibration using OEM diagnostic software, such as Cummins INSITE or Caterpillar ET, may be necessary. Technicians should escalate to OEM support if faults persist despite following diagnostic procedures, as proprietary insights from manufacturers like Bosch or Detroit Diesel might be required for complex issues.
Fault Codes for SPN 929
FMI 0: Data valid but above normal operational range (most severe)
SPN 929 FMI 0 relates to tire location data exceeding normal operational limits. This fault often occurs in vehicles with newly installed tire pressure monitoring systems, particularly after a calibration error. When this code appears, it can lead to incorrect tire location readings, affecting autom
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FMI 1: Data valid but below normal operational range (most severe)
SPN 929 FMI 1 indicates the tire location identification parameter has dropped below the normal operational range, preventing accurate tire position mapping within the TPMS network. This fault commonly appears after trailer disconnection/reconnection cycles when the ECM loses tire position reference
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FMI 2: Data erratic, intermittent or incorrect
SPN 929 FMI 2 indicates the Tire Location parameter in the Tire Condition Message 1 PG is erratic, intermittent, or incorrect. This fault often appears after a wheel-end service or tire rotation when the position identification wiring is disturbed. Technicians commonly see this code on trucks with T
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FMI 3: Voltage above normal or shorted high
SPN 929 FMI 3 is associated with an abnormal voltage reading in the tire location system, which can lead to incorrect tire position reporting. This fault often surfaces when there is a wiring issue or sensor malfunction following a tire change or during intense off-road conditions. Technicians will
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FMI 4: Voltage below normal or shorted low
SPN 929 FMI 4 indicates voltage below normal in tire location identification circuits within the tire pressure monitoring system. This fault commonly appears after trailer disconnection events or during cold weather startup when TPMS sensors experience voltage drops. The ECM cannot properly identify
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FMI 5: Current below normal or open circuit
This fault indicates the tire location identification circuit (high/low order nibble) has current below normal or an open circuit. The ECM detects a missing or high-impedance signal from the tire pressure monitoring system sensor. Commonly appears after a tire rotation or replacement when the sensor
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FMI 6: Current above normal or grounded circuit
SPN 929 FMI 6 indicates a current issue above normal or a grounded circuit in the tire location system. This fault is critical in vehicles with advanced tire monitoring systems, especially after a tire pressure sensor replacement or wiring harness inspection. Technicians often encounter this fault w
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FMI 7: Mechanical system not responding properly
SPN 929 FMI 7 indicates mechanical failure in the tire position identification system within the Tire Pressure Monitoring System (TPMS). This fault typically occurs when technicians replace wheel assemblies on commercial vehicles without properly programming new TPMS sensors, causing the ECM to lose
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FMI 9: Abnormal update rate
SPN 929 FMI 9 triggers when the Tire Condition Message 1 (PGN 65216) is not received at the expected periodic rate, typically 100 ms per SAE J1939-71. This fault commonly appears after a wheel-end service where the tire pressure sensor harness was left disconnected, or after ECM replacement without
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FMI 11: Root cause not known
The SPN 929 FMI 11 fault code is related to the tire location data, with an unknown root cause. This issue often surfaces after tire replacement or alignment services, when technicians fail to reset the tire location information correctly. The fault affects parametric data, crucial for systems relyi
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FMI 12: Bad intelligent device or component
SPN 929 FMI 12 indicates the Tire Location data in the Tire Condition Message 1 is flagged as a bad intelligent device or component. The ECU detects the position number (low nibble left-right, high nibble front-back) is either stuck, out of range, or failing internal validation. This code commonly a
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FMI 13: Out of calibration
SPN 929 FMI 13 indicates the tire position identification system is out of calibration, preventing accurate tire location mapping within the TPMS network. This fault commonly appears after tire rotation procedures when technicians forget to recalibrate the TPMS sensors, causing the ECM to receive po
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FMI 14: Special instructions
SPN 929 FMI 14 signals that the Tire Location parameter in the Tire Condition Message 1 PG is reporting a value of FFh (not available) or an invalid position number per SAE J1939/71. This fault often appears after a tire rotation or when a new ECU is installed without reconfiguring the tire position
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FMI 18: Data valid but below normal operating range (moderately severe)
SPN 929 with FMI 18 refers to the tire location sensor reading valid data but below normal operating range. This often occurs after tire rotations if the tire pressure monitoring system (TPMS) is not recalibrated correctly. In practice, technicians might see this fault after vehicle maintenance when
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FMI 31: Condition exists
SPN 929 FMI 31 indicates a tire location identification condition exists within the Tire Pressure Monitoring System (TPMS). This fault commonly appears during fleet maintenance when technicians replace tire sensors or perform wheel rotations on commercial vehicles. The ECM detects irregularities in