The air temperature sensor on the Mercury Marine EFI 150, 175, 200, 225 Pro Max, 300 X and Racing (ROS 260, 280, 300 Drag, S3000) EFI 2.0 Liter, 2.4 Liter, 2.5 Liter, and 3.0 Liter 2-stroke V6 outboard engines is a critical component of the engine's fuel management system. It plays a vital role in ensuring optimal performance by providing the ECU (Electronic Control Unit) with real-time data on the intake air temperature.
This data allows the ECU to adjust the air-fuel mixture to maintain efficiency and power output, particularly in high-performance applications like the Mercury Marine and Racing EFI engines.
The air temperature sensor transmits manifold absolute air temperature, through full rpm range, to the ECU. As air temperature increases “sensor”
resistance decreases causing the ECU to decrease fuel flow (leaner mixture).
Disconnecting the air temperature sensor (creating an open circuit) will increase fuel flow (richen mixture by 10%). Bypassing air temp sensor (creating a short in circuit) will cause fuel flow to decrease 10%.
The air temperature sensor, with part numbers 13221A1 and 13221T01, must be tested periodically to prevent performance issues and ensure the engine runs as intended.
Air Temperature Sensor Testing Procedure
Sensor Functionality:
The air temperature sensor (P/N 13221A1 or 13221T01) measures the intake air temperature and sends this data to the ECU (Electronic Control Unit). The ECU adjusts the air-fuel mixture based on the temperature.
Key Behavior:
As the air temperature increases, the resistance of the sensor decreases.
If the sensor circuit is open (disconnected), fuel flow increases by 10%.
If the sensor circuit is shorted (bypassed), fuel flow decreases by 10%.
Tools Needed:
Digital Multimeter (capable of measuring resistance in ohms).
EFI Tester (P/N 91-11001A2) for detailed ECU system checks.
Testing Steps:
Disconnect the Sensor:
Locate the air temperature sensor (part numbers 13221A1 or 13221T01) on the intake manifold and disconnect its wiring harness.
Measure Resistance:
Set the multimeter to the resistance (ohms) setting.
Place the meter leads on the sensor terminals.
Compare the resistance reading to the values provided in the service manual.
Resistance-to-Temperature Values:
At 32°F (0°C): Resistance is approximately 9,000–11,000 ohms.
At 77°F (25°C): Resistance is approximately 2,000–3,000 ohms.
At 100°F (38°C): Resistance is approximately 1,200–1,400 ohms.
Interpret Results:
If the resistance values fall within the specified range for the measured temperature, the sensor is functioning correctly.
If the resistance is infinite (open circuit) or outside the expected range, the sensor is faulty and needs replacement.
Testing with EFI Tester:
Connect the EFI tester to the engine's ECU system.
Follow the EFI tester instructions to verify the air temperature sensor's operation within the ECU's feedback loop.
Notes:
The air temperature sensor (P/N 13221A1, 13221T01) is critical for proper engine performance and fuel efficiency.
A faulty sensor can cause the engine to run rich or lean, leading to performance issues or potential engine damage.
Always verify the wiring connections and inspect for signs of corrosion or damage before replacing the sensor.
Testing the air temperature sensor on Mercury EFI 2.5L outboard engines is a straightforward yet essential procedure to ensure the optimal operation of the engine's fuel management system.
Regular inspection and maintenance of this sensor, identified by part numbers 13221A1 and 13221T01, not only enhance fuel efficiency but also prevent potential damage caused by improper air-fuel mixtures.
Following the outlined testing procedure with the right tools ensures that the sensor functions accurately, keeping your high-performance Mercury outboard engine running at its peak.
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