My car 1997 C5 / base / auto. The speedometer reads 15mph with the key on and when you begin to take off from a stop the ECBM thinks you are spinning the rear tires loose and pulses the traction control until you get going above 10mph. When you put the trans in 1st manually it takes off normally. Turning off the traction control does not affect it.
I checked all the grounds at the PCM and they are all under 5 ohms.
The VSS output wire has 5 volts when the key is on, and when you disconnect the ECBM the speedometer works normally.
I am thinking bad ECBM, but due to availability and cost I thought I would run it by you guys and see if anyone had any other ideas before I get involved with trying to get the module repaired.

 

No codes other than a couple climate control codes, it has stock tires

 

I have 5 volts on the VSS output wire (PCM to Speedo) with key on. Can anyone tell me if that is normal?

 

Here is the entire VSS write up:

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DTC P0500 Vehicle Speed Sensor (VSS) Circuit




Circuit Description
A toothed wheel mounted on the output shaft of the transmission generates an AC voltage pulse as it moves past the Vehicle Speed Sensor (VSS). The VSS contains a permanent magnet surrounded by a coil of wire, which produces a magnetic field. The toothed wheel interrupts the magnetic field this generates an AC voltage pulse. As the vehicle speed increases the AC voltage pulse rate also increases.

The PCM supplies this signal to the BCM and the instrument cluster.

Conditions for Setting the DTC
DTCs P0107, P0108, P0117, P0118, P0506, P0507, P1120, P1220, P1221 are not set.
ECT is greater than 35°C (95°F)
Engine speed is greater than 1000 RPM.
TP sensor angle is greater than 10%.
MAP is greater than 40 kPa when A/C is OFF.
MAP is greater than 45 kPa when A/C is ON.
All conditions met for 2 seconds.
PCM detects vehicle speed at 0 km/h (0 mph) for 50 seconds.
Action Taken When the DTC Sets
The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records.
Conditions for Clearing the MIL/DTC
The PCM turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
Use a scan tool in order to clear the MIL and the DTC.
Diagnostic Aids
Inspect all of the circuits to the PCM and the VSS. Inspect the terminal connections at the VSS harness connector and at the PCM harness connector. If all of the circuits and connections are OK, replace the PCM.

For intermittents, refer to Symptoms .

Test Description
The numbers below refer to the step numbers on the diagnostic table.

The ASR/TCS must be disabled when performing this step. Whenever the ignition key is cycled to the OFF position and then cycled back ON, the ASR/TCS system will default ON.

Step
Action
Value(s)
Yes
No

1
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check ?
--
Go to Step 2
Go to Powertrain On Board Diagnostic (OBD) System Check

2

Important
DO NOT perform this test on vehicles with Independent Suspension without supporting the lower control arms so that the drive axles are in a normal horizontal position. Running the vehicle with the wheels hanging down at full travel may damage the drive axles.


Turn OFF the ignition.
Raise the drive wheels. Refer to Lifting and Jacking the Vehicle .
Start the engine.
Disable ASR/TCS systems, if so equipped.
Idle the engine in gear.
Does the scan tool indicate a vehicle speed above the specified value?
0.0 mph (0.0 km/h)
Refer to Diagnostic Aids
Go to Step 3

3
Turn OFF the ignition.
Disconnect the VSS electrical connector.
Connect the Signal Generator Tester J 33431-B to the VSS harness.
Turn ON the ignition, tester ON and set to generate a VSS signal.
Does the scan tool indicate a vehicle speed above the specified value?
0.0 mph (0.0 km/h)
Go to Step 4
Go to Step 5

4
Repair the faulty VSS connection or faulty VSS.

Is the action complete?
--
Go to Step 12
--

5
Measure the voltage from the VSS signal circuit to a known good ground using the DMM J 39200 .

Does the DMM indicate the specified value?
5.0 V
Go to Step 6
Go to Step 9

6
Measure the voltage from the VSS signal circuit to the VSS ground circuit using the DMM J 39200 .

Does the DMM indicate the specified value?
5.0 V
Go to Step 7
Go to Step 10

7
Check the calibration ID for the most current and/or incorrect calibration.

Is the most current and proper calibration present?
--
Go to Step 8
Go to Step 11

8

Important:
Program the replacement PCM. Refer to PCM/TAC Module Replacement .


Replace the PCM.

Is the action complete?
--
Go to Step 12
--

9
Repair the signal circuit to the vehicle speed sensor.

Is the action complete?
--
Go to Step 12
--

10
Repair the ground circuit to the vehicle speed sensor.

Is the action complete?
--
Go to Step 12
--

11
Install proper calibration and/or most current.

Is the action complete?
--
Go to Step 12
--

12
Select the Diagnostic Trouble Code (DTC) option and the Clear DTC Information option using the scan tool.
Idle the engine at the normal operating temperature.
Select the Diagnostic Trouble Code (DTC) option and the Specific DTC option, then enter the DTC number using the scan tool.
Operate the vehicle within the conditions for setting this DTC as specified in the supporting text, if applicable.
Does the scan tool indicate that this test ran and passed?
--
Go to Step 13
Go to Step 2

13
Select the Capture Info option and the Review Info option using the scan tool.

Does the scan tool display any DTCs that you have not diagnosed?
--
Go to the applicable DTC table
System OK



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Document ID# 57853
1997 Chevrolet/Geo Corvette

 

Thanks Bill, this has been a great help! Looks like my next step is to replace the PCM. I have ordered one from Madtuner and will update this thread when it is installed.

 

Replaced PCM today, turned key on speedometer needle swept to 200 came back and rested at 15mph. ARGHHH!

 

Pulled the 10amp ign. ABS fuse and the speedo works normally, must be the ECBM.

 

Not sure if these speedometers suffer the same issues as some other GM products but I just shipped one out for repair for my daughter's Envoy. Her issue is the stepper motors that drive the needles.