Speedometer readout,91vette.

[polo91]

Hey Guys, great site you have here. I'm experencinga problem with the digital readout on my
91corvette. When it first started acting up the readout would stay at 0 for a few hundred
yards or so than work fine. As time went on, the distance before it would give a speed readout increased. Now it takes about 2miles of driving before it starts to work,
It will continue to work ok unless the car sits for a long period of time like over night.
Any thoughts on this would be appreciated. Save the wave.

[1990-C4-41001]

As you may know, there is no flex cable driving the speedometer, odometer or trip miles function. It's all electronically derived in the C4.

The speedometer's drive signal starts with a electro-mechanical speed sensor on the transmission. It puts out 4,000 pulses per mile driven. That signal is routed to the ECM where it is conditioned and from there to the CCM. At the CCM, the signal is merged into a data stream which goes to the cluster and then to the speedometer and odometer on the LCD display. A counter is incremented by the same pulses. It counts at 400 pulses input for each 1/10th of a mile driven and that's how the miles driven is captured for the distance side of the miles per gallon equation and how your cumulative odometer and your trip mileage counter portion of the LCD display works.

For your problem, I would suspect that there is something going on with the speed sensor or its associated wiring. Perhaps the mechanical portion has to spin awhile before it begins to make the pulses and it is getting worse with time.

One test would be to check if the trip odometer is being incremented when this problem occurs. If it is, the speed sensor is working and the problem lies within the ECM, CCM, Cluster or wiring. But if the trip odometer also doesn't work when the speedometer itself is out to lunch, I'd concentrate on the speed sensor or its connector & wiring as a prime suspect.

Hope this helps.

John

[polo91]

Thanks for your reply 1990-C4, it makes sense that the speed sensor could be the culpert.
As for the odometer regestering during the problem I'n not sure on that but I'll check.
Once again, thanks for the reply, I'll post my findingswhen solved.