Engine Coolant Temperature Sensor: Miata

NOTE: Coolant temperature sender is part of ECT sensor.

ECT Sensor Testing Procedure

Disconnect ECT sensor 3-pin harness connector. Remove ECT sensor. ECT sensor is located on left side of engine. Place ECT sensor and a thermometer in a container of water. Connect ohmmeter between ECT sensor terminals "A" and "B". See Fig 2. Slowly heat water. Observe resistance reading at specified temperatures. See ENGINE COOLANT TEMPERATURE (ECT) SENSOR SPECIFICATIONS table. If ECT sensor is okay, but PID value is not within specification, check for open or short circuit. Go to next step. If resistance reading is not as specified, replace ECT sensor.

Short Circuit Inspection

Check for continuity in reference voltage circuit (Red/Blue wire) between ECT sensor 3-pin harness connector terminal "A" and PCM 31-pin harness connector terminal No. 4P through common connector to ground. If continuity is present, repair short circuit in Red/Blue wire between ECT sensor 3-pin harness connector terminal "A" and PCM 31-pin harness connector terminal No. 4P or replace wire harness. If wire harness is okay, replace ECT sensor.

Engine Coolant Sender Testing Procedure

To check coolant temperature sender portion of ECT sensor, measure resistance between ground and ECT sensor terminal "C" (with ECT sensor installed). See Fig 2. Resistance should be 160-230 ohms with sensor temperature at 122°F (50°C). If engine coolant sender resistance is not within specification, replace ECT sensor.

ENGINE COOLANT TEMPERATURE (ECT) SENSOR SPECIFICATIONS

Fig 1: Identifying ECT Sensor & PCM 31-Pin Harness Connectors & Terminals (Miata)

Courtesy of MAZDA MOTORS CORP.

Fig 2: Identifying ECT Sensor Terminals (Miata)

Courtesy of MAZDA MOTORS CORP.