converted into a usable electrical signal. The ECA uses
this reference for altitude-dependent EGR flow
Coolant Temperature Sensor
This sensor is located at the rear of the intake
manifold and consists of a brass housing that contains a
thermistor. When reference voltage (about 9 volts,
supplied by the processor to all sensors) is applied to the
sensor, the resistance can be measured by the resulting
voltage drop. Resistance is then interpreted as coolant
temperature by the ECA. EGR flow is cut off by the ECA
when a predetermined temperature is reached. If the
coolant temperature becomes too high (due to prolonged
idling), the ECA will advance the initial ignition timing
to increase the idle speed. The increase in engine rpm
will increase coolant and radiator airflow, resulting in a
decrease in coolant temperature.
Inlet Air Sensor
Inlet air temperature is measured by a sensor
mounted in the air cleaner. It operates in the same
manner as the coolant sensor. The ECA uses its signal
to control engine timing. At high inlet temperatures
(above 90°F), the ECA modifies the engine timing to
prevent spark knock.
Crankshaft Position Sensor and Metal Pulse
The crankshaft is fitted with a four-lobe metal pulse
ring. Its position is constantly monitored by the
crankshaft position sensor. Signals are sent to the ECA
representing both the position of the crankshaft and the
frequency of the pulses (engine rpm).
Throttle Position Sensor
The throttle sensor is a rheostat connected to the
throttle plate shaft. Changes in the throttle plate angle
varies the resistance of the reference voltage that is
supplied by the processor. Signals arc interpreted by the
ECA in one of the following three ways:
1. Closed throttle (idle or decelcration)
2. Part throttle (cruise)
3. Full throttle (maximum acceleration)
EGR Valve and Sensor
A position sensor is built into the EGR valve. The
ECA uses the signal from the sensor to determine the
position of the valve. The EGR valve and position sensor
are replaced as a unit.
The distributor is locked in place during engine
assembly. Since all timing is controlled by the ECA,
there are no rotational adjustments possible for initial
ignition timing. There are no mechanical advance
adjustments so there is no need to remove the distributor
except for replacement.
Because of the complicated nature of this system,
special diagnostic tools are necessary for
troubleshooting. Any troubleshooting without these
special tools is limited to mechanical checks of
connectors and wiring.
DISTRIBUTORLESS IGNITION SYSTEM
Some later engines have no distributor as we know
it. The distributor and ignition timing are all a part of an
electronic control unit or ignition module (fig. 4-39).
This system totally eliminates any vacuum or
centrifugal advance mechanism and, in most cases, the
Figure 4-39.Distributorless ignition system wiring diagram.