TD5 Defender
ENGINE MANAGEMENT SYSTEM
7
DESCRIPTION AND OPERATION
SENSOR - MASS AIR FLOW (MAF)
The MAF sensor is located in the intake system between the air filter housing and the turbocharger. The ECM
uses the information generated by the MAF sensor to control exhaust gas recirculation (EGR).
The MAF sensor works on the hot film principal. The MAF sensor has 2 sensing elements contained within a film.
One element is controlled at ambient temperature e.g. 25
°
C (77
°
F) while the other is heated to 200
°
C (360
°
F)
above this temperature e.g. 225
°
C (437
°
F). As air passes through the MAF sensor the hot film will be cooled.
The current required to keep the constant 200
°
C (360
°
F) difference provides a precise although non-linear signal
of the air drawn into the engine. The MAF sensor sends a voltage of between 0 and 5 volts to the ECM,
proportional to the mass of the incoming air. This calculation allows the ECM to set the EGR ratio for varying
operating conditions.
Inputs / Outputs
The MAF sensor receives battery voltage (C0149-3) via the ECM on a brown/orange wire. Signal output on a
slate/light green wire from the MAF sensor (C0149-2) to the ECM (C0158-11) is a variable voltage proportional to
the air drawn into the engine. The MAF sensor is provided an earth (C0149-1) via the ECM (C0158-20) on a
pink/black wire.
The MAF sensor can fail or supply an incorrect signal if one or more of the following occurs:
•
Sensor open circuit.
•
Short circuit to vehicle supply.
•
Short circuit to vehicle earth.
•
Contaminated sensor element.
•
Damaged sensor element.
•
Damaged wiring harness.
•
MAF sensor supplies incorrect signal (due to air leak or air inlet restriction).
In the event of a MAF sensor signal failure any of the following symptoms my be observed:
•
During driving, engine speed may dip before recovering.
•
Difficult starting.
•
Engine stalls after starting.
•
Delayed throttle response.
•
EGR inoperative.
•
Reduced engine performance.
•
MAF signal out of parameters.
In the event of a MAF sensor failure, the ECM will use a fixed default value from its memory.