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ASEMaster35yrs, Chevy Technician
Category: Chevy
Satisfied Customers: 2095
Experience:  ASE Certified since 1972,Master Auto tech,Heavy Truck Tech ,chev dealer tech
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Chevrolet Corvette: I have an 84 Corvette Crossfire. I have

Resolved Question:

I have an 84 Corvette Crossfire. I have recently updated a lot of the emission controls including the EGR valve, Map Sensor, O2 sensor. I have also installed a new TPS and one IAC on the real TBI. My car has a vapor canister but is missing the purge solenoid. When warmed up the car will idle at around 1000 to 1200 rpm. Sometimes the idle will bounce between 700 and 1300 rmp. I need to take the car in for emissions but every time I idle the engine at 2500 rpms, the check engine light comes on which would be an automatic failure. The code is 24 which according to the manual is the VSS. According to my manual it says to ignore this code if the code was created while the car was not moving to prevent misdiagnosis. My question is, how do I get this code to stop popping up during the emissions check while the technician is idling the engine at 2500 rmps . When I drive the car, the light will go off and it drives normally. My fear is that the ECM is bad. Any help would be greatly appreciated.
Submitted: 4 years ago.
Category: Chevy
Expert:  Walt-Mod replied 4 years ago.

I'm a moderator for this topic. I've been working hard to find a Professional to assist you right away, but sometimes finding the right professional can take a little longer than expected.

I wonder whether you're ok with continuing to wait for an answer. If you are, please let me know and I will continue my search. If not, feel free to let me know and I will cancel this question for you.

Thank you!
Customer: replied 4 years ago.

Hello ,


I am willing to wait. I really need to get an answer so I can get this car back on the road.

Expert:  Walt-Mod replied 4 years ago.
Thank you for your continued patience. We will continue the search for a professional for you.
Expert:  ASEMaster35yrs replied 4 years ago.
Hi, glad to be of help.

I would check the wiring between the VSS and the ECM ... it
helps if you have the factory Helms service manual ... there are diagnostic
flow charts and wiring schematics that will make the job easier. If you
followed the trouble shooting charts in a Helms, you would see that one of the
causes for a code 24, is a bad IP. The VSS is incorporated into the IP. MY
guess, 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 Service
Manual at,

The vss is connected on the backside of the speed gauge, the
wire goes down on the drives side of the center console. You can take out the
hardboard with carpet to get under the console, the connector should be
somewhere under your lighter or on the left side of the radio. It's a blue (on
mine) plastic box with something like a little circuitboard sticking out on one
side. On that side there is another connector (from your main harness) clicked
on to the circuit board. Check if that is still in place and if the connectors
are good and clean (on mine I forgot to reconnected it after taking out the
speedgauge so that caused the code 24 to come up permanently. One of the
connectors can be corroded or not mating correctly. Since it works most of the
time I would focus on the connectors. Find all the connectors from the wiring
diagrams in the service manual and clean the connectors.

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ASEMaster35yrs, Chevy Technician
Category: Chevy
Satisfied Customers: 2095
Experience: ASE Certified since 1972,Master Auto tech,Heavy Truck Tech ,chev dealer tech
ASEMaster35yrs and other Chevy Specialists are ready to help you
Customer: replied 4 years ago.



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?

Expert:  ASEMaster35yrs replied 4 years ago.
If you are losing ground, try attaching another ground manually to see if the problem goes away. Look at the capacitors carefully and look for any bulges or color distortions that may be different from the rest.
Customer: replied 4 years ago.

I will try attaching a new ground line tonight after work. I will let you know the results

Expert:  ASEMaster35yrs replied 4 years ago.
Ok, let me know.