Circuit Description
Vehicle speed information is provided to the engine control module (ECM) by the vehicle speed sensor (VSS). The VSS is a permanent magnet generator that is mounted on the transmission and produces a pulsing voltage. The AC voltage amplitude and frequency increases with vehicle speed. The ECM converts the pulsing voltage into km/h (mph). The ECM supplies the necessary signal to the instrument panel for speedometer, odometer operation and to the cruise control module. The ECM also supplies the signal to the body control module (BCM), and the electronic suspension control module, if equipped.
If the ECM detects a vehicle speed drop for 6 seconds, DTC P0503 sets. DTC P0503 is a type B DTC.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0503 Vehicle Speed Sensor (VSS) Circuit Intermittent
Conditions for Running the DTC
| • | No MAP sensor DTC P0106. |
| • | No idle control system DTCs P0506 or P0507. |
| • | No TP sensor DTCs P0120 or P0220. |
| • | The vehicle speed is greater than 13 km/h (8 mph) for 6 seconds and has not increased by 6 km/h (4 mph) or greater for at least 0.5 seconds. |
| • | All conditions are met for more than 6 seconds. |
Conditions for Setting the DTC
The ECM detects a vehicle speed drop for 6 seconds.
Action Taken When the DTC Sets
| • | The ECM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the Conditions for Setting the DTC are met. |
| • | The ECM disables Cruise Control. |
| • | The ECM commands the coolant fan ON. |
| • | The ECM records the operating conditions when the Conditions for Setting the DTC are met. The ECM stores this information as Freeze Frame and Failure Records. |
| • | The ECM stores DTC P0503 in ECM history during the second consecutive trip in which the Conditions for Setting the DTC are met. |
Conditions for Clearing the MIL/DTC
| • | The ECM turns OFF the MIL during the third consecutive trip in which the diagnostic test runs and passes. |
| • | A scan tool can clear the MIL/DTC. |
| • | The ECM clears the DTC from ECM history if the vehicle completes 40 warm-up cycles without an emission-related diagnostic fault occurring. |
| • | The ECM cancels the DTC default actions when the fault no longer exists and the DTC passes. |
Diagnostic Aids
Ensure the VSS is correctly torqued to the transmission housing.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The vehicle speed detector in the ECM and the VSS are matched in such a way that an open or a short to ground in the VSS low signal circuit will not usually cause a loss of speed signal or a DTC P0503 to set.
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This step isolates the short between the VSS and the wiring.
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The DMM will detect AC voltage if the VSS high signal circuit is shorted to ground.
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | ||||||||
2 |
Notice: In order to avoid damage to the drive axles, support the lower control arms in the normal horizontal position. Do not run the vehicle in gear with the wheels hanging down at full travel. Important: Record the Failure Records before clearing the DTCs. Using the Clear Info function erases the Failure Records from the ECM. Does the scan tool display vehicle speed above the specified value? | 0 km/h (0 mph) | Go to Step 3 | Go to Step 4 | ||||||||
3 |
Does the scan tool display vehicle speed above the specified value? | 0 km/h (0 mph) | Go to Intermittent Conditions in Engine Controls - 6.0L | Go to Step 4 | ||||||||
Is the resistance within the specified range? | 966-2,200 ohms | Go to Step 6 | Go to Step 5 | |||||||||
5 | Test the VSS high signal circuit and the VSS low signal circuit for the following conditions:
Refer to Circuit Testing in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 10 | ||||||||
6 | Measure the resistance between the VSS high signal circuit and ground. Is the resistance greater than the specified value? | 50 K ohms | Go to Step 9 | Go to Step 7 | ||||||||
Is the resistance greater than the specified value? | 50 K ohms | Go to Step 10 | Go to Step 8 | |||||||||
8 | Repair the short to ground in the VSS high signal circuit. Refer to Testing for Short to Ground and Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 14 | -- | ||||||||
Is the AC voltage equal to or greater than the specified value? | 0.5 V | Go to Step 11 | Go to Step 10 | |||||||||
10 |
Refer to Vehicle Speed Sensor Replacement in Rear Drive Axle.
Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 12 | ||||||||
11 | Test the high signal circuit of the VSS for a short to voltage. refer to Testing for a Short to Voltage and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 13 | ||||||||
12 | Replace the VSS. Refer to Vehicle Speed Sensor Replacement in Rear Drive Axle. Did you complete the repair? | -- | Go to Step 14 | -- | ||||||||
13 | Replace the ECM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Is the action complete? | -- | Go to Step 14 | -- | ||||||||
14 | Perform the following procedure in order to verify the repair:
Has the test run and passed? | -- | Go to Step 15 | Go to Step 2 | ||||||||
15 | With the scan tool, observe the stored information, capture info and DTC info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | System OK |
