I am willing to wait. I really need to get an answer so I can get this car back on the road.
I would check the wiring between the VSS and the ECM ... ithelps if you have the factory Helms service manual ... there are diagnosticflow charts and wiring schematics that will make the job easier. If youfollowed the trouble shooting charts in a Helms, you would see that one of thecauses for a code 24, is a bad IP. The VSS is incorporated into the IP. MYguess, that is the cause. You may have a bad IP (IP= Instrument Panel),or just some bad connections. You can order a Helm's, Chevrolet Factory ServiceManual at, http://www.helminc.com.
The vss is connected on the backside of the speed gauge, thewire goes down on the drives side of the center console. You can take out thehardboard with carpet to get under the console, the connector should besomewhere under your lighter or on the left side of the radio. It's a blue (onmine) plastic box with something like a little circuitboard sticking out on oneside. On that side there is another connector (from your main harness) clickedon to the circuit board. Check if that is still in place and if the connectorsare good and clean (on mine I forgot to reconnected it after taking out thespeedgauge so that caused the code 24 to come up permanently. One of theconnectors can be corroded or not mating correctly. Since it works most of thetime I would focus on the connectors. Find all the connectors from the wiringdiagrams in the service manual and clean the connectors.
Sorry for the delay in getting back with you. As far as I can tell, the VSS is working properly on my car. When I drive , it is accurate when checked against my GPS. The VSS on the 84 corvette is not located behind the instrument cluster. It is located on the transmission. The 84 corvette has the digital IP. I have checked all connections and everything appears to check out. I have a chevy shop manual for the 84 corvette so I am able to follow the wiring diagrams and procedures within the manual. I believe the IP is loosing a ground somewhere within the cluster. The car drives normally and does not set the check engine light until i have it parked. The car is in neutral and i have the emergency brake on. The test for emissions here in Nevada will run the engine at 2500 rpm. This is when the check engine light is coming on. The light will go off once the car is put into gear and moved or the car is restarted. This one really has me stumped. Im thinking about replacing some of the electrolytic capacitors on the power supply in the IP. Most check good in circuit but my cap meter will read a shorted cap as a good cap sometimes. I will need to take some of them out of circuit to test.
Do you have any other ideas?
I will try attaching a new ground line tonight after work. I will let you know the results