Temperature Sensors; Cylinder Head Coolant Temperature Sensor - Mercury 200 OptiMax Jet Drive Service Manual

Ignition
Meter Test Leads
Red
A
A
B
B
C
C
5. If the ohm check of the MAP sensor indicates that the MAP sensor is serviceable, perform an ohm check on the wiring between
the MAP sensor connector and the PCM.

Temperature Sensors

Cylinder Head Coolant Temperature Sensor

The cylinder head coolant temperature sensor is a thermistor immersed in the engine coolant stream. Each cylinder head has a
sensor and it is located near the top of the cylinder head. It supplies the PCM with temperature information. The PCM adjusts the
timing and the amount of fuel delivered according to the water temperature in the head. Low coolant temperature produces high
resistance, while high temperature causes low resistance. The normal resistance value for the sensor at 20 °C (68 °F) is 12.5 kΩ
± 10%.
Cylinder Head Coolant Temperature Sensor Test
The PCM monitors the cylinder head coolant temperature sensors when the ignition key is in the run position. The computer
diagnostic system (CDS) can provide a numerical readout of the head temperature before and after the engine is started. With
the engine not running, cylinder head coolant temperature should be approximately the same as the ambient air temperature.
After the engine is started, the temperature sensor should indicate a rise in head coolant temperature to approximately 40–50 °C
(104–122 °F). Outside air temperature and the temperature of the water that the engine is operating in will directly affect the engine
block temperature.
a
1. With the engine running and the CDS connected to the engine, if the cylinder head coolant temperature sensor does not
appear to be indicating a temperature change, shake or move the sensor harness and connector. If the temperature begins
to change, look for a broken, loose, or corroded wire.
Computer Diagnostic System (CDS)
2. Disconnect the engine harness connector from the sensor.
3. Perform a visual inspection of the pins at the sensor and the wires coming from the connector. Look for broken, bent, or
corroded pins at the sensor; and loose, broken, or corroded wires at the connector.
Page 2A-26
Meter Scale
Black
B
C
A
C
A
B
b
Auto
Auto
Auto
Auto
Auto
Auto
a - Port cylinder head sensor
b - Starboard cylinder head temperature
sensor
45740
Order through SPX
Reading (Ω)
at 20 °C (68 °F)
4.28 kΩ ± 30%
98.6 kΩ ± 30%
4.28 kΩ ± 30%
102.6 kΩ ± 30%
98.6 kΩ ± 30%
102.6 kΩ ± 30%
31760
90-8M0050731 MAY 2011