AZ35440 John Deere Coolant Temperature Sensor (Water Temperature Sensor)—compatible with John Deere tractors, loaders, harvesters, and various diesel engines—monitors engine coolant temperature and provides temperature control signals to the ECU. I. Model and Classification

OE Part Number: AZ35440 (John Deere Original)

Replacement Part Number: AZ26997

Type: NTC Thermistor-type Water Temperature Sensor

Aliases: Coolant Temperature Sensor, Water Temperature Sending Unit

II. Applicable Models (Core)

Engine Series

4239D / 4239T (3.9L)

3029 (2.9L), 4039 (3.9L), 4045 (4.5L)

6059, 6068 (6.8L)

Complete Machinery/Equipment

Tractors: 2155, 2355, 2355N, 2555, 2755

Loaders: 324H, 344H, 605C

Combine Harvesters: 1032, 1042, 1052, 1065, 1072, 1075, 1155, 1165, 1170, 1450, 1550, etc.

III. Key Parameters (Industry Standard)

Table

Parameter Specification

Working Principle NTC (Negative Temperature Coefficient) Thermistor

Supply Voltage DC 12V / 24V (Construction Machinery Standard)

Measurement Range -40°C to +125°C

Output Signal Analog Resistance / Voltage Signal (Varies with temperature)

Electrical Interface 1-Pin / 2-Pin Waterproof Connector

Thread Interface M14×1.5 / PT1/8

Protection Rating IP67

Operating Temperature -40°C to +125°C

IV. Function and Principle

Core Function: Real-time monitoring of engine coolant temperature; converts the temperature signal into a resistance/voltage signal for transmission to the ECU. This signal is used to control fan activation/deactivation, fuel injection volume, and ignition timing. Additionally, it triggers a high-temperature alarm in the event of overheating, thereby preventing engine damage such as cylinder scoring or bearing seizure. Principle: The internal NTC thermistor's resistance value decreases as temperature rises; the ECU calculates the coolant temperature by monitoring the changes in resistance or voltage.

VI. Installation and Troubleshooting

Installation Guidelines

Install into the coolant passage located on the engine cylinder head or thermostat housing.

Clean the threads and sealing surfaces; replace the copper washer or O-ring.

Tightening Torque: 20–30 N·m.

Ensure the electrical connector is watertight and makes good contact.

Common Faults and Symptoms

Coolant temperature gauge is unresponsive / displays abnormal readings: Open circuit in the sensor, corroded connector pins, or short circuit in the wiring harness.

Cooling fan fails to activate / runs continuously: Abnormal sensor signal causing the ECU to misinterpret the coolant temperature.

Difficulty starting the engine / Unstable idle: Inaccurate signal when the engine is cold, leading to abnormal fuel injection volume.

False triggering of the high-temperature warning light: Internal short circuit within the sensor or resistance value drift.

Simple Testing Procedure

Measure the resistance using the resistance range on a multimeter:

At room temperature (25°C): Approx. 2000–3000 Ω

In hot water (90°C): Approx. 200–300 Ω

If the resistance value shows no change or is abnormal: Replace the sensor.

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