Hey all,
I have a 1990 base model 5.7 litre.
My speedometer constantly displays zero and my odometer displays the same constant mileage without updating.
My question to all is I removed my original stereo tuner located under the passenger side kick panel. This was done as the previous owner put in a cheap pioneer unit that had its own amp and tuner.
I figured I no longer needed this device. Just wondering if the signal goes through this unit for speed control volumn ? Could I have caused this problem? Need to know how fast im goin.
Thanks for any imput!

 

Your car likely has a VSS issue. The SCV is controlled by the VSS also but it's not the 'first stop' so that shouldn't have created any issue. How long has it been since it's worked? Do you have C68 auto AC?

 

Hi, I do have the C68 option for automatic climate control.
I checked the VSS signal from the sensor itself and I have a good signal. I can check it ( purple and Yellow ) wires at the ECM and its good. Also checked for a signal to the climate module. It good there too. I wish there was someway to check it at the cluster but this is a logic board and i'm not comfortable back probing this unit.
I posted before on this issue and followed all the recommended steps.
The fault tracing suggestions provided here were very good.
I might just have to bite the bullet and go to Chevrolet.

 

Can you extract the speed from the C68 module? Do you have a FSM?

 

I believe I pulled all the data that the climate module can provide.
As I recall I had no speed indicated from the Climate module. I had done this quite a while ago so my memory is a little hazy.
Secondly, I wish I had a FSM, would make fault tracing a little more easy.

 

The VSS signal from the transmission first goes to the FX3 adjustable suspension module if you have it and the ECM. The ECM processes the signal to a 4k pulse per mile. The signal comes out of the ECM then goes to the Cruise Control Module, the CCM which displays the speed on the LCD and the Radio Control Head for volume control.


Using an Oscilloscope would make tracing the VSS signal very easy.

If you look at the ECM located above the battery you will see it has
4 connectors plugged into it.

On the Green connector pin 8 is a Purple wire input signal from the VSS.

On the Green connector pin 2 is a Yellow wire input signal from the VSS.

If you were to remove the VSS sensor from the transmission and
spin it by hand or a drill, you should see a AC sine wave on the Purple
wire.

If the input signal is there the output signal from the ECM is on the Red connector Pin 9 a Dark Green/White wire. It's now a square wave.

The signal then goes to the CCM White connector Pin C2 Dark Green/White wire. The CCM process the signal and displays the speed on the LCD. Since the LCD is displaying a Zero that would indicated the problem is in the CCM if the VSS input signal is on Pin C2 of the CCM.

The VSS signal also goes to the Cruise Control Module Pin D Green/White wire.

***************************
Does the Cruise Control work?

The VSS signal also goes to the Radio Control Head connector C1 pin 5 Dark Green/White wire.

Removing either the Control Head or CDM (tuner) will not effect
the VSS operation for the other modules.

You can short pin A to pin G on the diagnostic connector located above the drivers right knee.

Turn the ignition On.
Look at the LCD and you will see numbers displayed.
See if any error codes are set.
Module 1 is the CCM
Module 4 is the ECM

The display will stop with 1.0 displayed.
Press the Trip/ODO button on the Driver Info. Center until you see 1.2

Press the Eng/MET button until you see 05 this displays the vehicle speed in MPH. Example 05 033 would be 33 MPH.

If you were to spin the VSS with a drill you should see MPH displayed
in the last three digits after 05 xxx.

 

Thanks so much for the tech assist. I"ll probably get to it this weekend,
hopefully will come to some definate conclusion.