Mercury 200 OptiMax Jet Drive, Service Manual, Page: 109 / 391

Ignition
Page 2A-28 90-8M0050731 MAY 2011
Manifold Intake Air Temperature (MAT) Sensor
The manifold intake air temperature sensor is a thermistor that controls a signal voltage to the PCM. It is located on the intake
manifold. It informs the PCM of the air temperature inside the intake manifold. The PCM adjusts the fuel injection duration needed
to run the engine at optimum efficiency according to the MAT information. When intake air is cold, the sensor resistance is high.
As the air temperature rises, resistance lowers.
15241
Manifold Intake Air Temperature Sensor Test
The computer diagnostic system (CDS) will determine the proper functioning of the MAT sensor by providing a numerical readout
of the sensor temperature before and after the engine is started. With the engine not running, intake air temperature should be
approximately the same as the ambient air temperature. After the engine is started, the temperature sensor should indicate a rise
in air intake temperature. Outside air temperature will directly affect the engine manifold air intake temperature.
1. With the engine running and the CDS connected to the engine, if the MAT sensor does not appear to be indicating a
temperature change, shake or move the sensor harness and connector. If the temperature begins to change, look for a broken,
loose, or corroded wire.
Computer Diagnostic System (CDS) Order through SPX
2. Disconnect the connector from the sensor.
3. Perform a visual inspection of the pins at the sensor and the wires coming from the connector. Look for broken, bent, or
corroded pins at the sensor; and loose, broken, or corroded wires at the connector.
4. The sensor can be tested with an ohmmeter by disconnecting it from the harness and heating or cooling the end of the sensor
at a controlled temperature. If the readings do not match those in the table, replace the sensor and retest.
Manifold Intake Air Temperature Sensor Ohm Test
Centigrade ‑10 °C 20 °C 40 °C 65 °C 95 °C
Fahrenheit 14 °F 68 °F 104 °F 150 °F 203 °F
Ohms 55.3 kΩ ± 10% 12.5 kΩ ± 10% 5.3 kΩ ± 10% 2.08 kΩ ± 10% 787 Ω ± 10%
5. If the wiring is serviceable, replace the PCM and recheck the sensor function using the CDS.
Guardian Protection System
The Guardian Protection System monitors critical engine functions and will reduce engine power accordingly in an attempt to keep
the engine running within safe operating parameters.
If a sensor fails, the PCM will compensate so the engine will not go into a rich condition. Disconnecting a sensor for troubleshooting,
may have no noticeable effect and be misleading the diagnosis of the engine. All sensor failures will be recorded and stored in
the PCM.