The engine must be completely cool before starting work.
The coolant temperature indication system consists of a temperature gauge mounted in the instrument panel and a coolant temperature sensor unit, usually located on the thermostat housing (see illustration). Some vehicles have more than one sensor, but only one of them is used by the temperature gauge. On M40 engines, the sensor block is mounted on the left rear side of the cylinder head.
If you suspect that the meter is not working properly, first check the coolant level in the system. Make sure the wiring between the sensor and the meter is intact and that the fuses are good. (If you suspect that the fuel level meter is also faulty, then the problem is almost certainly in the instrument panel or its wiring).
Refer to the relevant circuit diagrams before checking the electrical circuit (see chapter Onboard electrical system). If the sensor block is simply grounding the circuit, test by grounding the wire connected to the sensor with the ignition on (but without starting the engine, for safety reasons). If the meter goes off scale, replace the sensor assembly. If the meter does not respond, then either it or its wiring is faulty. Where the sensor block has two terminals, test the circuit by measuring the resistance of the block. No hard numbers were available at the time this manual was written, but normal resistance should be in the range of a few hundred to a few thousand ohms (temperature dependent). Readings equal to zero (short circuit) or infinity (open circuit) indicate a faulty sensor assembly.
Do not ground the wire for more than a couple of seconds, otherwise the meter may be damaged.
If a new sensor assembly needs to be installed, allow the engine to cool completely first. Removing the sensor will leak some coolant, so either get ready to assemble it, or keep the new block ready to install it immediately after removing the old one. Disconnect the wiring, then loosen the screws and remove the old sensor block from the engine, then install the new one. Apply sealant to the threads. Connect the wiring and finally check the coolant level.