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".
The scantool reading should closely match with speedometer reading with drive wheels turning.
To prevent misdiagnosis, identify the type of Vehicle Speed Sensor used prior to using this page. Disregard a Code 24 set when drive wheels are not turning.
Code 24 will set if vehicle speed equals 0 mph when:
The Scantool 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 the Scantool and check for proper signal while in drive.
Disregard Code 24 if set while drive wheels are not turning.
This test assumes the speedometer is working OK.
Cruise Control "off".
Raise drive wheels.
Notice: Do not perform this test without supporting the lower control arms so that the drive axles are in a normal horizontal position. Running the vehicle in gear with the wheels hanging down at full travel may damage the drive axles.
With engine idling in gear, the Scantool should display mph above 0.
Does it?