DTC P0502 Vehicle Speed Sensor (VSS) Circuit Low Input. Pontiac Firebird 1995

For 1990-2009 cars only

Makes 🢒 Pontiac 🢒 1995 🢒 Firebird 🢒 DTC 🢒 DTC P0502 Vehicle Speed Sensor (VSS) Circuit Low Input

Circuit Description

The VSS provides vehicle speed information to the PCM. The VSS is a Permanent Magnet (PM) generator. The VSS produces a pulsing AC voltage. The AC voltage level and the number of pulses increase as the speed of the vehicle increases. The PCM converts the voltage pulses to vehicle speed. A scan tool can display the calculated vehicle speed.

When the PCM detects a low vehicle speed when there is a high engine speed in a drive gear range, then DTC P0502 sets. DTC P0502 is a type A DTC.

Conditions for Setting the DTC

•	No MAP DTCs P0107 or P0108

•	No TP DTCs P0122 or P0123

•	No TFP Valve Position Switch DTC P1810

•	Not in Park or Neutral

•	The TP angle is greater than 20%.

•	The engine vacuum is 0-106 kPa.

•	The engine speed is greater than 4000 RPM.

•	The engine torque is 54-542 N·m (40-400 lb ft).

•	The transmission output speed is less than 150 RPM.

•	All conditions met for 2.5 seconds

Action Taken When the DTC Sets

•	The PCM commands second gear only

•	The PCM commands maximum line pressure

•	The PCM freezes shift adapts from being updated.

•	The PCM inhibits TCC Solenoid Valve engagement.

•	The PCM illuminates the Malfunction Indicator Lamp (MIL).

Conditions for Clearing the MIL/DTC

•	The PCM turns OFF the MIL after three consecutive ignition cycles without a failure reported.

•	A scan tool can clear the DTC from the PCM history. The PCM clears the DTC from the PCM history if the vehicle completes 40 warm-up cycles without a failure reported.

•	The PCM cancels the DTC default actions when the fault no longer exists and the ignition is OFF long enough in order to power down the PCM.

Diagnostic Aids

•	A VSS DTC P0502 will set when no vehicle speed is detected at start OFF.

•	Inspect the wiring for poor electrical connections at the PCM. Look for the following conditions:

-	A bent terminal

-	A backed out terminal

-	A damaged terminal

-	Poor terminal tension

-	A chafed wire

-	A broken wire inside the insulation

•	When diagnosing for an intermittent short or open condition, massage the wiring harness while watching test equipment for changes.

•	Test circuits 400 and 401 for Electromagnetic Interferences (EMI) induced by a wiring harness incorrectly routed too near the spark plug wires.

•	Incorrect VSS calibration may set DTC P0502. Refer to Service bulletins to ensure that the calibration is current.

•	Inspect for a damaged VSS or transmission rotor teeth.

•	Ensure that the VSS is correctly aligned and torqued securely to the transmission case.

Test Description

The numbers below refer to the Step numbers on the diagnostic table.

This step verifies the integrity of the VSS.

This step verifies the integrity of the VSS circuit.

This step tests the resistance of the VSS.

DTC P0502 Vehicle Speed Sensor (VSS) Circuit -- Low Input (Transmission Output Speed Signal)

Step	Action	Value(s)	Yes	No
1	Was the Powertrain On-Board Diagnostic (OBD) System Check performed?	--	Go to Step 2	Go to Powertrain On Board Diagnostic (OBD) System Check , Section 6
2	Install the scan tool. With the engine OFF, turn the ignition switch to the RUN position. Important:: Before clearing DTCs, use the scan tool in order to record Freeze Frame and Failure Records for reference. The Clear Info function will erase the data. Record the DTC Freeze Frame and Failure Records. Raise the drive wheels. Start the engine. Disable the traction control system. Place the transmission in any drive range. With the drive wheels rotating, does the Trans Output Speed increase with the drive wheel speed?	--	Go to Diagnostic Aids	Go to Step 3
3	Turn the ignition switch to the OFF position. Disconnect the C1 (blue) PCM connector. Connect the J 39200 DVOM between harness connector terminals C1-6 and C1-47. Select the AC Volts option. Rotate the rear wheels. Ensure that the driveshaft is turning. Is the voltage measurement greater than the specified value?	0.5V	Go to Step 4	Go to Step 6
4	Measure the resistance between the harness connector terminals C1-6 and C1-47. Is the measured resistance greater than the specified value?	2700ohms	Go to Step 5	Go to Step 6
5	Inspect circuit 400 and circuit 401 for a poor connection at the PCM. Inspect circuit 400 and circuit 401 for a poor connection at the VSS. Inspect circuit 400 and circuit 401 for an open circuit. Refer to Electrical Diagnosis, Section 8. Did you find and correct a problem?	--	Go to Step 13	Go to Step 8
6	Is the resistance measured between the harness connector terminals C1-6 and C1-47 less than the specified value?	1480ohms	Go to Step 7	Go to Step 8
7	Inspect circuits 400 and 401 for a short to ground. Inspect circuits 400 and 401 for a shorted together condition. Refer to Electrical Diagnosis, Section 8. Did you find and correct a problem?	--	Go to Step 13	Go to Step 8
8	Reconnect the C1 (blue) PCM connector. Disconnect the wiring harness from the VSS. Measure the resistance between the VSS terminals A and B. Use the J 39200 DVOM. Is the measured resistance within the specified range?	1480-2700ohms	Go to Step 11	Go to Step 9
9	Remove the VSS. Inspect the output shaft speed sensor rotor for damage or misalignment. Did you find a problem?	--	Go to Step 13	Go to Step 10
10	Replace the VSS. Refer to Vehicle Speed Sensor Replacement . Is the replacement complete?	--	Go to Step 13	--
11	Inspect the PCM connector for the following conditions: Damaged connector pins Backed out connector pins Weak terminal tension Did you find and correct a problem?	--	Go to Step 13	Go to Step 12
12	Replace the PCM. Refer to Powertrain Control Module Replacement/Programming , Section 6. Is the replacement complete?	--	Go to Step 13	--
13	After the repair is complete, select DTC. Select Clear Info. Operate the vehicle so that the transmission output speed is greater than 151 RPM for 3 seconds. Select Specific DTC. Enter DTC P0502. Has the test run and passed?	--	System OK	Begin the diagnosis again. Go to Step 1