John Deere Engines SPN733FMI4 Fault Code: Rack Position Voltage Low
Rack Position Voltage Low · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 733 FMI 4 means the ECU has detected that the internal rack position voltage, fed back from the injection pump's rack position sensor, is lower than the normal operating range. When this sets, the ECU shuts off the actuator solenoid and fuel shut-off solenoid, the engine dies, and it will not restart.
Critical severity. This code causes the ECU to shut the engine down and prevents it from restarting. Any occurrence of SPN 733 FMI 4 stops the machine immediately and requires diagnosis before operation can resume.
What does John Deere Engines error code SPN733FMI4 mean?
SPN 733 FMI 4 relates to the rack position sensor circuit inside the mechanical fuel injection pump. The ECU controls fuel delivery by energizing an actuator solenoid inside the injection pump, which drives a spring-loaded rack toward full fuel delivery. The rack position sensor, made up of two inductive coils called the rack position coil and rack reference coil, reports back to the ECU where the rack actually is so the ECU can confirm the rack moved to the commanded position.
FMI 4 specifically means the voltage the ECU sees from this feedback circuit is lower than the normal operating range. Since this voltage is used by the ECU to know the actual rack position, a low reading tells the ECU it can no longer trust the fuel delivery feedback.
Because the ECU can no longer verify rack position, it responds by shutting off current to both the actuator solenoid and the fuel shut-off solenoid. The result is the engine dies and will not restart until the fault is fixed.
Common causes of SPN733FMI4
- Dirt, damage, or poor terminal positioning on the ECU 30-way connector and the 6-way pump connector
- Open or short circuit in the rack position sensor located inside the injection pump
- A bad injection pump
- Open in the rack position reference circuit
- Open in the rack position common circuit
- Short to ground in the rack position common circuit
- A bad ECU
How to troubleshoot John Deere Engines SPN733FMI4: first checks
- Inspect the ECU 30-way connector and the injection pump's 6-way connector for dirt, corrosion, damaged pins, or terminals that are not seated properly
- Check the rack position reference and rack position common circuits for open or shorted wiring, including shorts to ground
- Use the JT07328 Connector Adapter Test Kit to check voltages at the connectors, never force probes directly into connector terminals since this can damage the terminals
- If wiring and connectors check out, suspect the rack position sensor inside the injection pump or the injection pump itself
- If the pump and sensor circuit test good, consider the ECU as a possible cause
How the code clears
No separate clearing step is listed. Since the engine dies and will not restart once this code sets, the underlying wiring, connector, sensor, pump, or ECU fault must be repaired before the engine will run again.
Frequently asked questions
What does SPN 733 FMI 4 mean on a John Deere engine?
It means the ECU has detected that the internal rack position voltage, which comes from the rack position sensor inside the injection pump, is lower than the normal operating range. This is an internal ECU voltage tied to fuel delivery feedback.
Why did my engine die and not restart after this code?
When SPN 733 FMI 4 sets, the ECU shuts off current to the actuator solenoid and the fuel shut-off solenoid as a protective response. This kills the engine and prevents it from restarting until the fault is diagnosed and repaired.
Can I test the connectors myself?
You can inspect them visually for dirt, corrosion, or damaged terminals, but when taking voltage measurements you should use the JT07328 Connector Adapter Test Kit. Do not force probes into the connector terminals since that can damage them.
Is this a wiring problem or a pump problem?
It can be either. Listed causes include dirty or damaged connector terminals, open or shorted wiring in the rack position reference or common circuits, a short to ground in the common circuit, a faulty rack position sensor inside the injection pump, a bad injection pump itself, or a bad ECU.
Is it safe to keep running the engine if this code appears intermittently?
No. Once this code sets, the ECU is designed to shut the engine off and it will not restart, so there is no safe way to keep operating through this fault. Treat any occurrence as a reason to stop and diagnose.
Do I need to replace the injection pump to fix this?
Not necessarily. The pump and its internal rack position sensor are two possible causes, but connector and wiring problems on the ECU 30-way connector, the pump's 6-way connector, and the rack position reference or common circuits should be checked first since those are often the source of the issue.