rickbraley

I have an 85 with the 4+3 manual trans. Car is completely stock with a little over 90K miles.
I am having trouble with the speedometer quitting after driving the car for a few miles. Code 24 is set. If I let it set for a while in between runs it will start working again and then stop after a short period. Once it stops it doesn't start again until I shut down the car and let it set for several minutes.
I have searched this forum and based on my findings I decided to replace the speed sensor on the transmission. This did not solve the problem.
I need some ideas on how to troubleshoot this problem; I obviously need the speedometer to work!
Any ideas and/or feedback etc will be greatly appreciated!
Thanks in advance.

Fasterer

My 85 has the same mileage with the same problem but in reverse. I had the motor replaced and since the first startup the spedo will show nothing for a few miles and all of a sudden the speed will show variances till it settles down to correct speed. If it sits it goes crazy again. I'm begining to think its a ground somewheres in the engine removal. You didn't by chance have any head work or tranny work done. If you find the answer i would be very interested as they may have the same origin
Bill

rickbraley

I will investigate the engine ground(s). I did put a clutch in it a while back but I don't think the problem started occuring until a long time after this work was done.

I wouldn't think it would be the ECM or the IP since it does work for a while?

Frankly I am just not sure.

Hopefully some other Vette enthusiasts out there will offer us some help!!

MaSTeRofDZaSTeR

Fasterer, I'm having the same problem you did it seems on my 90 for the past 2 days. It came back tonight. Speedos going crazy until I stop or accelerate at higher speeds after the vettes been running for a while or in very hot temps. I just had a new tranny put in. I heard about the speed sensor. Could this be related? Open to any suggestions and thinking on having my tranny people take a look at it.

rickbraley

My engine ground appears to be connected properly so I don't think this is my problem.

I did some more troubleshooting per the factory service manual and per the manual I have a faulty IP cluster. The manual states to check the voltage at one of the wires on the IP cluster connector and when the speedo stops working the voltage exceeds 10 volts supposedly indicating the IP cluster is not "switching to ground" properly.

How does the speed sensor fit into this? Does it supply the periodic ground?

Anyone have any insight on getting an IP cluster tested and/or repaired? Who does this, what's the average cost, etc?

thanks.

rickbraley

Determined to figure something out on this I just finished another troubleshooting session and I narrowed down the problem to the output of the speed sensor.

While running the car on stands in gear and the speedo working I can measure an alternating voltage at the output of the speed sensor. The speedo works as long as the voltage is present. It varies a bit but there is clearly something there per my DVM.

The speedo quits working when the voltage drops to 0. I can disconnect the connector that runs to the IP from the sensor and duplicate the voltage present/not present scenario so I don't think it is anything upstream of the sensor causing the problem.

Now my next question is what could be causing this? As stated earlier I replaced the speed sensor with a new unit and the problem remained unchanged. It would seem reasonable then to assume that whatever is driving the speed sensor in the transmission is stopping spinning the sensor?

Any ideas on where to go next? Is there something in the tranny that could be causing this and if so what has to be done to fix it?

Any input will be greatly appreciated!!

masterprice33

I'm having the same exact problem with my 85, most of the time it happens when I have been driving it for awhile or when I start the car up after driving it for awhile it will stick at 0 mph. can't be much help but I'm having the same problem i have the VSS code that comes on usually when the car has been driven for awhile and the speedometer will either jump around or stay at zero, and the car will start bucking a little bit at high speeds (40-80 and higher mph). was actually looking for some help with this issue on whether or not it's the voltage to the sensor or the sensor itself. So i'm just a bystander waiting for answers just as you are, I don't want to spend the money on the sensor if the sensor installed now is fine.

rickbraley

I believe the sensor actually creates a small pulsating signal and therefore there is no voltage to it. I took my old sensor and drove it with a hand drill and there was an induced voltage across the two terminals of the sensor which supports this position.

So, if there is no voltage across the two terminals when the car is running and in gear it has to mean either the circuitry in the sensor is damaged or it is not being spun.

Am I missing something here or does this sound correct?

I am surprised we are not getting more feedback on this from the forum? Is this really an uncommon problem or what?

Any transmission guys out there that might have some ideas on why my speed sensor might stop producing a signal?

Again, any help would be appreciated...

masterprice33

If it were in fact a faulty IP cluster would that still make a code 24 set in? doesn't a code 24 mean it's something directly related to the performance of the actual speed sensor? yeah usually the forum guys would be jumping at it...what you say makes sense though, but we'll wait and see what they say.

rickbraley

I don't think my problem is a faulty IP cluster. I originally followed one of the troubleshooting processes in my factory service manual for DTC 24 and it led me to believe it might be my IP but my additional efforts that identified the lack of output from the speed sensor makes me think this is the problem and not a faulty IP. I believe the IP cluster is not "switching to ground" as the troubleshooting process for DTC 24 concludes but I think the IP is not "switching to ground" because it is not getting the signal from the speed sensor.

Again, just guesses on my part at this time. I guess the next thing to do is to remove the speed sensor and see if I can find anything with whatever mechanism drives it in the transmission.

Still hoping for some outside assistance... anyone know anything on this?

toptechx6

Can't say I've seen this on a Vette but have on other cars. The plastic/nylon gears inside the tailshaft that drive the sensor partially strip, I'm not sure if temperature plays a roll but have seen them drive the sensor intemittantly.

rickbraley

Hey thanks, that is where I am heading... it does seem like the problem should persist regardless of temp, etc though... hmmm...?

I assume I have to remove the tailshaft housing to get to this gear; anyone know if I can do this with the trans in the car? Anything to watch out for?

thanks.

rickbraley

I believe I have found the source of my problem. Upon removing the speed sensor to examine the speedo gears at the tranny I found the speedo drive gear clip has broken. I believe what is happening is the gear is on the output shaft just tight enough to drive the sensor when the gear is cool but as it heats and expands it starts slipping on the output shaft thus not driving the sensor, etc.

Will I will be able to replace this clip with the tranny assembled and in the car?

Does the clip go on the output shaft then the gear slide up over it or what? I believe there is a small pilot hole in the output shaft that the clip hooks on then the gear goes over the clip? Is this correct? If not how does it go together?

Anyone have any feedback on this?

Thanks.

rickbraley

Anyone?

Fasterer

Wow, Finally found what was causing mine. Was a connection in the plug for the speed sensor at the tranny. Must have been stressed when the removed the engine and let the tranny drop. There sure is not much slack wire bcak there. Had to push the plug back in with flat side of screw driver to get it to latch in. Been drivig for a week now ,no more problem. Course i've got everyting crossed i can. If it makes it to Carlisle and back i'm gold.

toptechx6

Rick as far as I know it will at least be necessary to remove the tailshaft, the nylon gear slides over the clip from the rear on most transmissions, can't swear it is the same on a 4/3.

Bill Bamberg

I've been having this problem for a long time. I've been able to change the symtoms slightly, but never fix the problem. The general symptom is that after a few miles of driving, when everything is heated up, the speedo will stop working and read zero. All speed sensor related functions cease, including overdrive and cruise control. I've:

1. Changed the sensor. I now have two working sensors.
2. Changed the green gear when the clutch was rebuilt. No fix.
3. Installed a new pigtail when the sensor was changed the second time.
The original plug was scorched, so I rebuilt the heat shield.
4. Replaced the ECM.

Yesterday I pulled the IP cluster for one more round before sending it off for repair. Since the ECM is still out of the dash, I pulled out the the connector that connects the yellow/purple leads from the VSS to the black/purple leads going to the cluster. I connected my DMM, set to read Hertz, across the yellow/purple wires with the car hot and speedo reading zero. The frequency reading just flashed random numbers regardless of car speed.

Since I have a spare sensor, I hooked it to a drill and measured the output. The FSM describes the sensor as a signal generator, so I used the Hertz setting on my meter to measure the output. The signal output registers as a positive frequency proportional to rotational speed, whether forward or reverse. With no rotation, it reads a steady zero. The overall resistance across the output of the sensor is about 420 Ohms.

With the cluster removed, I opened the connector and exposed the yellow/purple wires. I then measured the resistance across the new sensor and again got 420 Ohms and a steady zero Hz. I then cranked the engine, which wouldn't start, but immediatelly got the same random flashings for a signal out, even though the tranny was still locked in reverse with the clutch in. This would appear to rule out both the cluster and the sensor. It almost appears that there is an intermittant filter capacitor that is heat sensitive. I'd expect something like that to ruin radio reception.

Today, I'm going to buy a new connector for the VSS and wire around the car wiring to see if I can get a clean signal to the cluster.