SPN 605 FMI 4: Meaning and Fix
SPN 605 FMI 4 signals that the refrigerant high pressure switch circuit has a voltage below normal or is shorted to ground. This fault commonly appears after a technician replaces the ECM or performs a battery jump-start, inadvertently grounding the signal wire. The ECM detects the implausibly low voltage and disengages the compressor clutch to prevent damage, even if the A/C pressure is actually normal.
Common Symptoms
- No A/C Cooling: Compressor clutch remains disengaged; cabin air is not cooled despite normal refrigerant charge.
- ECM Fault Lamp: Amber or red warning lamp illuminates on the dash; fault code stored in J1939 memory.
- Intermittent Clutch: Compressor clutch may briefly engage then drop out when the shorted circuit fluctuates.
- Derate Active: Engine torque may be reduced slightly as a safety fallback for perceived high-pressure condition.
Probable Causes
- Shorted Signal Wire: Insulation chafed against chassis ground; wire pinched during component installation.
- Faulty Pressure Switch: Internal short to ground within the switch due to moisture ingress or thermal stress.
- ECM Pin Corrosion: Moisture or green corrosion at the ECM connector pin for the switch input.
- Aftermarket Harness: Improperly pinned aftermarket harness or splice that grounds the signal line.
Advanced Technical Analysis
The ECM monitors the switch input as a discrete digital signal pulled high via an internal 10 kΩ resistor to 5 V reference. When the circuit is shorted low, the voltage drops below 0.8 V for more than 50 ms, triggering FMI 4. The ECM then sets the ‘Error’ state (10b) and inhibits compressor clutch engagement until the fault clears and an ignition cycle occurs. This logic prevents rapid cycling that could damage the compressor.
A short-to-low condition must be distinguished from a legitimate high-pressure reading. The switch is normally open when pressure is below ~28 bar. A shorted wire mimics a closed switch (grounded signal), causing the ECM to think pressure is too high. Debouncing timers (typically 100 ms) filter noise; if the short persists, the fault is latched. Real-world testing shows that a 1.5 kΩ pull-down resistor in the harness can falsely trigger this code.
Upon detecting SPN 605 FMI 4, the ECM activates a safety fallback: it sets the ‘Shutdown’ PG to prevent compressor operation. Engine torque may be derated by up to 15% to reduce load and prevent overheating. The ECM also logs the event with a timestamp and freeze-frame data, including A/C request status and engine speed. This data is critical for diagnosing intermittent faults that occur only under vibration.
Long-term prevention includes dielectric grease on switch connectors and ECM pins. In one documented Deutz workshop case, a technician found that a loose ground strap caused intermittent shorts only during engine idle. Replacing the harness section and applying conductive shielding solved the recurring fault. Always verify the switch resistance (open circuit >1 MΩ) before condemning the ECM; a simple continuity test with a multimeter can save hours of troubleshooting.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check the switch connector and harness for chafing, corrosion, or pinched wires near the A/C compressor.
- Voltage Measurement: With ignition ON and switch disconnected, measure signal pin voltage; should be 4.5-5.0 V.
- Switch Resistance Test: Disconnect switch; measure resistance between signal and ground. Normal is >1 MΩ; short indicates internal fault.
- ECM Pin Check: Unplug ECM connector, inspect pin for corrosion, then verify continuity between ECM pin and switch connector.