MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION SERVICE MANUAL NUMBER 23
Page 5B-16 90-861326--1 MARCH 1999
TWO-WIRE SENSORS (ECT AND IAT)
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.
a
b
c
d
e
2-Wire Sensor
a-Typical Sensor
b-ECM
c-Signal Sensor
d-5 Volt
e-Sensor Ground
Digital Signals
Digital signals are also variable, but not continuously. They can only be represented by
distinct 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
positive voltage, the other is no voltage (zero volts). As you can see in the following figure,
a digital binary signal is a square wave.
a
b
c
d
e
f
g
h
Digital Binary Signal
a-Voltage
b-Time
c-Lo
d-Hi
e-On
f-Off
g-Yes
h-No
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