Pat, You have been able to help me out with some complicated car issues since 1987 and it never ceases to amaze me of your knowledge.

Well, I have another puzzling issue with a 1992 Corvette base coupe with 80,000 miles.

The LCD display for the speedometer started to fade a few years ago and although it still displayed the mph, it was getting hard to see. I procured a replacement LCD display and carefully installed it. Afterwards, the LCD was bright and clear for a couple of months without issue.

After months of working properly, then when the car sat overnight or in the covered garage at work for 9 hours, the LCD would only display zero (0) mph for the first half to one mile before starting to register mph. Over the next few months, it got progressively worse to the point where now I drive up to 20 miles before the speedometer starts to show the mph. All the other gauges, including the gas on the LCD displays fine, but the speedometer will show 0. When the speedometer is not displaying the mph, the cruise control and the radio speed controlled volume do not work either.

Occasionally, the ECM shows a Code 24 (VSS) and I just had the VSS replaced and it didn’t fix the speedometer and still set the same Code 24. My local Corvette Specialist is at a loss. I’ve had this car since new and would not want to part with, so again, I could use your vast expertise.

Thank you in advance, Lonny

Answer

Thanks for the compliment hope I can help again. First you have to know that codes do not tell you what to replace only what test to perform. In this case even the code description attempts to tell techs that it is VSS circuit not VSS. “Code 24 Vehicle Speed Sensor circuit malfunction”. Fortunately for you the test is really simple.

Begin by testing for an output signal at the VSS with the wire harness disconnected. If there is no signal test the VSS (new doesn’t mean good it just means new) and if the sensor is good look for gearing or spacing problems inside the transmission.

If there is a strong VSS signal at the sensor reconnect the wire harness and if you have Selective Ride Control unplug the ride control module and see if the speedometer works. If the speedometer does work you have a bad ride control computer.

If the speedometer still doesn’t work trace the VSS harness to the next connector and test for a signal at that connector. If you had a signal at the VSS and none at the first connector the wire between the VSS is damaged.

If you do have a signal at the first connector go to the next connector which will usually be the PCM (Powertrain Control Module) unplug and check for a signal. No signal there and good signal at the other end means a wire problem between the two points.

If you have a good signal going into the PCM test for a signal out of the PCM going over to the instrument cluster. If you have a good signal going into the PCM and none coming out you have a bad PCM!

If you have a good signal out of the PCM test for a signal at the other end of the harness where it plugs into the cluster. Again if you had a good signal going into the harness at the PCM and none at the instrument cluster there is a wiring problem between the two points.

Finally if there is a good signal at the end of the harness where it plugs into the instrument cluster and the speedometer still doesn’t work test the cluster ground. Not likely in your case because the other gauges are working normally. If input and ground are good you have a bad instrument cluster.

Easy peasy as they say.

Also this is the general procedure for many fault code testing procedures so you don’t wind up replacing parts needlessly.

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