Mercury MerCruiser D2.8L D-Tronic Service Manual page 476

DESCRIPTION
SENSORS WITH MORE THAN TWO-WIRES (MAP / IAT AND TP)
As an example, the following figure shows a schematic representation of a 3-wire sensor.
All 3-wire sensors have a reference voltage, a ground and a variable wiper. The lead coming
off of the wiper will be the signal to the Engine Control Module (ECM). As this wiper position
changes, the signal voltage returned to the computer also changes.
3-Wire Sensor
TWO-WIRE SENSORS (ECT)
The following figure is the schematic of a 2-wire type sensor. This sensor is basically a
variable resistor in series with a fixed-known resistor within the computer. By knowing the
values of the input voltage and the voltage drop across the known resistor, the value of the
variable resistor can be determined. The variable resistors that are commonly used are
called thermistors. A thermistor's resistance varies inversely with temperature.
2-Wire Sensor
Page 5A-10
SERVICE MANUAL NUMBER 22
90-860074--1 FEBRUARY 2002