Pontiac Fiero 1988 Service Manual page 621

6 E 3 - A - 3 8 DRIVABILITY A N D EMISSIONS - FUEL INJECTION (PORT)
VEHICLE SPEED
SENSOR
P.M. GENERATOR
(MOUNTED IN TRANS)
12V IGN. —
IICCCB / J N
TO CRUISE
CONTROL
VSS BUFFER O f C
(MOUNTED IN I.P.)
12V IGN.
ECM
i.p.
CONNECTOR
— «
BRN 437
A10
1 5 - W A Y .LP. CONNECTOR
I
P/N SWITCH
BLK 450 - — ^ ^ ORN/BLK434
CLOSED IN
PARK OR NEUTRAL
B10
VSS SIGNAL
P/N SIGNAL
-A/VV— « v
7-8-87
2-16-87
7S3378-6E
C O D E
24
VEHICLE SPEED SENSOR (VSS) CIRCUIT
2.8L "P" SERIES (PORT)
C i r c u i t D e s c r i p t i o n :
The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the vehicle speed sensor which
alternately grounds CKT 437 when drive wheels are turning. This pulsing action takes place about 2000 times
per mile and the ECM will calculate vehicle speed based on the time between "pulses".
"Scan" reading should closely match with speedometer reading with drive wheels turning.
** To prevent misdiagnosis, the technician should review electrical Section "8A" or the electrical trouble
shooting manual, and identify the type of vehicle speed sensor used prior to using this chart. Disregard a Code
24 set when drive wheels are not turning.
T e s t D e s c r i p t i o n :
Numbers below refer to circled
numbers on the diagnostic chart.
1. Code 24 will set if vehicle speed equals 0 mph
when:
• Engine speed is between 2000 and 4400 rpm
• TPS is less than 2% (closed throttle)
• Low load condition (high vacuum)
• Not in park or neutral
• All conditions met for 5 seconds
These conditions are met during a road load
deceleration. Disregard Code 24 that sets when
drive wheels are not turning.
2. 8-12 volts, at the I.P. connector, indicates CKT 437
is open between the I.P. connector and the VSS, or
there is a faulty vehicle speed sensor. A voltage of
less than 1 volt, at the I.P. connector, indicates
that CKT 437 wire is shorted to ground. If after
disconnecting CKT 437 at the vehicle speed sensor
the voltage reads above 10 volts, the vehicle speed
sensor is faulty. If voltage remains less than 8
volts, then CKT 437 wire is grounded. If 437 is not
grounded, there is a faulty connection at the ECM,
or a faulty ECM. The I.P. connector is located in
the center console near the ECM.
D i a g n o s t i c A i d s :
"Scan" should indicate a vehicle speed whenever
the drive wheels are turning greater than 3 mph.
A faulty or misadjusted park/neutral switch can
result in a false Code 24. Use a "Scan" and check for
proper signal while in drive. Refer to CHART C-1A
for P/N switch diagnosis check.
If all OK, refer to "Intermittents" in Section "B".