Case IH 3678 – Engine Control Unit (ECU) / Electronic Diesel Control (EDC): Temperature sensors : below plausible temperature

This fault code indicates that one of the engine's temperature sensors is transmitting a reading that is unrealistically low for the current operating conditions. This anomaly suggests a malfunction within the sensor itself (such as an internal short or failure) or an issue with its associated circuitry, such as a broken wire or corroded connector. When this code is triggered, the system may illuminate the STOP light on the dashboard as a precautionary measure to prevent potential engine damage.

• Farmall 85C
• Farmall 95
• Farmall 105U
• Farmall 115U
• Farmall 120U
• Maxxum 100
• Maxxum 110
• Maxxum 115
• Maxxum 120
• Maxxum 125
• Maxxum 130
• Maxxum 140
• Maxxum 150
• Puma 130
• Puma 140
• Puma 150
• Puma 165
• Puma 170
• Puma 185
• Puma 200
• Puma 240
• Magnum
• Steiger
• Quadtrac

1. 1

   Connect a diagnostic scan tool and monitor live data for engine temperature sensors (Coolant, Intake Air, Fuel, Exhaust Gas) to identify which sensor is reading below plausible values.

2. 2

   Inspect the wiring harness leading to the suspect sensor for chafed wires, rodent damage, or pinched lines.

3. 3

   Disconnect the sensor and inspect the connector pins for corrosion, moisture ingress, or pushed-back terminals.

4. 4

   Measure the electrical resistance (Ohms) across the sensor's terminals with a multimeter and compare against OEM specifications for the current ambient temperature to check for an open circuit or dead sensor.

5. 5

   Turn the key to the ON position (engine off) and probe the harness connector to ensure the ECU is supplying the correct 5V reference voltage to the sensor.

1. 1

   Safety First

   Turn off the engine, remove the key, and disconnect the battery to prevent electrical shorts and comply with safety regulations. Ensure the engine is completely cool before working on the cooling system.

2. 2

   Locate the Sensor

   Locate the faulty temperature sensor. The engine coolant temperature sensor is typically positioned directly on the engine thermostat housing or the cylinder head water jacket.

3. 3

   Prepare for Fluid Loss

   If replacing a fluid sensor (coolant or oil), place a clean drain pan underneath the work area. You may choose to partially drain the system to a level below the sensor to minimize spills.

4. 4

   Remove the Sensor

   Depress the locking tab and disconnect the electrical pigtail. Use a deep socket or wrench to carefully unthread the old sensor from the housing.

5. 5

   Install the New Sensor

   Thread the new OEM sensor into the housing by hand to prevent cross-threading. Torque the sensor to the OEM specification (e.g., M14x1.5 threaded temperature sensors on these engines are typically torqued to 30.2 to 36.9 lb-ft).

6. 6

   Reassembly

   Reconnect the electrical connector until it clicks securely. Refill any lost coolant or fluids and bleed the system of air if necessary.

7. 7

   Clear Codes

   Reconnect the battery, start the engine, and use your diagnostic tool to clear the 3678 fault code. Monitor the live data to ensure the new sensor is reading plausible temperatures as the engine warms up.