The SAE J1939 Suspect Parameter Number (SPN) 1387, labeled “Auxiliary Pressure #1,” is a standardized data parameter used to monitor a secondary pressure source within a heavy-duty vehicle or equipment system. Unlike primary pressure parameters such as engine oil pressure (SPN 100) or intake manifold pressure (SPN 102), this SPN is designated for an auxiliary pressure sensor that is not critical to the core engine control functions. This parameter is commonly utilized in applications where a secondary hydraulic, pneumatic, or liquid pressure reading is required for system monitoring, operator feedback, or implement control. For example, on a PACCAR MX-13 engine installed in a Kenworth T680, SPN 1387 might be configured to monitor the pressure in a secondary cooling circuit or a hydraulic fan drive system. On a Caterpillar 336E excavator, it could represent the pressure in a pilot control circuit or a specific implement hydraulic line. The diagnostic significance of SPN 1387 lies in its auxiliary nature: while a fault on this parameter will not typically derate or shut down the engine, it can indicate a developing problem in a supporting system that, if ignored, could lead to secondary damage, reduced machine performance, or operator safety concerns. Understanding this parameter is critical for technicians because it often represents a non-standard application-specific sensor, requiring careful cross-referencing with OEM wiring diagrams and vehicle configuration data.
Technical Overview
From an engineering perspective, SPN 1387 represents a pressure measurement derived from an analog voltage signal generated by a dedicated auxiliary pressure transducer. The sensor is typically a three-wire device: a 5-volt reference supply from the Electronic Control Module (ECM), a signal return (ground), and a variable voltage output that is linearly proportional to the applied pressure. The ECM reads this analog voltage via an internal Analog-to-Digital Converter (ADC) and converts the voltage to a pressure value in kilopascals (kPa) using a calibration curve stored in the ECM’s memory. The specific sensor type can vary widely by application. For instance, a Cummins ISX15 might use a 0-5 volt output sensor with a range of 0-1000 kPa for a hydraulic fan drive circuit, while a Volvo D13 could employ a similar sensor for an auxiliary air suspension system. On John Deere construction equipment, the sensor might be a 4-20 mA current loop sensor converted to a voltage input at the ECM. The normal operating range for SPN 1387 is entirely application-dependent. In a hydraulic system, the range might be 0-35000 kPa (0-5000 psi), while in a pneumatic system, it could be 0-1200 kPa (0-174 psi). The ECM continuously monitors this signal and compares it to expected values based on engine speed, load, and system state. A reading outside of a programmed threshold for a defined duration will trigger a diagnostic trouble code (DTC) associated with SPN 1387, such as “Data Valid But Above Normal Operational Range” or “Voltage Below Normal.”
J1939 Network Behavior
On the J1939 Controller Area Network (CAN) bus, SPN 1387 is transmitted as part of a specific Parameter Group Number (PGN). The most common PGN for this parameter is PGN 65164 (Auxiliary Analog Information), which is a broadcast message typically sent by the engine ECM or a dedicated vehicle control unit. This PGN contains multiple SPNs, including SPN 1387 (Auxiliary Pressure #1) and potentially SPN 1388 (Auxiliary Pressure #2), SPN 1385 (Auxiliary Temperature #1), and others. The transmission rate for this PGN is generally 100 milliseconds (10 Hz), providing near real-time data for other controllers on the network. The source address (SA) for the message is usually the primary engine ECM (SA 0), but on some vehicles with a separate body controller or implement ECU, the source address may be different (e.g., SA 33 for a transmission controller or SA 208 for an auxiliary control module). Other ECUs on the network, such as a transmission controller (TCM), a body control module (BCM), or an instrument cluster, use this data for various purposes. For example, a BCM might use the auxiliary pressure reading to control a warning lamp on the dashboard, or a hydraulic pump controller might use the data to adjust pump displacement. The data is broadcast globally on the bus, meaning any ECU that needs the information can receive it without a request. The parameter is transmitted as a 16-bit unsigned integer, with a resolution of 0.125 kPa per bit and a data range of 0 to 32127.5 kPa, allowing for high precision across a wide pressure range.
Diagnostic Importance
Faults associated with SPN 1387 are critical to address because they often indicate a failure in a system that, while not directly affecting engine combustion, is essential for the proper operation of auxiliary equipment. The ECM’s response to a fault on this parameter is typically non-derating; that is, the engine will not enter a reduced power mode. However, the ECM may activate engine protection strategies related to the auxiliary system. For example, if SPN 1387 is monitoring a hydraulic fan drive pressure and the sensor fails, the ECM may command the fan to run at a fixed speed (e.g., 100% duty cycle) to ensure adequate cooling, which can lead to increased fuel consumption and fan noise. On a Detroit Diesel DD15, a fault on this parameter in a secondary air system could cause the ECM to disable an emissions control device, leading to a Check Engine lamp and possibly a derate if the system is critical for emissions compliance. The consequences of ignoring active fault codes for SPN 1387 include progressive mechanical damage. In a hydraulic system, a faulty sensor reading could mask a low-pressure condition, leading to pump cavitation, overheating, or component seizure. In a pneumatic system, it could result in a loss of brake assist or suspension control. Technicians must not dismiss these faults as “just a sensor” because the auxiliary system it monitors often has a direct impact on vehicle operability and safety, particularly in vocational trucks, construction equipment, and agricultural machinery.
Common Failure Patterns
In real-world service environments, technicians encounter several recurring failure patterns for SPN 1387. The most frequent is a wiring or connector issue: the sensor’s 5-volt reference or signal return wire can chafe against a chassis component, leading to an intermittent open or short circuit. This is especially common in applications where the sensor is mounted on a vibrating component, such as a hydraulic pump or engine block. Another common pattern is sensor degradation due to contamination. In hydraulic systems, the pressure sensor’s diaphragm can become coated with oil sludge or debris, causing a slow drift in the output signal. This is often seen in older Cummins ISX engines where the auxiliary pressure sensor is located in a high-temperature area near the exhaust manifold. Calibration drift is another issue: over time, the sensor’s internal strain gauge can lose accuracy, resulting in a reading that is consistently 5-10% low or high. This is difficult to detect without a known reference pressure. Mechanical failures, such as a blocked pressure port or a ruptured diaphragm, occur less frequently but are catastrophic. On Volvo D13 engines, a common failure is a cracked sensor housing due to thermal cycling, which allows moisture ingress and causes a short circuit. Finally, on PACCAR MX engines, technicians have reported that the auxiliary pressure sensor can fail due to over-pressurization from a system pressure spike, often caused by a faulty relief valve in the auxiliary circuit.
Diagnostic Approach
When diagnosing a fault code involving SPN 1387, a structured approach is essential. Begin by connecting a J1939 diagnostic tool (e.g., a Cummins INLINE 6 or a Noregon JPRO) to read the active and inactive fault codes. Record all associated Failure Mode Identifiers (FMIs) and the occurrence count. Next, consult the OEM wiring diagram to identify the specific sensor location, wire colors, and ECM pin assignments. The first circuit check is to measure the sensor’s 5-volt reference voltage at the connector with the ignition on and the engine off. This should be within 4.75 to 5.25 volts. A low voltage indicates a short to ground or a failing ECM internal supply. Next, measure the signal return (ground) circuit for continuity to chassis ground (less than 5 ohms). Then, measure the signal voltage between the signal wire and ground. With zero applied pressure, the sensor output should be approximately 0.5 volts; with maximum system pressure applied, it should approach 4.5 volts. A clamped reading at 0 or 5 volts indicates a short circuit or open circuit. If the voltage is within range but the displayed pressure is incorrect, perform a pressure calibration check using a mechanical pressure gauge tee’d into the system. Compare the gauge reading to the diagnostic tool reading. A mismatch of more than 2-3% indicates sensor drift. If all electrical values are correct and the sensor is properly calibrated, the issue may be in the ECM’s internal ADC or a software configuration problem. In such cases, escalate to OEM-specific software (e.g., Cummins Insite, Detroit Diesel Diagnostic Link, or P
Fault Codes for SPN 1387
FMI 0: Data valid but above normal operational range (most severe)
SPN 1387 FMI 0 indicates the Auxiliary Pressure #1 sensor reports a value above the calibrated maximum, typically >450 kPa. This code often appears after a forced DPF regeneration or when an air compressor unloader valve sticks closed, causing pressure spikes in auxiliary circuits like air brakes or
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FMI 1: Data valid but below normal operational range (most severe)
SPN 1387 FMI 1 refers to an issue with auxiliary pressure sensor #1, where the pressure is valid but below normal operational levels. This fault frequently arises after maintenance activities involving hydraulic systems, such as replacing hoses or valves, where air might have entered the system. Tec
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FMI 2: Data erratic, intermittent or incorrect
SPN 1387 FMI 2 indicates erratic, intermittent, or incorrect data from auxiliary pressure sensor #1, which monitors supplementary hydraulic or pneumatic systems. This fault commonly appears during implement operation when pressure fluctuations exceed normal parameters. Technicians frequently encount
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FMI 3: Voltage above normal or shorted high
The SPN 1387 FMI 3 fault indicates that the voltage from Auxiliary Pressure Sensor #1 is above normal, likely due to a short circuit. This is often encountered after maintenance involving wiring or sensor replacement. Technicians find that this fault can lead to incorrect pressure readings, impactin
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FMI 4: Voltage below normal or shorted low
This fault indicates the ECM detected the voltage on the Auxiliary Pressure #1 sensor signal circuit is below the normal operating range, typically less than 0.25 V for over 1.5 seconds. This code commonly appears after a forced DPF regeneration when the sensor harness is accidentally pinched during
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FMI 5: Current below normal or open circuit
The SPN 1387 FMI 5 fault indicates an open circuit or current below normal in the auxiliary pressure sensor #1, often leading to inaccurate pressure readings. Technicians frequently encounter this code after replacing sensor wiring harnesses, particularly in environments with high vibration or therm
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FMI 6: Current above normal or grounded circuit
This fault indicates excessive current flow or grounded circuit condition in auxiliary pressure sensor #1 circuit. Common scenario occurs during hydraulic system troubleshooting when technicians accidentally ground sensor wiring while testing auxiliary attachments on excavators or when moisture infi
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FMI 7: Mechanical system not responding properly
SPN 1387 FMI 7 indicates the Auxiliary Pressure #1 sensor signal is valid but the mechanical system fails to respond properly. In practice, this code frequently appears after a hydraulic pump replacement when trapped air or debris prevents the pressure from stabilizing within expected ranges, trigge
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FMI 9: Abnormal update rate
SPN 1387 with FMI 9 indicates an abnormal update rate from Auxiliary Pressure Sensor #1. This fault is frequently observed in machinery post-ECM replacement, often leading to erratic performance and inaccurate pressure readings. Technicians should monitor this issue, especially during post-maintenan
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FMI 11: Root cause not known
SPN 1387 FMI 11 indicates an auxiliary pressure sensor #1 fault with unknown root cause, typically occurring in specialized hydraulic systems or pneumatic circuits. This fault commonly appears during complex diagnostic scenarios where multiple system interactions create intermittent pressure anomali
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FMI 12: Bad intelligent device or component
SPN 1387 FMI 12 indicates the Auxiliary Pressure #1 sensor has failed its internal self-check, reporting a bad intelligent device. This fault commonly appears after a sensor is physically damaged during engine bay cleaning or when moisture ingression causes internal short circuits. The ECM detects c
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FMI 13: Out of calibration
SPN 1387 FMI 13 often appears when the auxiliary pressure sensor #1 readings deviate from expected parameters, indicating calibration issues. This code is typically encountered after ECM replacements or updates where calibration data is not transferred correctly. Technicians also report seeing this
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FMI 14: Special instructions
SPN 1387 FMI 14 indicates special instructions are required for auxiliary pressure sensor #1, typically manifesting during hydraulic system calibration procedures or after ECM software updates. This fault commonly appears when technicians perform hydraulic pump replacements on construction equipment
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FMI 18: Data valid but below normal operating range (moderately severe)
This fault indicates the auxiliary pressure #1 sensor signal is valid but below the normal operating range, typically between 0-200 kPa. Technicians often see this on mobile cranes or refuse trucks after a cold start when hydraulic oil is viscous, causing a slow pressure rise. The ECM registers the
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FMI 31: Condition exists
Auxiliary Pressure #1 (SPN 1387, FMI 31) indicates a condition with the pressure sensor output. This fault often appears in heavy-duty vehicles when the auxiliary pressure sensor #1 is malfunctioning. Technicians commonly encounter this issue after replacing the ECM or performing system upgrades, as