DTC 24

Vehicle Speed Sensor (VSS):

Circuit Description:
The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the Vehicle Speed Sensor (VSS) buffer, which alternately grounds CKT 437, when receiving voltage pulses from Vehicle Speed Sensor (VSS) 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".

A Tech 1 "Scan" tool reading should closely match the speedometer reading with drive wheels turning.

Test Description:
Numbers below refer to circled numbers on the diagnostic chart.

1. DTC 24 will set if:

^ CKT 437 voltage is constant
^ Engine speed between 1500 - 4400 RPM.
^ Vehicle speed signal indicates less than 1 mph (2 km/H) on Tech 1
^ Automatic transmission in drive
^ MAP <23 kPa.
^ All conditions must be met for 4 seconds

2. This test determines if the VSS buffer is receiving the A/C signal from the VSS.

3. This test monitors the VSS buffer voltage on CKT 437. With the wheels turning, the pulsing action will result in a varying voltage. The variation will be greater at low wheel speeds to an average of 4-6 volts at about 20 mph (32 km/H).

Diagnostic Aids:
1. Tech 1 "Scan" tool reading should closely match speedometer reading, with drive wheels turning.

2. Check Park/Neutral (P/N) switch diagnosis chart if vehicle equipped with automatic transmission.