Engine Diagnosis

Diagnostic Trouble Code 24:

Vehicle Speed Sensor Circuit:

CIRCUIT DESCRIPTION
The ECM applies and monitors 12 volts on the Vehicle Speed Sensor (VSS) signal circuit. The VSS signal circuit connects to the VSS buffer which alternately grounds the VSS signal circuit, when receiving voltage pulses from VSS while 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 scan tool reading should closely match the speedometer reading with drive wheels turning.

DTC 24 WILL SET WHEN
^ Automatic transmission in drive.
^ Coolant temperature above 65°C (149°F).
^ Throttle position below 2%.
^ MAP limit 20 kPa.
^ All conditions must be met for five seconds.
^ CKT 1716 voltage is constant.
^ Engine speed between 1400 - 4400 rpm.
^ Vehicle speed signal indicates less than two mph (three km/h) on Tech 1 scan tool.

ACTION TAKEN (ECM WILL DEFAULT TO)
The Malfunction Indicator Lamp (MIL) will illuminate.

DTC 24 WILL CLEAR WHEN
Conditions for fault are no longer present and Tech 1 "Clear DTCs" function is used or 50 ignition switch keycycles have passed with no further faults or ignition switch is turned "OFF" and battery feed voltage is removed for 10 seconds.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Checks to see if DTC was set as result of a hard failure or intermittent condition.

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 the VSS signal circuit. 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. Scan tool reading should closely match speedometer readings with drive wheels turning.
2. Refer to Diagnosis By Symptom / Intermittents - Intermittent Malfunctions