Toro Workman HDX Service Manual page 43

Engine Coolant Temperature Sensor
The engine coolant temperature sensor plays an impor-
tant role in communicating the engine's current operat-
ing temperature to the ECU. This sensor is protected
within a brass body making it less responsive compared
to the air temperature sensor. This sensor is most bene-
ficial in cold start conditions by telling the ECU to make
corrections to the base injection time by keeping the in-
jectors open longer, thus delivering more fuel during en-
gine warm up.
When the coolant temperature reaches about 140
o
(60 C), the engine is considered to be at sufficient oper-
ating temperature to turn on the "closed loop" function.
This allows the engine to run at a much leaner air fuel
ratio for economy and exhaust catalyst performance.
This sensor is a NTC (negative temperature coefficient)
type. At cold temperatures the NTC type sensor's resist-
ance is very high, but reduces as coolant temperature
increases.
Heated Oxygen Sensor
The oxygen sensor is the last in--line device to provide
input for the ECU. It is used to inform the ECU about the
effectiveness of its output commands. The oxygen sen-
sor is placed in the exhaust system to detect whether or
not the engine is operating in a rich, lean or stoichiomet-
ric state.
The sensor works by creating an electrical voltage sig-
nal (0 to 1V DC) when it senses a difference in the level
of oxygen in the exhaust compared to the level of oxy-
gen in the outside atmosphere. When the mixture is rich,
meaning there is more fuel than oxygen present in the
exhaust system, the voltage output from the sensor will
increase. The voltage will lessen when a leaner mixture
is present. The heating element of this sensor is de-
signed to bring the sensor up to operating temperature
quickly. This sensor provides the most accurate signal
within a temperature range window normally seen dur-
ing most engine loads and RPM's.
Workman HDX
o
F
Rev. A
Page 3 - - 25
Figure 24
Figure 25
Briggs/Daihatsu Gasoline Engine