Manifold Air Temperature (Mat) Sensor; Throttle Position Sensor (Tps); Oil Pressure Switch; Actuators - Mercury 25 EFI Service Manual

Manifold Air Temperature (MAT) Sensor

The Manifold Air Temperature (MAT) sensor is a thermistor that controls a signal voltage to the ECM. It is located outside the
intake manifold, near the throttle body/shutter. It informs the ECM of the air temperature inside the engine cowl. The ECM
adjusts the fuel injection volume 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.

Throttle Position Sensor (TPS)

The Throttle Position Sensor (TPS) is located on the throttle body, and is in direct contact with the throttle shaft. The TPS
transmits throttle angle information to the ECM which regulates the fuel injector volume (pulse width) and ignition timing. Should
the sensor fail, a warning horn will sound and the engine RPM will be reduced by the ECM.

Oil Pressure Switch

The oil pressure switch is located on the port side of the engine, and protrudes into the pressurized oil galley after the oil filter.
The sensor sends a signal to the ECM indicating a low oil pressure condition, which activates a warning horn. Engine RPM will
automatically be reduced when the oil pressure is below the minimum specification.
During a low oil pressure condition for more than 5 seconds, the warning horn will sound continuously, the warning lamp will be
illuminated continuously, and the engine RPM will be reduced to 2800 or less. The engine must be stopped to cancel the
warning.

Actuators

Function of Actuators

Actuators receive output signals from the ECM, and perform functions which control air‑fuel ratios, spark advance, and idle
RPM. The actuators maintain optimal fuel economy as a direct relation to the engine horsepower.

Engine Control Module

The ECM requires 5 volts to operate. If the ECM should fail, the engine will stop running. Inputs to the ECM can be monitored
and tested by the computer diagnostic system (CDS).
CDS G3 Diagnostic Tool Kit
The ECM performs the following functions:
• Calculates precise fuel and ignition timing requirements based on engine speed, throttle position, manifold pressure,
manifold air temperature, and engine coolant temperature.
• Directly controls the ground circuit of fuel injectors, ignition coils, high‑pressure fuel pump, and idle air control.
• Indirectly controls the positive circuit of fuel injectors, ignition coils, and idle air control.
• Controls alarm horn and warning functions.
90-8M0105568 eng MARCH 2016
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© 2016 Mercury Marine
Fuel System Operation
Page 3A-5