Digital Signals - MerCruiser 305 CID (5.0L) Service Manual

SERVICE MANUAL NUMBER 24
TWO-WIRE SENSOR (ECT)
The following figure is the schematic of a 2-wire type sensor. This sensor is basically a vari-
able 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 therm-
istors. A thermistor's resistance varies inversely with temperature.
2-Wire Sensor

Digital Signals

Digital signals are also variable, but not continuously. They can only be represented by dis-
tinct voltages within a range. For example, 1 V, 2 V or 3 V would be allowed, but 1.27 V or
2.65 V would not. Digital signals are especially useful when the information can only refer
to two conditions - YES and NO, ON and OFF, or HIGH and LOW. This would be called a
digital binary signal. A digital binary signal is limited to two voltage levels. One level is a posi-
tive voltage, the other is no voltage (zero volts). As you can see in the following figure, a
digital binary signal is a square wave.
Digital Binary Signal
Index
90-861327 OCTOBER 1999
a - Typical Sensor
b - ECM
c - Signal Sensor
d - 5 Volt
e - Sensor Ground
d
a
c
a - Voltage
b - Time
c - Lo
d - Hi
e - On
f - Off
g - Yes
h - No
FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
a
g
e
f h
b
b
c
d
e
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