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The diagram on the previous page shows that the sensors are not wired to the solenoid valve or any control actuator as in a conventional system. Instead, each sensor goes to an ECM input pin and each actuator is wired to an ECM output pin. The ECM computer monitors the data from the sensors and sends the appropriate signals to the controls and actuators based on programming logic. The ECM also sends status information to the operator's warning lights and gauges.

In an ECM controlled system, one sensor may affect several actuators and one actuator may be affected by several sensors, depending on how the ECM is programmed.

Sensors used in ECM systems are different than those used in independent systems. Instead of the simple open/close type of sensor, ECM systems use thermistors and sending units to send signals to the ECM (temperature, pressure, speed, or whatever function is being sensed).

EXAMPLE. Instead of a thermal switch opening or closing at a preset temperature, ECM's use a temperature sensor to provide voltage which the logic program converts into an actual temperature measurement. Instead of simply knowing if the coolant temperature is above or below the set point (i.e., hotter that 190f F [88f C]), the program knows the actual temperature (i.e., 196.4f F [91.3f C]).

The fan drive solenoid is not wired to the sensors as it is in a conventional system but instead wired to a relay controlled by the ECM. The ECM computer program looks at the data from several sensors and decides when to engage and disengage the fan drive. The program considers engine coolant temperature, air-conditioner's refrigerant pressure, intake-manifold air temperature, engine speed, and the engine brake status, and possibly other factors depending on engine configuration.

New troubleshooting techniques may be necessary when working on a truck with an ECM control system. A vehicle's wiring diagram no longer indicates which sensor affects which actuator. The diagram only shows which ECM pin each sensor and actuator is connected. To determine the relationships between the sensors and actuators, refer to the vehicle or engine service manual for descriptions of exact conditions under which each actuator is engaged (i.e. Fault Codes).

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