Detailed guide for MID 128 PID 84 FMI 9
Back to top ↑MID 128 PID 84 FMI 9 - Vehicle Speed
**Fault Type:** Abnormal update rate
Fault Condition
- Missing signal from VECU (Vehicle Electronic Control Unit)
Possible Symptoms
- Engine derate
Possible Causes
- J1708 vehicle speed message does not exist (VECU error)
- Communication failure between EMS and VECU
- VECU not broadcasting vehicle speed data
System Information
The Vehicle Electronic Control Unit (VECU) directly monitors the Vehicle Speed Sensor (VSS). The VECU broadcasts this data on the serial data lines to other ECUs including the EMS.
Diagnostic Tips
1. Check VECU for fault codes
2. Verify VECU power and ground connections
3. Inspect J1939 and J1708 data link wiring for damage
4. Check termination resistors on data links
5. Verify vehicle speed sensor operation and wiring to VECU
6. Use Premium Tech Tool (PTT) to monitor network communication
Mechanic community notes for MID 128 PID 84 FMI 9
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