A friend has purchased an unrestored nice driver 71/270 horse convertible, all stock. I'm creeping over from the C2 side for some help

A few days ago his speedo became erratic, I told him not to worry his cable was about to break, we'd replace it. The speedometer quite altogether, except for the unusual activity that it would, just once in a while, try to flutter off it's stop.

I pulled his cable connection at the speedo and set it up where you could see it while the car was in motion, the cable wasn't broken. I wondered if it was just too worn at the end to seat properly, so I installed a tach cable briefly and ran it with the drill motor- no speedometer action.

I have a few questions and I promise I tried some searching first...

1) could he have an odometer issue that is causing interference with the speedometer operation? How can I check?

2) is there anything else I can check before I pull the unit?

3) if the speedometer needs rebuilt, I have an excellent shop a few minutes from town (old timer), but I haven't taken one of these out in 15 years (my old '69) so if anyone has any cheats please let me know. I have small hands.


Thanks guys,

Benton

 

First of all a speedometer can be spun with an electric drill but only if the drill is spun BACKWARDS from its usual rotation. If the speedometer is the original it'll have a nylon worm gear inside that will get stripped over time which causes the odometer and trip meter to quit but the rebuilt units use a steel gear. Better to rebuild or replace it.

 

Thank you for the reply. I had not thought of running the drill the other way.

 

Do the drill test and see what you get.

Can the odometer cause an issue with the speedo... unlikely. Usually the odometer will stop working first, then the speedo will tank.

If you run the speedo in reverse with a drill and it works then I would go to the tranny and inspect the driven gear there.

If you find out the speedo is back, use this page below to remove the left dash pad. It is the best and safest way. We always remove the steering column from the car (only 5 bolts) and doing so will prevent you from damaging the dash pad.

It's also possible if the needle isn't moving... that something broke in the speedcup or the jewel bushing is binding up the speed cup... Too many things inside the speedo could be broke and I'm sure your guy will be able to fix it...

When you get to the link below, click on supporting document and a pdf will open.

1968-1977 Corvette Left Hand Dash Pad Removal

This is a copy of the thread post I did two days ago... it'll explain to you how the speedometers fail (I hope).

On any speedometer.. 1953-1977....

If the odometer is not working.. the speedometer needs to come out and be serviced.

When the first worm bushing wears out.. it puts the first worm (A) in a bind with the second worm (B). The first, second, third and fourth gears are what drive the odometer and the trip odometer. When the first worm binds with the second worm it eats the teeth off the gear. This stops the function of the odometer.

Once the bushing gets worn enough the first worm magnet will start hitting (scrapping) the speed cup... This cause erratic needle jumping and moving. The speed cup has a shaft on it that the needle attaches to and when the first worm bushing gets bad enough it will eventually grab the speed cup slamming the needle and in most cases breaking it and the speed cup.

Replacing (and they are available) the second worm gear with the plastic replacement is only a band-aide and actually a huge waste of time considering how hard it is to remove the speedometer from the car.

Other items that can cause a speedometer to be jumpy would include: Defective or binding speedometer cable, worn driven gear in the transmission, cruse control transducer(on equipped cars). The previous items can cause bouncing but anytime you have a working speedo with a non working odometer you know there is a first worm issue in the head.

I'd also like to remind everyone to NEVER put grease, wd-40 or other types of grease (lube) on a speedo cable. The cables are reverse wound and this will pack the grease in the speedo head.. The firstworm bushing is made from oylite and the lube will cause premature failure.

If you decide to remove the speedometer, here is a great page on how to get the dash pad out of the car... the easy way.

1968-1977 Corvette Left Hand Dash Pad Removal

 

Willcox...this is invaluable. Thank you so much for the reply and the reposting. I read your steps (I did this long ago but as the saying goes I've slept since then).

I will try to spin the speedo with a reverse drill but I'm betting in a few days I will just pull the left side as you outline! -Benton