With Speed Density Update

Chart - Code 24 for Vehicle Speed Sensor Circuit:

Vehicle Speed Sensor Circuit:

CIRCUIT DESCRIPTION:
The Electronic Control Module (ECM) applies and monitors 12 volts on CKT 437. CKT 437 connects to the Vehicle Speed Sensor (VSS) 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.Disregard a Code 24 set when drive wheels are not turning.

TEST DESCRIPTION:
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 1400 and 3600 rpm
- Throttle Position Sensor (TPS) is less than 2%
- Low load condition (Low Air Flow)
- 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 IP connector, indicates CKT 437 is open between the IP connector and the VSS, or there is a faulty VSS. A voltage of less than 1 volt, at the IP connector, indicates that CKT 437 wire is shorted to ground. If, after disconnecting CKT 437 at the VSS, the voltage reads above 10 volts, the VSS 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.

DIAGNOSTIC AIDS:
"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. Chart C-1A Park/Neutral Switch Diagnosis
If all OK, see "DIAGNOSIS BY SYMPTOMS - INTERMITTENT CONDITION DIAGNOSIS." - Intermittent Malfunctions