SPN 986 FMI 19 – J1939 CM1 SPN986 Signal from source #1 or #2 is erratic Fault Code (Symptoms, Causes & Fix)
The SPN 986 FMI 19 fault code indicates an erratic signal from source #1 or #2 on the J1939 communication network. This diesel engine fault code suggests that the Engine Control Module (ECM) is detecting inconsistent data transmission, which can affect the performance of the truck's communication system.
What SPN 986 FMI 19 Means
The SPN 986 FMI 19 fault code is related to the J1939 communication network, specifically indicating that the signal from source #1 or #2 is erratic. This means the ECM is receiving inconsistent or unstable data from one of the components on the J1939 network, which can lead to communication errors and potential performance issues.
Common Causes of This Fault Code
- Intermittent connection in the J1939 data link
- Faulty or loose J1939 terminating resistor
- Damaged or corroded wiring harness
- Faulty ECU or module on the J1939 network
- Connector corrosion
Symptoms Drivers May Notice
When the SPN 986 FMI 19 fault code is active, drivers may notice erratic behavior in the vehicle's electronic systems. This can include intermittent loss of communication with certain modules, unexpected warning lights on the dashboard, or even temporary loss of certain functionalities controlled by the affected modules.
How Technicians Diagnose This Fault
Technicians begin diagnosing the SPN 986 FMI 19 fault by performing a visual inspection of the J1939 data link connectors for any signs of corrosion or damage. Next, they check the integrity and resistance of the J1939 terminating resistors to ensure they are within specifications. A continuity test on the J1939 wiring harness is also conducted to identify any breaks or shorts. Using a diagnostic tool, technicians monitor the J1939 network traffic for erratic signals and inspect ECUs and modules for proper operation and communication.
Common Misdiagnosis
Common misdiagnoses include assuming a specific sensor failure without checking the network, replacing ECUs or modules without verifying network integrity, and overlooking simple wiring issues such as loose connections. These mistakes can lead to unnecessary part replacements and prolonged downtime.
Can You Keep Driving?
While the truck may continue to operate with the SPN 986 FMI 19 fault code active, it is not advisable to ignore the issue. Erratic signals on the J1939 network can lead to unpredictable behavior of electronic systems and may eventually cause the engine to enter a derate or protection mode, affecting vehicle performance.
Mechanic's Diagnostic Tip
When troubleshooting the SPN 986 FMI 19 fault code, always start with the simplest checks. Ensure all connectors are secure and free from corrosion before moving on to more complex diagnostics. This can save time and prevent unnecessary part replacements.
Frequently Asked Questions
What tools are needed to diagnose SPN 986 FMI 19? A multimeter and a diagnostic tool capable of reading J1939 network traffic are essential for diagnosing this fault.
Can a faulty sensor cause SPN 986 FMI 19? While a faulty sensor can contribute to erratic signals, the issue is more commonly related to network communication problems.
Is it safe to drive with SPN 986 FMI 19 active? Driving with this fault code active is not recommended as it can lead to further issues and potential engine derate.
