Pontiac Fiero 1988 Service Manual page 499

6E2-A-44 DRIVEABILITY AND EMISSIONS - FUEL INJECTION (TBI)
MANIFOLD
ABSOLUTE
PRESSURE
(VACUUM)
5-6-87
MAP SENSOR
A B C
ECM
TO TPS
SENSOR
416
GRY
432
LT. GRN
— 469
BLK *
W11
B20
W14
TO MAT
SENSOR *
—^VVv^—
5VREF
— MAP SIGNAL
THIS WIRE MAY ALSO HAVE A TRACER COLOR
I SENSOR
-ZT
GROUND
4-21-87
* 7S 3208-6E
C O D E
34
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR CIRCUIT
(SIGNAL VOLTAGE LOW - HIGH VACUUM)
2.SL "P" SERIES (TBI)
Circuit Description:
The manifold absolute pressure sensor (MAP) responds to changes in manifold pressure (vacuum). The
ECM receives this information as a signal voltage that will vary from about 1 to 1.5 volts at closed throttle idle,
to 4- 4.5 volts at wide open throttle.
If the MAP sensor fails, the ECM will sustitute a fixed MAP value and use the throttle position sensor (TPS)
to control fuel delivery.
Test Description:
Numbers below refer to circled
numbers on the diagnostic chart.
1. This step determines if Code 34 is the result of a
hard failure or an intermittent condition.
A Code 34 will set when:
• MAP signal voltage is too low.
• The ignition is "ON".
2. Jumpering harness terminals "B" to "C", 5 volt to
signal, will determine if the sensor is at fault, or if
there is a problem with the ECM or wiring.
3. The "Scan" tool may not display 12 volts. The
important thing is that the ECM recognizes the
voltage as more than 4 volts, indicating that the
ECM and CKT 432 are OK.
Diagnostic Aids:
With the ignition "ON", and the engine stopped,
the manifold pressure is equal to atmospheric
pressure and the signal voltage will be high. This
information is used by the ECM as an indication of
vehicle altitude and is referred to as BARO.
Comparison of the BARO reading with a known good
vehicle with the same sensor is a good way to check
accuracy of a "suspect" sensor. Readings should be the
same ± .4 volt.
A Code 34 will result if CKTs 416 or 432 are open
or shorted to ground.
If CKT 416 is open or shorted to ground, there may
also be a stored Code 22.
If Code 34 is intermittent, refer to Section "B".