ECM Code 24 Question

[Tommy Almond]

Okay I replaced the O2 sensor and for now not getting the Code 13 so hopefully that is taken care of. But I am once in a while getting a Code 24, Vehicle Speed Sensor. I know this is mounted behind the speedo cluster and I beleive works off of a spinning reflective wheel or sort to get it readings and send series of pulse voltage to the ECM to determine the vehicle's speed......am I in the ballpark?

Should I get a Code 24 while engine is running, parked and not going down the highway?

Any thoughts, comments, idea will be great and thanks in advance. I have the '81 Service Manual and also the CCC Service Manual and see the troubleshooting chart, just haven't jacked the vette up yet to start that process.

[Peer81]

Hello Tommy,

Yes you're in the ballpark!
I don't remember it perfectly but I think the ECM only sets a code 24 when the car is in motion.
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 circuitboard. 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.

If I understand correctly you only have the code so now and then? That isn't the easiest to look for the problem! Good luck

Greetings Peter

[Tommy Almond]

Peter, yes I got the Code 24 shortly after cranking engine while sitting in the garage after a couple of minutes and no movement of the car. Per the manual that code should not come on until after 5 mins of road running so sorta loss on why would come up while sitting still. I will check the wiring and other item you mentioned located under the center console to see if a loose wire etc and also check the connection at the ECM, I believe that is port 16.

The 24 code has been rare but has been and since I am trying to make and keep this 81 as close to the assembly line as can......I get a little picky if not all doing as should. Call it anal but trying to make everything right.

The 24 code has never locked on, just lites up for a few seconds and back off, but enough to store a code in the ECM.

[Wajulia]

The Speed sensor is a rotating mirror behind the speedo that is rotated by the speedo cable. There is a light and a sensor mounted to the back of the speedo. When the mirror passes the light it reflects back to the sensor and the ECM counts the number of times a second the sensor sees the light


Not much that can go wrong, the sensor can fail or 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.

[Tommy Almond]

The Speed sensor is a rotating mirror behind the speedo that is rotated by the speedo cable. There is a light and a sensor mounted to the back of the speedo. When the mirror passes the light it reflects back to the sensor and the ECM counts the number of times a second the sensor sees the light


Not much that can go wrong, the sensor can fail or 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.

That's what I was thinking so gonna tackle that this weekend if have time. It is a rare code that only pops up once in a blue moon.....but I am anal on wanting everything to be right as came from factory....even if it is a 31 year old lady.