My speedo seems to intermittently not work. The milescounter does work, but the speedo seems not to indicate speed. I already verified the cable (which is new btw) and it's ok.

Any thoughts ?

 

GM changed the design of the speedometer in 1978 and went with what was called the Series AK speedo. This was an entirely different head configuration than previously used.

Below is an exploded view of the speedo. Guessing would be about all one could do without having the head in front of them to see what is causing it.

Common issues would be the bearing (plastic) made inside the "Frame & Bearing Assembly" If this bearing wears out or is breaking it could be allowing the first worm and magnet to ride to far inward and be binding up the speed cup.

Or you could have an issue with the Spindle retainer, which is just a clip that holds the spindle in place. If this is binding up on the speed cup spindle this could be your issue.

Inside the first worm and magnet is a brass bushing and that could be your issue.. There are just to many things to list that could cause this without seeing it.

Your best bet would be to remove it and have it serviced or replace it. Replacement parts are becoming more and more scarce for this head as well. Buying a good used one and making one out of two may be another choice for you. What you do not want to mess with would be the Hair Spring Adjuster.. this is what controls the calibration of the head.

I have instructions on my site on how to remove the dash cluster, follow these steps and once out then remove the speedometer from the housing. http://willcoxcorvette.com/repairand...lp.php?hID=146

 

Is there a difference in cables since you say the head is different ?

 

Could it be the driven gear in the tranny?

 

I already checked that and it seems to be fine.

 

It can't be the tranny gear if the odometer is working...