Pontiac Fiero 1988 Service Manual page 485

6E2-A-30 DRIVEABILITY AND EMISSIONS - FUEL INJECTION (TBI)
COOLANT
TEMPERATURE
SENSOR
ECM
452
r
410 YEL
• 452 BLK
B8
B12
TO TPS
SENSOR
5-1-87
COOLANT SENSOR
SIGNAL
5 V
SENSOR
"GROUND
4-6-87
*7S3205-6EA
CODE 15'
COOLANT TEMPERATURE SENSOR CIRCUIT
(LOW TEMPERATURE INDICATED)
2.5L "P" SERIES (TBI)
Circuit Description:
The coolant temperature sensor uses a thermistor to control the signal voltage to the ECM. The ECM
applies a voltage on CKT 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high,
therefore, the ECM will see high signal voltage.
As the engine warms, the sensor resistance becomes less, and the voltage drops. At normal engine
operating temperature, the voltage will measure about 1.5 to 2.0 volts at the ECM terminal "B8".
Coolant temperature is one of the inputs used to control:
© Fuel delivery • Convertor Clutch (TCC)
• Electronic Spark Timing(EST) • Idle (IAC)
9 Cooling Fan
Test Description:
Numbers below refer to circled
numbers on the diagnostic chart.
1. Checks to see if code was set as result of hard
failure or intermittent condition.
Code 15 will set if:
• Signal Voltage indicates a coolant
temperature below -30°C
(-22°F)
for 60
seconds.
2. This test simulates conditions for a Code 14. If the
ECM recognizes the grounded circuit (low
voltage), and displays a high temperature, the
ECM and wiring are OK.
3. This test will determine if there is a wiring
problem or a faulty ECM. If CKT 452 is open,
there may also be a Code 21 stored.
Diagnostic Aids:
A "Scan" tool reads engine temperature in
degrees centigrade. After the engine is started,
the temperature should rise steadily to about 90°,
then stabilize, when the thermostat opens.
If the engine has been allowed to cool to an
ambient temperature (overnight), coolant and
MAT temperatures may be checked with a "Scan"
tool and should read close to each other.
When a Code 15 is set, the ECM will turn
"ON" the Engine Cooling Fan.
A Code 15 will result if CKTs 410 or 452 are
open.
If Code 15 is intermittent refer to Section "B".