DIAGNOSTICS
SERVICE MANUAL NUMBER 24
Code 25 (1 of 3): Intake Air Temperature (IAT) Sensor Circuit
ECM
B
A
MEFI 3
J2-30
J2-3
CIRCUIT DESCRIPTION:
The Intake Air Temperature (IAT) Sensor uses a thermistor to control the signal voltage to
the ECM. The ECM applies a voltage on CKT 472 to the sensor. When the intake air is cold,
the sensor (thermistor) resistance is high, therefore, the ECM will see high signal voltage.
As the intake air warms, the sensor resistance becomes less, and the voltage drops. At nor-
mal engine operating temperature, 160-180°F (71-82°C), the voltage will measure about
1.5
to 2.0 volts.
DIAGNOSTIC AIDS:
Check for the following conditions:
•
Poor connection at ECM. Inspect harness connectors for backed out terminals,
improper mating, broken locks, improperly formed or damaged terminals, and poor
terminal to wire connection.
•
Damaged harness. Inspect the wiring harness for damage. If the harness appears to
be OK, observe the IAT display on the scan tool while moving connectors and wiring
harnesses related to the IAT sensor. A change in the IAT display will indicate the
location of the fault.
•
•
The scan tool displays intake air temperature in degrees celsius and fahrenheit. If
the engine is cold (not running within 8 hours), the scan tool should display a IAT
sensor value within a few degrees of outside air temperature. This may help aid in
diagnosing a “shifted” sensor. After engine is started, the temperature should rise
steadily and then stabilize at operating temperature.
Check harness routing for a potential short to ground in CKT 472.
After repairs, clear DTC following “Clearing Trouble Codes” procedure at the front of this
section. Failure to do so may result in DTC not properly being cleared.
Index
Page 5G-66
90-861327--1 OCTOBER 1999
| Categories | Mercury MerCruiser Manuals |
|---|---|
| Tags | Mercury MerCruiser 305 CID, Mercury MerCruiser 350 CID |
| Model Year | 1998, 1999, 2000, 2001 |
| Download File |
|
| Document File Type | |
| Copyright | Attribution Non-commercial |
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