Speedometer cable troubleshoot C3

[Charles P.]

I have a question regarding my speedometer. The speed no longer seems to register above 30MPH unless I'm doing say 60 where it might say 50MPH. It seems to bounce especially at lower speeds on deacceleration. It also seems to be getting noisy.

Could this be the cable or speedo and is the fix as easy as replacing the cable or will this require other parts at the level of the tranny such as the gear seal w/ o-ring? Several catalogs list more than one length of AC repacement cable ie 70, 71 or 72 1/2 inches -- which length is correct?
Lastly do you have to drop the speedo/tach panel or can the cable come off the rear of the speedo with patience and working your hands from under?

Any input or tips would be appreciated. I hate jobs like this as replacing a blown bulb behind a tach on my 72 took better than an hour.

[John Chesher]

Re: Speedometer cable troubleshoot C3

Sounds likely to be the cable to me. A common problem on the C3s... I loosened up the instrument panel by removiong the 4 screws you can see from the front and pulling it out a bit. I don't remember that I removed anything else, but it was a TREMENDOUS pain to get the old cable detached and the new one attached and I have very little hands. One tip, the cable does not screw off the back of the speedo. There is a little tab right beside where you attach the cable. If my memory is correct, you can just push the new one in and it will click when installed all the way and you are done. To remove, I think you have to press the tab in while pulling out on the cable. Requires the fingers of a contortionist to pull that off in such tight quarters.

[Michael W.]

Re: Speedometer cable troubleshoot C3

Based on your description, my guess is that it's the instrument itself and not the cable. I can't see how a damaged cable can cause the speedo to indicate low.

[Charles P.]

Re: Speedometer cable troubleshoot C3

If it is the speedometer itself I guess this will be apparent after the cable is replaced---or should I attack the speedo first? Would this require replacement of the speedo or is there someone out there that repairs them?

Thanks

[Dick W.]

Re: Speedometer cable troubleshoot C3

I believe that if you search the archives you will find several recommendations for companies that repair instruments.

[Jim T.]

Re: Speedometer cable troubleshoot C3

Charles I don't know about the 4-speed 70 Corvettes, but my 70 Corvette with the turbo 400 has a two piece speedometer cable. I can actually disconnect the speedometer at the bottom of the engine firewall. Very easy to do to stop putting miles on the cars odometer. Takes about a minute. My 70 came from the factory like this. I would recommend you find a vender to sell you a new speedometer gearing assembly. It is really easy to change out. The facing is held on with two screws and you replace everything behind the facing. The needle is easy to remove, just gently twist counter clockwise and gently reinstall. Remove and lubricate your cables. To get the speedometer out, remove the plastic kick panel after removing the door sill. Loosen/remove the two bolts at the base of the steering wheel shaft going through the firewall, remove the bolts supporting the steering wheel assembly to the dash and support the steering wheel on the seat. Remove the dash screws on the left, top, and right side and pull the dash back enough to remove the speedometer. When you remove the speedometer look for the little black round insulating
things and reinstall them. I lost a couple and used very very small vacuum line to replace them. You will see where they were positioned by the markings they leave.