Without Speed Density Update

Chart - Code 24 for Vehicle Speed Sensor Circuit:

Wiring Diagram for Vehicle Speed Sensor Circuit:

Circuit Description:
This vehicle will have one of two types of vehicle speed sensor (VSS)depending on the type of instrument cluster used. On vehicles with digital clusters, a permanent magnet (PM) generator, located on the transaxle is used. The pulsating output of the PM begins at about 3 mph. The output of PM is input to VSS buffer. The output of this buffer is a pulse at about 2000 times per mile. This is input to the vehicle speed input of the Electronic Control Module (ECM). The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the VSS which alternately grounds CKT 437 when drive wheels are turning and toggles the 12V speed input of ECM low between pulses. The ECM then determines the vehicle speed based on the time between these pulses. On vehicles with-out digital clusters, vehicle speed is determined by an optical scanner at the speedometer head, driven by cable. The output of the optical scanner is buffered and is input to ECM . The ECM then determines the vehicle speed as above with the digital cluster type.

"Scan" reading should closely match with speedometer reading with drive wheels turning. Disregard a Code 24 that is "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 2200 and 4400 rpm
- Throttle Position Sensor (TPS) is less than 2%
- Low load condition (Manifold Absolute Pressure (MAP) less than 30 kPa)
- 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 Vehicle Speed Sensor, or buffer. 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 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.

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." Testing and Inspection