Crank Position Sensor (Cps); Engine Coolant Temperature (Ect) Sensor; Manifold Absolute Pressure (Map) Sensor - Mercury 25 EFI Service Manual

Fuel System Operation

Crank Position Sensor (CPS)

The Crank Position Sensor (CPS) is a hall effect switch. A hall effect switch senses the collapse of a magnetic field. The outer
ring of the flywheel has a raised key or tooth on it. As the flywheel rotates, the CPS senses the location of the crankshaft via the
collapse of the magnetic field caused by the tooth passing the CPS. The CPS supplies the trigger signal information to the
ECM. The ECM utilizes the CPS information and determines when to trigger each ignition coil and fuel injector. The CPS
provides the ECM with engine RPM information, which the ECM uses in determining fuel delivery, spark timing, and over
revolution protection.

Engine Coolant Temperature (ECT) Sensor

The Engine Coolant Temperature (ECT) sensor is located at the top of the cylinder block, aft of the flywheel. It is a thermistor
immersed in the engine coolant path, after the thermostat. It supplies the ECM with engine temperature information. The ECM
adjusts the timing, and the volume of fuel delivered via the fuel injectors, according to the water temperature in the cylinder
block. Low coolant temperature produces high resistance, while high coolant temperature causes low resistance.
When an engine coolant temperature condition occurs over 90 °C (194 °F), the warning horn will sound continuously and the
warning light will be illuminated continuously. When the engine coolant temperature is below 90 °C (194 °F), the warning horn
and the warning lamp turn off.
Should the sensor fail or become disconnected, the warning horn will sound intermittently, the warning lamp will be intermittent
and the engine RPM will be reduced to 2800 or less. The engine must be stopped to cancel the warning.

Manifold Absolute Pressure (MAP) Sensor

The Manifold Absolute Pressure (MAP) sensor measures relative changes in the intake manifold, whether in a vacuum
condition or pressure condition. It is mounted on the intake manifold. A hose connected to the intake manifold, after the throttle
body/shutter, is connected to the MAP sensor. When the engine is rotating during start‑up, the rotating flywheel generates
voltage through the stator to energize the ECM. The ECM then reads the ambient atmospheric pressure through the MAP
sensor. This information is used by the ECM as an indication of altitude and/or the density of the air. The manifold absolute
pressure will change as a direct result of engine load and RPM. This information is used by the ECM to calculate optimum fuel
delivery volume, and spark timing.
Should the sensor fail or become disconnected, the warning horn will sound intermittently, the warning lamp will be intermittent,
and the engine RPM will be reduced to 2800 or less. The engine must be stopped to cancel the warning.
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90-8M0105568 eng MARCH 2016