converted into a usable electrical signal. The ECA uses this reference for altitude-dependent EGR flow requirements.

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 Ring

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.

Distributor

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.