I have a 1991 camaro RS. Some times the speedo works and sometimes it does'nt. Does anyone no where my speedo gets its signal from and could this be my problem ? also the odometer does'nt work if the speedo does'nt work. ANY HELP WOULD BE GREAT Thanks ahead of time.

 

Your speedometer gets a 12V pulsed signal from the tailshaft. The VSS (Vehicle Speed Sensor) sends the signal through yellow and purple wires to the ECM. The ECM then conditions and processes the signal and sends it to the instrument cluster and your cruise control module.

If your cruise control cuts out when the speedo stops working, the problem is in or before the ECM. If not, it's in the instrument cluster or wires from the ECM to the cluster.

You can put a scope (or just a voltmeter) on the yellow and purple wires from the VSS to test for a signal.

What engine model do you have? Then I can tell you what ECM pins to measure.

Lou

 

I have the 305 V8 TBI motor. Yeah my cruise control works when the speedo works. but when the speedo does'nt work I have no cruise control.

 

Do you have a scope or at least a voltmeter to test some signals?

On your TBI computer, look at the smaller connector and find pins A and C, the wire colors will be yellow and purple. Put a scope on them (positive on the yellow wire) and you should see a 12V pulse signal where frequency increases with the speed (duty cycle should remain around 50%). This is the signal that comes from the VSS to the ECM. If that doesn't show any dropouts (when your speedo does), then it's a problem in the ECM, connectors or harness after the ECM.

You can then put a scope on the bigger connector on the ECM and measure pin C (brown wire) against ground. This wire supplies your instrument cluster with the VSS signal. You should again see a pulsed signal.

The cruise control gets the speed signal from the ECM from the same connector, pin E (red wire). The frequency, however, is half.

Let us know what you find out.
Lou

 

I'm having the same problem, basically. I don't have cruise control in the equation though. But, with my laptop/datalogger hooked up, I'm still getting a speedo reading on the computer. So, my problem must be wiring between the ecm and cluster or the guage itself, I guess. Any ideas, anyone?

 

I am have speedo issues, so I have tried to do some data logging, but as of yet I can't get any software to talk to my ECM so I do not know if I am getting a good VSS or not. Anyway one cluster the speedo does nothing and on the other (just bought from a yard) the speedo goes to 72 mph as soon as the key turns to on, and won't budge after that.

I never tried the cruise control thing, I will try it when I get home. Any thoughts on the two responses from the two different speedo's??

 

A cluster that does nothing sounds bad. A cluster that shows 72mph and doesn't move, sounds bad as well. Seems like the drivers on these clusters are shot.

Lou

 

Just to clarify, only the speedos are screwing up. Well, and the odometer. The little circuit board that controls them has no missing conections or burn marks, but the IC's could be screwed.

I guess I will see if i can find another in the area and try it.

 

OK, cruise control does not work. I really like that test. I will try a few more things before I try and find a speed sensor. Thanks for the input.