2016 CASE 750M STARTING ISSUE POWER FROM KEY TO CPU BUT NOT BACK TO STARTER SOLENOID

[MikePhua]

Machine Background and Historical Context
The CASE 750M is a medium‑size crawler dozer widely used in construction, landscaping, and roadwork applications. Manufactured by CASE Construction Equipment, part of CNH Industrial, it represents a modern generation of M Series machines that balance power, efficiency, and operator comfort. Introduced with Tier 4 Final compliant engines around the mid‑2010s, the 750M typically features an FPT 4‑cylinder turbocharged diesel engine delivering about 92 horsepower and weighing around 20,000 lb in operating configuration. This model is engineered with a hydrostatic transmission, variable pitch blade, and robust undercarriage designed to last in tough working conditions. Sales of the M Series have been strong globally due to their reliability and telematics support via integrated systems that help fleet managers monitor machine health and usage.
In heavy equipment, electrical issues that prevent a machine from starting are not just inconvenient; they can halt entire operations. Over the past decade, operators and technicians have increasingly encountered electrical puzzles where modern control units interplay with safety circuits, making fault diagnosis more complex than simply replacing a starter or battery.
Understanding the Electrical Start Circuit
To grasp the issue, it helps to understand key terminology:

• Ignition Switch – A multi‑position switch that directs battery voltage into the machine’s electrical network, including the engine control unit (ECU, also called CPU in this context).
  
• CPU/ECU – The computerized control unit that manages engine functions, safety interlocks, and start logic.
  
• Starter Solenoid – A heavy‑duty electromagnetic switch that connects the high‑current path from the battery to the starter motor when energized.
  
• Relays and Fuses – Intermediate electrical components that protect circuits and provide switching via low‑current control signals.
  

In the reported situation, turning the key sends voltage successfully to the CPU, indicating the ignition switch and primary wiring are functioning up to that point. However, voltage does not return from the CPU or associated safety logic back to the starter solenoid, so the starter motor never engages to crank the engine. This pattern clearly shows the machine goes part‑way through its start logic but gets interrupted before engaging the high‑current starter circuit.
Common Causes and Diagnostic Paths

1. Blown or Damaged Fuses and Relays
   Heavy equipment has multiple protective fuses and relays in the starter and control circuits. A blown fuse or failed relay can allow key voltage to reach the CPU while blocking the path to the starter. Always check both engine bay and cab fuse boxes. Replace suspected blown fuses with correct amp‑rated ones and test relays by swapping with identical known‑good units.
   
2. Battery and Ground Integrity
   Even with a new battery installed, poor ground connections or corrosion at battery terminals, frame ground straps, or engine ground points can prevent sufficient current flow to the solenoid. Use a multimeter to verify solid voltage at both battery posts, ground straps, and the starter solenoid input. Clean terminals and tighten connections as needed.
   
3. Safety Interlocks and Neutral Switches
   CASE machines include safety interlocks: operator presence (seat switch), transmission neutral sensor, and other conditions that must be satisfied before a start signal reaches the solenoid. If a safety switch is open or faulty, the ECU may block the starter signal. Inspect and test these circuits, as they can interrupt the return path.
   
4. Wiring Harness and Connector Issues
   Flexing, vibration, and contamination over time can degrade wiring harnesses and connectors. Look for broken wires, pinched harness segments, or corroded connectors especially on paths from the CPU to the starter solenoid. Repair or replace damaged harness sections.
   
5. Starter Solenoid and High‑Current Path
   Sometimes the starter solenoid itself is the culprit. If it clicks but does not provide power to the starter motor, internal contacts may be worn, or the solenoid coil could be failing. Applying a direct battery feed to the solenoid terminals can help isolate whether the solenoid is switching correctly.
   
6. Control Unit Faults
   The CPU (engine control unit) may detect an internal fault or safety violation and simply not send the starter enable signal even though it has power. ECU faults can be diagnosed with a proper CASE diagnostic tool or electrical schematic, isolating error codes or communication issues.
   

Practical Steps and Solutions

• Verify Electrical Schematic: Obtain the machine’s electrical schematic for the 750M to know exact pinouts and paths for the start circuit; CASE schematics specify connectors and wire colors.
  
• Test Safety Switches: Bypass or simulate closed switch conditions (only temporarily) to determine if a safety interlock is preventing start signals.
  
• Clean and Secure Grounds: Heavy equipment often suffers from voltage drop due to ground issues; cleaning and reattaching grounds improve overall electrical performance.
  
• Check Starter Solenoid Directly: Use a test jumper to feed battery voltage directly to the solenoid control terminal—if the starter engages reliably, the issue is upstream in the control circuit.
  
• Consider ECU Diagnostics: If other steps fail, connecting a CASE dealer or technician with proper scan tools can identify ECU‑level faults.
  

Real‑World Anecdotes and Industry Notes
A contractor in Texas reported similar symptoms on an 850K dozer: after months of intermittent no‑start events, the crew installed a simple push‑button start at the starter and the machine ran reliably, illustrating that sometimes creative bypasses can keep jobs moving, though they aren’t proper long‑term solutions. Such workarounds, while practical under deadline pressure, underscore the value of diagnosing and repairing electrical logic circuits properly for safety and longevity.
Conclusion and Recommendations
When a CASE 750M dozer shows power from the ignition switch to the CPU but fails to energize the starter solenoid, the issue almost always lies in the intermediate circuitry: safety interlocks, relays, fuses, grounds, or wiring harness problems. A structured diagnostic approach—starting at the battery, moving through relays and safety switches, and concluding with starter and control unit evaluation—can isolate the fault efficiently. Keeping comprehensive schematics and a multimeter ready on site greatly reduces downtime from electrical mysteries in modern heavy equipment.