speedo rebuild or replace?
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Burning Brakes
Joined: Jul 2004
Posts: 1,192
Likes: 111
From: If your not the lead dog the view never changes Boise, Id
Before buying a new or rebuilt speedo hook your speedo cable to a drill and see if the speedo works, might be the speedo gear in the transmission.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,851
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
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
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
Last edited by Willcox Corvette; Oct 31, 2015 at 08:26 PM.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Wilcox, you continue to impress! Thank you for that very detailed picture and instructions to remove the dash pad. I don't need to do that job yet, but I know it's only a matter of time before I do.
Le Mans Master
Joined: Jan 2001
Posts: 6,722
Likes: 1,376
I would get it rebuilt.....
I bought a new one about 26 years ago, and while wonderful, and has worked perfectly since then; I still have the old one, and I probably will have for life.....
And that is really the only reason I can provide for why to have it rebuilt. I would hate to toss it.
And that is really the only reason I can provide for why to have it rebuilt. I would hate to toss it.
Le Mans Master
Joined: Oct 2007
Posts: 7,445
Likes: 971
From: Tennessee/Rhode Island
I would have the OEM speedo rebuilt if you have it...don't toss it....
