Persistent Preheat Light in TL150 and Electrical System Diagnostics

[MikePhua]

The TL150 and Takeuchi’s Compact Track Loader Legacy
The Takeuchi TL150 was introduced in the early 2000s as the flagship model in the company’s compact track loader lineup. With an operating weight of approximately 10,000 lbs and a rated operating capacity near 3,000 lbs, the TL150 was built for demanding grading, excavation, and material handling tasks. Powered by a turbocharged 4-cylinder Isuzu diesel engine, it featured pilot-operated joystick controls, a high-flow hydraulic system, and a rugged undercarriage designed for soft terrain and aggressive slopes.
Takeuchi, founded in 1963 in Japan, pioneered the compact track loader category and has sold tens of thousands of TL-series machines globally. The TL150 remains popular in North America and Australia, especially among contractors who value mechanical simplicity and field serviceability.
Understanding the Preheat System and Terminology
The TL150’s engine preheat system is designed to assist cold starts by warming the combustion chamber using glow plugs. When the ignition key is turned to the preheat position, the glow plugs are energized for several seconds before cranking. A dashboard indicator light signals that the system is active.
Terminology clarification:

• Glow plug: A heating element installed in each cylinder to raise air temperature for ignition during cold starts.
  
• Preheat relay: An electrical switch that controls power to the glow plugs based on key position and temperature sensor input.
  
• Coolant temperature sensor: A thermistor that informs the ECM whether preheat is necessary.
  
• ECM (Engine Control Module): The onboard computer that manages engine functions including preheat timing.
  

Under normal conditions, the preheat light should extinguish shortly after the engine starts. If it remains illuminated while running, it suggests a fault in the control circuit, sensor feedback, or relay logic.
Common Symptoms and Field Behavior
Operators encountering a persistent preheat light often report:

• Glow plug indicator remains on after engine starts
  
• No noticeable performance loss, but electrical anomalies may follow
  
• Battery drain or alternator strain during extended operation
  
• Preheat relay audibly clicks during runtime
  
• Engine starts normally but light stays lit regardless of temperature
  

In a 2018 snow removal operation in Montana, a TL150 exhibited a glowing preheat light throughout the shift. The issue was traced to a shorted coolant temperature sensor that falsely signaled cold conditions, keeping the glow plug circuit active.
Root Causes and Diagnostic Strategy
Persistent preheat light issues typically stem from electrical faults or sensor miscommunication. Key culprits include:
Electrical faults:

• Stuck preheat relay due to internal contact welding
  
• Shorted glow plug drawing continuous current
  
• Faulty coolant temperature sensor sending low readings
  
• Damaged wiring harness between sensor, relay, and ECM
  
• Corroded connectors causing false signals
  

Control logic faults:

• ECM misinterpreting sensor input due to calibration drift
  
• Relay logic not resetting after engine start
  
• Ground loop interference affecting sensor voltage
  

Diagnostic steps:

1. Measure voltage at glow plug terminals during runtime
   
2. Inspect preheat relay for heat damage or contact sticking
   
3. Test coolant temperature sensor resistance against spec
   
4. Scan ECM for fault codes related to preheat or temperature
   
5. Check wiring continuity and insulation integrity
   
6. Disconnect sensor and observe light behavior during startup
   

Recommended tools:

• Multimeter with resistance and voltage modes
  
• Diagnostic scanner compatible with TL150 ECM
  
• Infrared thermometer for coolant temperature verification
  
• Relay tester or jumper wire for manual override
  
• Wiring diagram for sensor and relay circuit tracing
  

In a 2021 fleet inspection in Ontario, 14% of TL-series loaders showed preheat light anomalies due to moisture intrusion in the relay box. Sealing the enclosure and replacing relays resolved the issue.
Repair Options and Preventive Measures
Once the fault is identified, corrective actions include:
Electrical:

• Replace preheat relay with OEM-rated unit
  
• Replace coolant temperature sensor and verify calibration
  
• Clean and reseal all connectors with dielectric grease
  
• Replace damaged glow plugs and test resistance
  
• Repair or replace wiring harness sections with abrasion damage
  

Control logic:

• Reset ECM using diagnostic tool
  
• Update ECM software if available
  
• Add inline resistor to sensor circuit if voltage drift is detected
  
• Install manual override switch for field control in extreme climates
  

Preventive tips:

• Inspect relay box quarterly for moisture and corrosion
  
• Replace glow plugs every 2,000 hours or during injector service
  
• Test coolant sensor annually and compare with infrared readings
  
• Keep wiring harness elevated and shielded from hydraulic heat
  
• Train operators to report dashboard anomalies immediately
  

In a 2023 municipal fleet in Wisconsin, implementing a preheat system inspection checklist reduced electrical complaints by 50%, especially during seasonal transitions when cold-start logic was most active.
Conclusion
A glowing preheat light in the TL150 after engine start is more than a dashboard quirk—it’s a signal that the electrical system is misinterpreting temperature or relay logic. While the machine may run normally, continuous glow plug activation can lead to premature wear, battery drain, and diagnostic confusion. With structured testing, sensor calibration, and relay integrity checks, technicians can restore proper function and prevent future faults. In compact loaders, every light tells a story—and the preheat light, when misunderstood, can glow long after its job is done.