Im new on here and the first day i own my GTP i have a problem.. Can anyone help?

**azmattaz06** · 02-04-2009

Ok it starts like this... out of nowhere my car starts tripping like the speedometer goes crazy and up and down and my car feels like it doesn't know what gear to go to so it goes in and out.. i run the diagnostics at Autozone and come up with a code p0502 and I'm told its the Vehicle speed sensor so i go to buy one and they say there's two different ones so i got the one with the metal thing attached to it and i put it in and speedometer doesn't move much and its still acting up.. Im going to get the other sensor tomorrow morning i just wanted to ask if i was on the right track or if anyone can tell me anything else it might be.. Thanks you very much -Matthew

TDCRacing · 02-04-2009

Circuit Description
The vehicle speed sensor (VSS) system is a pulse generator consisting of a speed sensor assembly, located in the case extension, and a toothed vehicle speed sensor reluctor wheel, which is pressed onto the final drive carrier assembly. As the vehicle drives forward, the vehicle speed sensor reluctor wheel also rotates. This rotation produces a variable AC signal in the pickup coil that is proportional to vehicle speed.

The AC signal generated has a frequency and amplitude (voltage). The PCM uses the frequency portion of the AC signal to calculate the vehicle speed. The system is calibrated to produce 4000 pulses per mile and it is the amount of time it takes to produce those pulses that determines the vehicle speed. The voltage portion of the signal is used for diagnostic purposes only. The voltage will vary from 0.5 volts AC at 100 RPM to more than 200 volts AC at 6000 RPM.

This information is then sent to the PCM to indicate how fast the vehicle is traveling and to develop the shift pattern for the transmission. Other systems also use VSS information:

TCC apply and release
Cruise control system
Fuel delivery systems
If the PCM detects a low vehicle speed with a high engine speed while in a drive range, then DTC P0502 sets. DTC P0502 is a type B DTC.

Conditions for Running the DTC
No MAP sensor DTCs P0106, P0107, P0108, P1106 or P1107.
No TP sensor DTCs P0121, P0122, P0123, P1121 or P1122.
The transmission is not in PARK or NEUTRAL.
The input shaft speed is greater than 1500 RPM.
The TP angle is greater than 12%.
The MAP is 0-105 kPa (0-15 psi).
The engine torque is 55-200 N·m (40-150 lb ft).
Conditions for Setting the DTC
The output shaft speed is less than 150 RPM for 2.5 seconds.

Action Taken When the DTC Sets
The PCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the conditions for setting the DTC are met.
The PCM commands maximum line pressure.
The PCM freezes shift adapts.
The PCM calculates vehicle speed from the A/T ISS sensor and the commanded gear.
The PCM stores DTC P0502 in PCM history.
Conditions for Clearing the MIL/DTC
The PCM turns OFF the MIL during the third consecutive trip in which the diagnostic test runs and passes.
A scan tool clears the DTC from the PCM history.
The PCM clears the DTC from the PCM history if the vehicle completes 40 consecutive warm-up cycles without an emission related diagnostic fault occurring.
The PCM cancels the DTC default actions when the fault no longer exists and the ignition switch is OFF long enough in order to power down the PCM.
Diagnostic Aids
Check for electromagnetic interference (EMI) in circuits 400 (YEL) and 401 (PPL). EMI may be induced by a misrouted wiring harness along the spark plug wires.
Inspect the wiring at the PCM, the VSS connector and all other circuit connecting points for the following conditions:
A backed out terminal
A damaged terminal
Reduced terminal tension
A chafed wire
A broken wire inside the insulation
Moisture intrusion
Corrosion
When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
Ensure that the VSS is securely tightened to the case extension.
Test Description
The numbers below refer to the step numbers on the diagnostic table.

This step verifies that the PCM is receiving a signal from the VSS.

This step tests the ability of the VSS to produce an AC voltage as well as the integrity of the wiring from the VSS to the PCM connector.

This step performs a resistance test on the VSS circuits from the PCM connector.

A YES answer indicates an intermittent condition.

A NO answer indicates an intermittent condition.

DTC P0502 Vehicle Speed Sensor Circuit Low 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 in Engine Controls

2
Install the Scan Tool .
Turn the ignition switch to the RUN position.

Important
Before clearing the DTCs, use the scan tool in order to record the Freeze Frame and Failure Records for reference. Using the Clear Info function will erase the stored Freeze Frame and Failure Records from the PCM.

Record the Freeze Frame and Failure Records.
Clear the DTCs.

Notice
Support the lower control arms in the normal horizontal position in order to avoid damage to the drive axles. Do not operate the vehicle in gear with the wheels hanging down at full travel.

Raise and support the drive wheels.
Start and idle the engine.
Place the transmission in any drive range.
Monitor transmission OSS on the scan tool.
With drive wheels rotating, does transmission output shaft speed increase when wheel speed increases?
--
Go to Diagnostic Aids
Go to Step 3

3
Turn the ignition switch to the OFF position.
Disconnect PCM connector C1.
Connect a J 39200 digital multimeter (DMM) from terminal 64 to terminal 65 of PCM connector C1.
Select AC volts on the J 39200 DMM.
Place the transmission in NEUTRAL.
Rapidly rotate one drive wheel and observe the display.
Is the measured voltage greater than the specified value?
0.5 volts
Go to Step 15
Go to Step 4

4
Select ohms on the J 39200 DMM.
Measure the resistance between terminal 64 and terminal 65 of PCM connector C1.
Is the resistance greater than the specified value?
1864 ohms
Go to Step 5
Go to Step 6

5
Inspect circuits 400 (YEL) and 401 (PPL) for an open.

Refer to General Electrical Diagnosis Procedures in Wiring Systems.

Was a condition found?
--
Go to Step 12
Go to Step 10

6
Does the J 39200 DMM display a value less than the resistance specified?
981 ohms
Go to Step 7
Go to Step 8

7
Inspect circuits 400 (YEL) and 401 (PPL) for a short together.

Refer to General Electrical Diagnosis Procedures in Wiring Systems.

Was a condition found?
--
Go to Step 12
Go to Step 10

8
Connect one lead of the J 39200 DMM to a good ground.
Connect the other lead to terminal 64, then to terminal 65.
Is either resistance less than the specified value?
50 K ohms
Go to Step 9
Go to Step 11

9
Inspect circuits 400 (YEL) and 401 (PPL) for a short to ground.

Refer to General Electrical Diagnosis Procedures in Wiring Systems.

Was a condition found?
--
Go to Step 12
Go to Step 14

10
Disconnect the VSS.
Connect the J 39200 DMM, set to measure ohms, between VSS terminal A and terminal B.
Measure the resistance.
Is the resistance within the specified range?
981-1864 ohms
Go to Diagnostic Aids
Go to Step 14

11
Remove the VSS.
Inspect the vehicle speed sensor reluctor wheel on the final drive carrier assembly for damage or misalignment.
Was a condition found?
--
Go to Step 13
Go to Diagnostic Aids

12
Repair the circuit as necessary.

Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?
--
Go to Step 16
--

13
Correct the vehicle speed sensor reluctor wheel alignment or replace the vehicle speed sensor reluctor wheel.

Refer to Unit Repair.

Is the alignment correct or the replacement complete?
--
Go to Step 16
--

14
Replace the Vehicle Speed Sensor.

Refer to Vehicle Speed Sensor (VSS) Replacement .

Is the replacement complete?
--
Go to Step 16
--

15
Replace the PCM.

Refer to PCM Replacement/Programming in Engine Controls.

Is the replacement complete?
--
Go to Step 16
--

16
Perform the following procedure in order to verify the repair:

Select DTC.
Select Clear Info.
Drive the vehicle at 30 mph for at least 2 seconds.
Select Specific DTC.
Enter DTC P0502.
Has the test run and passed?
--
System OK
Go to Step 1

TDCRacing · 02-04-2009

This is out of a GM service manual. If you don't understand anything or need something cleared up let me know. This is the info thet the Tech's at GM go by when they receive the code you did.