Discovery 2. Manual - part 73

ENGINE MANAGEMENT SYSTEM - V8

DESCRIPTION AND OPERATION 18-2-21

Thermostat Monitoring sensor

The thermostat monitoring sensor is located in the radiator, adjacent the bottom hose. The ECM compares the
temperature measured by the thermostat monitoring sensor to the temperature measured by the ECT sensor. If the
difference between the two readings is too great, the ECM determines the thermostat is stuck. In this case, the ECM
registers a fault code in its memory.

The thermostat monitoring sensor works as a Negative Temperature Coefficient (NTC) sensor. As temperature rises,
the resistance in the sensor decreases, as temperature decreases, the resistance in the sensor increases. With this
information, the ECM is able to monitor the performance of the thermostat. The normal operating parameters of the
thermostat monitoring sensor are as follows:

ENGINE MANAGEMENT SYSTEM - V8

18-2-22 DESCRIPTION AND OPERATION

Input/Output
The ECM provides the thermostat monitoring sensor with a 5 volt reference via pin 21 of connector C0635 of the ECM,
and an earth via pin 5 of connector C0635 of the ECM.

There are three types of thermostat monitoring sensor diagnostic checks:

Sensor signal is above maximum threshold. For the ECM to register this as a fault, and illuminate the MIL, the
temperature registered by the thermostat monitoring sensor must be above 140

C (284

F) for more than 1

second.

Sensor signal is below minimum threshold. For the ECM to register this as a fault, and illuminate the MIL, the
temperature registered by the thermostat monitoring sensor must be below -33

C (-27

F) for more than 1

second, while the inlet air temperature reading is greater than -32

C (-25

F).

Signal difference between ECT sensor and thermostat monitoring sensor is below maximum threshold. For the
ECM to register this as a fault, and illuminate the MIL, the following conditions must exist:

No maximum or minimum threshold signal faults exist.

No faults are recorded against the thermostat monitoring sensor or vehicle speed signal.

Engine not in idle speed control.

Fuel cut-off not active.

Engine speed is greater than 400 rpm.

Road speed is greater than 0 mph.

Integrated mass air flow from engine start to fuel cut-off is greater than set value (between 3 kg and 10 kg
dependent upon engine coolant temperature at engine start).

Engine coolant temperature at engine start is between 9

C and 39

C (48

F and 102

F).

High range is selected.

Delay time before thermostat monitoring is enabled is between set limits (between 50 and 500 seconds
dependent upon engine coolant temperature at engine start).

Engine coolant temperature is greater than 90

C (194

F).

The difference between the ECT sensor reading and the thermostat monitoring sensor reading is less than
39

C (102

F).

ENGINE MANAGEMENT SYSTEM - V8

DESCRIPTION AND OPERATION 18-2-23

Should a malfunction occur, the following fault codes may be evident and can be retrieved by TestBook:

Mass Air Flow (MAF)/ Inlet Air Temperature (IAT) sensor (C0149)

The MAF/ IAT sensors are combined into a single unit and located between the air filter housing and the inlet manifold.
The ECM receives input signals from the MAF/ IAT sensor to calculate the mass of air flowing into the engine inlet
manifold.

Input/Output
The MAF sensor has both electrical input and output. Input to the MAF sensor comes from two different sources.
Battery voltage is supplied to the MAF sensor via fuse 2 of the engine compartment fuse box. The MAF sensor also
utilises a 5 volt reference input via pin 7 of connector C0636 of the ECM. The MAF sensor output voltage is measured
via pin 23 of connector C0636 of the ECM.

The IAT sensor has only electrical output. Output from the IAT sensor is measured at pin 34 of connector C0636 of
the ECM, this is a variable voltage/ resistance measured by the sensor to provide air temperature information to the
ECM.

The MAF/ IAT sensor share the same sensor earth. Sensor earth is via pin 9 of connector C0636 of the ECM.

The MAF/ IAT sensor and its connector has silver plated terminals for its low current signals to protect against
corrosion. DO NOT apply 12V to the 5V supply, as this will destroy the internal circuitry. The MAF/IAT sensor should
not be dropped or roughly handled and should be kept free from contamination.

P code

J2012 description

Land Rover description

P1117

Radiator outlet temperature

thermister low

Thermostat reading below -33

C (-

P1118

Radiator outlet temperature

thermister high

Thermostat reading above 140

(284

P0126

Engine thermostat defective

Difference in radiator and engine

coolant temperatures too small

ENGINE MANAGEMENT SYSTEM - V8

18-2-24 DESCRIPTION AND OPERATION

Mass Air Flow (MAF) Sensor
The MAF sensor utilises a “hot film” element contained in the air intake duct to monitor the mass of the air flow being
drawn into the engine. The MAF sensor contains two sensing elements, one element is controlled at ambient
temperature (e.g. 25

C (77

F)), while the other is heated to 200

C (360

F) above the ambient temperature (e.g. 225

(437

F)).

When the intake air passes the heated element, it cools it down, so lowering the resistance of the hot film element. In
order to maintain the same temperature, the circuit to the heated element has to supply more current. The change in
current causes a corresponding change in potential difference to be detected in the monitoring circuit. This change is
supplied to the ECM as a voltage between 0 and 5V, where it is processed by the ECM's internal mapping to interpret
the data as a measure of the mass of air flow.

The measured air mass flow is used by the ECM to determine the fuel quantity to be injected in order to maintain the
stoiciometric air:fuel mixture for optimum engine performance and low emissions.

Normal operating parameters of the MAF sensor are as follows:

MAF output

If the MAF sensor fails, the ECM implements a back up strategy which is based on throttle angle. Poor throttle
response and reduced performance will result.

The MAF sensor can fail the following ways or supply incorrect signal:

Sensor open circuit.

Short circuit to vehicle supply.

Short circuit to vehicle earth.

Contaminated sensor element.

Damaged sensor element.

Air leak after the MAF sensor.

Inlet air restriction.

Resistance in wiring harness causing signal offset.

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