Bouncy speedometer

[charlie D]

My speedometer has gotten bouncy at speeds under 40. Seems accurate and steady over 40. I have checked that the cable is well seated in the back, and that seems OK. Not sure that is a symptom of the cable starting to fail or the speedometer itself? So any diagnosis is helpful. Also, if needed, is there a recommended rebuilder or just replace it? Thanks
Charlie D

[rhjanes]

When mine have done that, it is the inside of the cable.  The part that spins.  It is a replaceable part.  You will need to remove the cable.  Get the correct inside part.  It pulls out.  There is information somewhere about how to clean it out.  Brake cleaner perhaps?  Dry.  Compressed air.  Graphite lube I think?  Replace the inside, install the cable and it should be fine. 

[Bill Collins]

The difficulty is seldom in the speedometer head itself. I would first pursue inspecting and lubricating or replacing the cable as suggested. If this does not solve the problem, the most likely cause is a worn out speedometer drive gear.

The driven gear, which is attached to the cable on the trans end, meshes to a circular drive gear pressed onto the transmission output shaft. This gear is plastic and over time the center of it wears down, creating a "dip" in the splines. It then makes only skipping contact with the driven gear, resulting in the needle bounce effect that you describe.

The gear itself is readily available from various toploader transmission suppliers. Installation requires removal of the transmission tailshaft housing. Exact positioning of the new gear onto the output shaft is critical for proper operation.

I have had several of these replaced over the years and it always fixed the problem.

[Bob Gaines]

Determine where the problem is . The most labor intensive is the drive gear in the tailshaft that Bill describes IMO. It could be as simple as the nylon gear at the end of the speedo cable has its gears damaged . Pull it out and inspect it. It should be evident if damaged or not .if damaged count the teeth and get a replacement . The gear is held on with a clip.  As mentioned you can take out the inside core of the speedo cable then lubricate and reinsert the core .That is not to mention the process of taking loose the dash cluster and also loose at the transmission. Sometimes that core which is like a tightly wound spring gets stretched from lack of lubricant and ruins the core because it causes symptoms like you described which I will not go into explaining. The problem is the lubrication process with taking the core out lubricating and reinstalling is fairly labor intensive and may or may not make things better and you will not be sure if the problem is the cable or other places mentioned.  The couple extra steps in replacing the entire cable with the core inside the first time is the sure fire way of solving the problem if that is what is causing it . At the very least you can move onto another possible culprit.  It isn't any easier but you are only going to do the job once rather then twice by doing the job of lubricating finding out it didn't work then having to do it again when you install a new cable.  I just thought I would mention these things in case it makes a difference to you.