303-14C Electronic Engine Controls- 2.7L Diesel: Description And Operation (page 14 of 23)

sensor and has a cooling effect on the film. The ECM monitors the current required to maintain the 200°Celsius (392°F) 
differential between the two elements and uses the differential to provide a precise, non-linear, signal which equates to 
the volume of air being drawn into the engine. 
 
The MAF sensor output is a digital signal proportional to the mass of the incoming air. The ECM uses this data, in 
conjunction with signals from other sensors and information from stored fuelling maps, to determine the precise fuel 
quantity to be injected into the cylinders. The signal is also used as a feedback signal for the EGR system. 
 
The IAT sensor incorporates a Negative Temperature Coefficient (NTC) thermistor in a voltage divider circuit. The NTC 
thermistor works on the principle of decreasing resistance in the sensor as the temperature of the intake air increases. As 
the thermistor allows more current to pass to ground, the voltage sensed by the ECM decreases. The change in voltage is 
proportional to the temperature change of the intake air. Using the voltage output from the IAT sensor, the ECM can 
correct the fuelling map for intake air temperature. The correction is an important requirement because hot air contains 
less oxygen than cold air for any given volume. 
 
The MAF sensor receives a 12V supply from the Battery Junction Box (BJB) and a ground connection via the ECM. Two 
further connections to the ECM provide a MAF signal and IAT signal. 
 
The IAT sensor receives a 5V reference voltage from the ECM and shares a ground with the MAF sensor. The signal 
output from the IAT sensor is calculated by the ECM by monitoring changes in the supplied reference voltage to the IAT 
sensor voltage divider circuit.  
 
The ECM checks the calculated air mass against the engine speed. If the calculated air mass is not plausible, the ECM 
uses a default air mass figure which is derived from the average engine speed compared to a stored characteristic map. 
The air mass value will be corrected using values for boost pressure, atmospheric pressure and air temperature. 
 
If the MAF sensor fails the ECM implements the default strategy based on engine speed. In the event of a MAF sensor 
signal failure, any of the following symptoms may be observed: 
 

Difficult starting  
Engine stalls after starting  
Delayed engine response  
Emission control inoperative  
Idle speed control inoperative  
Reduced engine performance.  

If the IAT sensor fails the ECM uses a default intake air temperature of -5°Celsius (23°F). In the event of an IAT sensor 
failure, any of the following symptoms may be observed: 
 

Over fuelling, resulting black smoke emitting from the exhaust.  
Idle speed control inoperative.  

ENGINE COOLANT TEMPERATURE SENSOR 

 
The engine coolant temperature sensor is located in the top hose at the coolant manifold junction. The ECT sensor 
provides the ECM and the instrument cluster with engine coolant temperature status. 
 
The ECM uses the temperature information for the following functions: 
 

Fuelling calculations  
Limit engine operation if engine coolant temperature becomes too high