SPN 1590 FMI 20: Meaning and Fix
This fault indicates the ACC system reports a mode value that has drifted above the maximum valid threshold, typically seen after a radar sensor misalignment or CAN bus voltage spike. In practice, technicians encounter this after a front-end collision repair where the ACC sensor bracket is bent, causing erratic mode signals. The ECM logs the fault and disables cruise control, requiring the driver to manually deactivate the system before resuming normal operation.
Common Symptoms
- ACC Unavailable: Cruise control is deactivated by ECM; driver sees ‘ACC Not Available’ on the cluster display.
- Warning Lamp Active: Amber ACC warning lamp illuminates continuously, indicating a system fault requiring driver attention.
- Erratic Mode Jumps: ACC mode sporadically switches between ‘Off’ and ‘Active’ without driver input, causing unexpected deceleration.
- Speed Loss on Hills: Truck loses speed on slight grades because ACC fails to maintain set speed due to corrupted mode data.
Probable Causes
- Sensor Drift: Radar sensor internal voltage reference drifts high due to temperature cycling or aging components.
- CAN Bus Noise: High-frequency electrical noise on the CAN bus causes the ACC mode byte to be read as a higher value.
- Ground Offset: Poor sensor ground connection introduces a voltage offset, raising the signal above the valid range.
- ECM Software Bug: Firmware error in the ACC module causes occasional mode value overflow after a reset sequence.
Advanced Technical Analysis
The ECM monitors the ACC mode signal on a dedicated CAN message (PGN 0x00F002) every 100 ms. When the raw byte value exceeds 0x0B (Off state) or the defined upper limit of 0x14, the FMI 20 is set. The ECM compares the mode to a plausibility table; if the value is outside all known states, it flags a drift-high fault. This prevents the ACC from entering an undefined state that could cause unintended acceleration.
Electrically, the ACC sensor’s internal pull-up resistor may degrade, causing the output voltage to rise above 4.5 V on a 0-5 V analog line. The ECM’s ADC sees this as a value > 1023 counts. A debouncing timer of 500 ms must elapse with the value continuously high before the fault is confirmed. This prevents transient noise from triggering false positives, but a sustained spike from a failing regulator will pass the timer.
Once FMI 20 is active, the ECM forces the ACC mode to ‘Off’ and sets the torque derate to 0% for cruise control. The driver is alerted via a dedicated warning lamp per ISO 2575. The system cannot self-recover; the ignition must be cycled after the root cause is fixed. This fallback ensures the driver regains full manual control, as per SAE J1939 safety requirements for adaptive cruise systems.
Long-term prevention involves checking the ACC sensor alignment with a factory scan tool after any front-end service. In one real workshop case, a MAN TGS had recurring SPN 1590 FMI 20 after a radar replacement; the issue was a loose ground terminal at chassis ground. Cleaning and torquing the connection to 8 N·m resolved it permanently. Always verify sensor supply voltage (4.75-5.25 V) and CAN termination resistance (60 Ω) as part of diagnosis.
Step-by-Step Troubleshooting Guide
- Verify Sensor Voltage: Measure supply voltage at ACC sensor connector; should be 5.0 V ±0.25 V with ignition on.
- Inspect CAN Bus: Check CAN-H and CAN-L for shorts to battery or ground; resistance between them must be 60 Ω.
- Check Ground Path: Measure voltage drop between sensor ground pin and chassis ground; must be below 50 mV.
- Relearn ACC Module: Use diagnostic tool to perform ACC sensor alignment and clear fault after repair is verified.