I was driving my '77 Corvette today and during an acceleration the speedometer failed and dropped.  The tachometer still works and I checked the fuses and they are all good.  I have only had this car for a short time.  Being a new member I really appreciate this website.

-Brint

 Shoot, that's bummer!  Did you check the cable?  My '69 has a cable that attaches to the distributor.

Thanks for the reply, the cable (according to my repair manual) is hooked up to the transmission.

-Brint

 

check the cable look and see if it still in the speedometer your tech is not gear drive

You can also pull the cable from the trans and check the gear on the end of cable. Make sure the gear has its teeth and does not spin freely on the cable. For that matter, make sure the cable and gear are still in the trans. If the retainer bolt fell out, the gear could have come out of the trans.

[QUOTE=greypoupon69]  Shoot, that's bummer!  Did you check the cable?  My '69 has a cable that attaches to the distributor.[/QUOTE]

And that would effect the speedo HOW?

Chances are one or both of your speedo gears are damaged. It could also be the cable, or the speedo head itself, but the gears would be more likely.
Does your car have cruize control? If so, then the cruize transducer could have locked up, and stripped the speedo gears, or the ends of the cables. Isn't this fun!!!
The speedo gears can be accessed by raising the car up safely, and take the speedo housing(bullet) off on the driver's side rear of the trans. tailhousing. It will require a 10mm, or a 7/16" wrench.('77 should be metric, but could be either way) Unscrew the speedo cable,remove the 10mm(7/16) headed bolt that retains the clip for the housing, and then pull the housing out of the tailhousing.(You will lose some fluid here, so be sure to have a catch pan handy!)
Once you have the housing out, you will see the plastic, driven gear. If it is damaged, you will see broken teeth, or other damage. Then, look into the hole where the housing was, and look at the driven gear, on the tailshaft. Sometimes, the driven gear will be damaged, but the drive gear is still OK. If the drive gear is also damaged, it will require removing the tailhousing to replace.
If both of the speedo gears are in good shape, it would be time to check the cable. See if you can spin the cable with your fingers. If you have a reversible drill, chuck the end of the cable into the drill, and spin it at a low speed, and have someone check to see if the speedo is moving. If not, and you have cruize, disconnect the lower cable from the transducer, mounted on the drivers side fenderwell. Check the end of the cable, and see that it has a square end on it. If not, then that is the prob. If the cable end has spun off, or is not square, the cable will need to be replaced. The cruize transducer will most likely be locked up, and that is what has damaged either the gears, or the cable.
If it is square, disconnect the other cable from the transducer. Try to spin the transducer with your fingers. If it is hard to turn, or is locked up, the transducer must be replaced.
If the gears, cable, and transducer are good, then spin the cable that goes to the inside of the car with the drill. If the speedo doesn't work now, then you will need to check the other end of that cable for a square end.(the end that goes into the speedo head itself rarely fails) If the other end of that cable is good, the only thing left is the speedo itself.

My money is on the transducer locked up, damaging the gears.
Speedo??!! We don' needs no stinkin speedos!!! Adams' Apple38867.8358564815

In my '78 when the speedometer didn't work, it was due to the cable between the transmission and the transducer. Replacement of the cable fixed the problem. The transducer was bad as well, causing the speedometer to fluctuate. Whether that was the root cause of the cable going bad I don't know. Either way, replacing it fixed that problem.

And if none of that makes sense, come by Autozone on Sunday and we can make a time next week to take a look at it.  (Can't spend more than a few minutes while at zone.)  I'll help you find the problem, and you can learn a bit at the same time.   Most of these items won't take long to check.

Hey thanks for the replies.  I was underneath the car yesterday looking for the problem and I noticed the cable that runs to the transmission for the speedometer was sitting right on top of the exhaust pipe.  The cable was melted in a spot about halfway through and you could see the wire inside of it.  Could this be the problem?

By the way, I tried to unscrew the cable from the transmission and I couldn't get my pliers to grip it because of the little space to work with, the exhaust was in the way.

-Brint

Yes the melted cable would do it.  You will need to replace both the cable and housing.  Before you buy one, check to see if you have crusie control.  If you do, the cable from the trans to the cruise is fairly easy to replace.  Without cruise, you need to pull the dash forward a bit, or perhaps remove the dash pad to get access to replace the cable.

With a good grip and twist, the cable should come off of the trans.  If worse comes to worse, remove the one bolt holding the driven gear from the transmission along with the cable, and then seperate them.  But in reality it's probably easier with the holder in the trans.

A good pair of Channel Locks (water pump pliers) and you should be able to get it loose.