Troubleshooting Temperature Sensor and Wiring Issues on a CAT 3116 in Medium-Duty Trucks

[MikePhua]

The CAT 3116 and Its Role in Medium-Duty Applications
The Caterpillar 3116 engine was introduced in the late 1980s as a mid-range diesel powerplant designed for vocational trucks, buses, and marine applications. Built with mechanical fuel injection and a cast-iron block, the 3116 was known for its durability and torque delivery in the 170–300 horsepower range. It powered vehicles like the Chevrolet Kodiak and GMC Topkick, often paired with Allison automatic transmissions and used in utility fleets, dump trucks, and bucket trucks.
Despite its mechanical simplicity, the 3116’s integration into GM chassis introduced electrical complexities, especially in sensor wiring and dashboard instrumentation. Many of these trucks used hybrid systems where Caterpillar components interfaced with GM electronics, leading to confusion during diagnostics and repairs.
Locating and Testing the Coolant Temperature Sensor
One common issue involves the coolant temperature gauge remaining at the low end, even after the engine reaches operating temperature. This typically points to a faulty temperature sending unit or disconnected wiring. On the 3116, the coolant temperature sensors are mounted near the upper radiator hose outlet. Two sensors are often present:

• Grey sensor: Typically connected to the dashboard temperature gauge
  
• Black sensor: Often used for air intake heater control or ECM input
  

Terminology:

• Temperature sending unit: A sensor that converts coolant temperature into an electrical signal for the gauge
  
• ECM (Engine Control Module): The electronic brain that manages engine parameters
  
• Air intake heater: A cold-start aid that warms incoming air to improve combustion
  

To test the gauge circuit, technicians can short the wire leading to the grey sensor with the ignition on. If the gauge needle spikes, the sensor is faulty. If the needle remains static, the issue may lie in the wiring or gauge itself.
Wiring Hazards and Preventative Measures
Several photos of the truck revealed bare wires draped across the frame rail and other metal surfaces. This is a serious concern, as vibration and movement will inevitably cause insulation to wear through, leading to shorts, erratic sensor readings, or even electrical fires.
Preventative steps:

• Use wire loom or spiral wrap to protect exposed wires
  
• Secure wiring with zip ties or clamps to prevent movement
  
• Avoid routing wires over sharp edges or heat sources
  
• Inspect wiring annually for signs of abrasion or corrosion
  

One technician recalled his father’s advice: “Any wire rubbing on metal will wear through—no exceptions.” This principle remains true across all vehicle platforms, especially in heavy-duty environments.
Parts Availability and Cross-Referencing Challenges
When attempting to replace the coolant sensor, the owner contacted a Caterpillar dealer, only to be told the part was a GM component. The Chevrolet dealership then reported that most parts for the 1991 Kodiak were discontinued, though they eventually located a compatible sensor under part number 15684629.
This highlights a recurring issue with medium-duty trucks from the 1990s: parts fall into a grey zone between OEM engine suppliers and chassis manufacturers. Cross-referencing becomes essential, especially when dealing with sensors, wiring harnesses, and dashboard components.
Solutions:

• Use online databases to cross-reference OEM and aftermarket part numbers
  
• Search by year, make, model, and part description on e-commerce platforms
  
• Consult forums and technical bulletins for verified substitutions
  
• Keep a log of replaced parts and their sources for future reference
  

In one case, the same sensor was found online for half the dealership price, demonstrating the value of independent sourcing.
Oil Pressure Sensor and Additional Diagnostics
The final image in the diagnostic sequence showed the oil pressure sending unit area. Like the coolant sensor, this component converts mechanical pressure into an electrical signal for the dashboard gauge. Faulty readings can result from:

• Failed sending unit
  
• Grounding issues
  
• Damaged wiring
  
• Incorrect sensor type for the gauge
  

Testing involves using a mechanical gauge to verify actual oil pressure, then comparing it to the dashboard reading. If discrepancies exist, the sending unit should be replaced with a verified compatible part.
Conclusion
Maintaining a CAT 3116 engine in a GM medium-duty truck requires a blend of mechanical knowledge and electrical troubleshooting. From locating the correct temperature sensor to protecting vulnerable wiring and sourcing discontinued parts, each step demands attention to detail and resourcefulness. With proper diagnostics and preventative care, these trucks can continue serving reliably—even decades after their original build date. In the world of mixed-manufacturer platforms, success lies in knowing where to look, how to test, and when to adapt.