Driving home tonite I noticed a clicking noise from the dash, and then noticed the speedometer needle jumping up and down with each click. Doesn't do it when stopped. Also, the odometer is not working (neither is the trip meter, which I reset to zero).

Does this sound like a cable problem, or has the speedo unit in the dash gone bad? How difficult is the cable and/or unit to replace?

This is on a 78 with a 350 auto tranny.

Thanks.

 

Check this article out on the Corvette Faq, http://www.corvettefaq.com/c3/RebuildSpeedo.htm

The article doesn't tell much about speedo repair, other than saying that lubing the cable with white lithium grease, to cure jumpy speedometers.
But it does tell you how to get it out, better than I could.

 

The clicking noise would tell me it's the cable. But that should not effect the odometer.

Usually when the cable is sticking or tweeked it will make the speedo bounce.

I would first check and see that the cable is clipped in properly to the speedo and check to see if there is a crimp in the cable. The cable is a PITA in that you have to route through the firewall.

Hope this helps.

Terry

 

I would of thought it was the cable, but when you mentioned the non-functioning odometer I'd suspect the speedometer. The plastic gears in the speedometer head might of started to deteriorate and lost some teeth.
If that is what it turns out to be, there was a how-to on replacing just those gears posted a couple months ago. I hope it's the cable, much easier fix for you.

Scott

 

Does the car have cruise control? If so, you can disconnect (and lubricate)the speedometer cables at the sending unit. Cruise control sending unit is on the driver's side sort of tucked under the master cylinder. There will be one long) cable from the transmission going into the sending unit and another (short) cable from the sending unit that goes through the firewall and connects to the speedometer.

After you disconnect (unscrew) the sending unit to speedometer cable, you can twist (quickly spin) the internal wire to simulate motion. When you twist the cable the speedometer indicator should move - the faster you twist the "faster" the speedometer will show. If a sharp twist of the wire (try both directions) does not show "speed" then the speedometer unit itself may be the problem or the speedometer cable may have worked itself loose from the speedometer.

Twisting this cable should be EASY, if not, then lack of lubrication might be causing it to bind. You might try to dribble some "Dri Slide" (graphite in a light oil) into the cable ends between the wire cable and its sheath.

The transmission to cruise control sending unit cable (internal wire) should not turn as it is attached to a gear in the transmission. If it does turn, the problem is likely to be the transmission speedometer gear.