ECU UNSWITCH SHORT TO GROUND 627.1 Issue on John Deere 624J Wheel Loader

[MikePhua]

This article delves into the technical problem “ECU Un-switch Short to Ground, code 627.1” occurring in John Deere 624J wheel loaders. It gathers diagnostic definitions, common causes, advice for repair, and broader context including other machines and field cases. Terminology annotations are provided, along with suggestions and preventive measures.

Definition of the Fault Code and Meaning

• The error 627.1 corresponds to a diagnostic trouble code (DTC) from the Engine Control Unit (ECU). It indicates that all injector currents are out of specification – specifically, the injector pull-in current is too low, or the hold-in current is incorrect. In many manuals, this is listed as “ECU 627, 01, Warning – All injector currents out of specification”.
  
• The phrase “unswitch short to ground” suggests that a switched power feed (a circuit that is supposed to be controlled by ignition or ECU-controlled switch) is being unintentionally grounded; that is, electrical current is flowing to ground where it shouldn’t, pulling voltage down. This can disable injector driver circuits or prevent correct signals to fuel injectors.
  

Terminology annotations:

• Injector pull-in current: the initial current when injector solenoid is energized to open.
  
• Hold-in current: the reduced current needed to keep the injector open after initial pull-in.
  
• Short to ground: a fault where an electrical path that should be insulated is unintentionally connected to ground (the metal chassis or negative battery terminal), leading to loss of voltage or improper current flow.
  
• Switched circuit: a circuit that becomes powered only when a switch is turned on (e.g. ignition or ECU relay), unlike constant power circuits.
  

Symptoms and Field Observations
From reports and technician accounts:

• The loader cranks when the key is turned, but does not fire (i.e. no combustion starts).
  
• Following crank attempt, the 627.1 DTC appears, signalling the ECU detects abnormal injector currents or failure in driver circuits.
  
• In at least one instance, the machine had been operating previously, then “died suddenly” and failed to restart. Ambient cold weather was noted; attempts to warm the machine had been made before diagnosing.
  
• Technicians have inspected the wiring harness, connectors, grounds, and visible wiring for damage, moisture, or insulation breakdown but often find nothing visibly at fault. This tends to lean suspicion toward a fault internal to the ECU.
  

Possible Causes and Diagnostic Checklist
To uncover the root cause of 627.1 with “unswitch short to ground” behavior, here are possible sources and a step-by-step checklist:
Possible causes:

• Damaged or chafed wiring in injector circuit causing insulation wear, leading to connection to ground.
  
• Corroded, loose, or missing ground connections for the ECU or injector driver circuits.
  
• Faulty switched power supply (relay, fuse, ignition switch) feeding the ECU or injector driver circuits, being shorted or not delivering correct voltage.
  
• Internal ECU failure: damaged driver circuits that can’t maintain correct pull-in or hold-in currents.
  
• Moisture ingress or cold-induced contraction causing contact failures.
  

Diagnostic checklist:

1. Battery & Supply Voltage Check
   • Measure battery voltage at rest and during cranking. Low voltage can affect injector driver behavior.
     
   • Measure voltage at the ECU power inputs (switched and constant).
     
2. Ground Integrity
   • Inspect ground leads from the ECU to chassis/battery negative.
     
   • Clean and tighten ground connections.
     
3. Wiring Harness Inspection
   • Trace injector wires from injector to ECU, especially near high-movement points or near heat sources.
     
   • Look for rubbed insulation, melted wire, pinched areas.
     
4. Switch-Relay-Fuse Circuits
   • Check fuses related to ECU power supply.
     
   • Check relays that supply switched power feeding ECU or injector circuits.
     
5. Injector Resistance & Current Draw
   • Measure coil resistance of each injector (cold). Compare to manufacturer spec.
     
   • With diagnostic tool, observe the pull-in & hold-in current readings from ECU (if possible) to see if any injector is significantly divergent.
     
6. ECU Bench Test / Replacement
   • If all wiring, grounds, power supply, and injectors seem good but fault persists, suspect ECU internal driver failure.
     
   • Swapping in a known good ECU (if available) may confirm diagnosis.
     

Repair Solutions and Suggestions
Based on what is typically found, here are solutions:

• Repair or replace damaged wiring: use correct gauge wire, proper insulation, keep wires away from heat and abrasion points.
  
• Ensure grounds are clean, tight, with correct gauge, and free of corrosion. For intermittency, removal and cleaning of ground terminated surfaces makes big difference.
  
• If switched supply relay or ignition switch is weak or pitted, replace. Bad connections can drop voltage under load.
  
• In cold climates, consider warming ECU enclosure or ensuring sealing to prevent condensation/icing that could cause shorts.
  
• If ECU is faulty: replace with correct model ECU specified for 624J, program as needed. Ensure ECU part number correct (matches firmware, engine version).
  

Case Study and Related News
An operator story: a wheel loader in a northern U.S. region stopped during winter. It cranked, but didn’t start; cold weather suspected. After thawing the machine indoors, the code 627.1 persisted. The wiring near injector banks was intact. Ground strap between engine block and chassis was corroded. Replacing that and cleaning ground points restored proper injector current, cleared code, and machine started normally.
In another instance, a dealership reported similar faults in several machines. They noticed that relays supplying power to ECU under-hood were often exposed to engine wash and moisture, causing internal corrosion. After replacing those relays with sealed units and re-routing wiring harnesses, recurrence rate of 627.x codes dropped significantly (estimated 60-70% reduction).

Wider Context and Technical Reference

• John Deere diagnostic manuals list “ECU 627,01,2” as “Warning – All injector currents are out of specification.”
  
• They also list “Injector Shorted to Ground” (ECU 611.04) separately. The fact that 627.1 is triggered suggests it is downstream of or related to injector driver circuits, often electrical load / current faults rather than pure sensor mis-reading.
  

Preventive Maintenance and Recommendations

• Regular inspection of engine bay wiring harness protective sheaths and routing, especially around injectors and near sharp edges or heat.
  
• Scheduled cleaning or replacement of ground straps, battery negative connections, and body/chassis ground points (every 500-1000 hours in harsh conditions).
  
• Use of ECU boxes or relays with moisture- and vibration-resistant seals.
  
• During cold months, warm the machine before starting, ensure batteries are in good condition (cold reduces battery capacity), check that fuel is not gelling.
  
• Keep documentation of codes and past repairs to detect patterns; if many machines same model have 627.x issues, check component batches (relays, ECUs) for defects.
  

Conclusion
The ECU 627.1 “Un-switch short to ground” issue on John Deere 624J loaders is a serious fault that prevents engine start because injector currents are not correct. While many assume the ECU itself is bad, often the problem lies in wiring, grounds, or power supply circuits. A careful diagnostic approach—verifying voltage, ground integrity, wiring condition, relay/fuse health—can often isolate the culprit without expensive ECU replacement. When ECU replacement is required, using the correct part and ensuring proper installation is crucial. With preventive maintenance and attention to wiring and power circuit details, such faults can be minimized.