How to tell if speedometer head bad?

[tampabronco]

How to determine if the cable is the problem or cluster heads are bad?

Have replaced the cable and plastic gear in the trans. Rerouted the cable to ensure it's not binding and have tried different snugness of cable to cluster

The needle bounces all over the place from 0 to 100 and is never close. Wide swings in speed

 

[anoblefox]

Mine did the same thing just before 300k miles. I was always able to lube the cable from the top (head unit) before that stopped the wide swings before, but finally had to replace the head.

 

[DirtDonk]

If we had access to the back of the speedometer more easily than we do in a bronco, you could attach a drill and run it up to a steady speed.
If the needle still bounces, then there’s a problem in the instrument. If it works smoothly, then the problem is still within the cable.

Did you replace just the inner cable core, or the entire cable assembly including the outer jacket?

 

[tampabronco]

If we had access to the back of the speedometer more easily than we do in a bronco, you could attach a drill and run it up to a steady speed.
If the needle still bounces, then there’s a problem in the instrument. If it works smoothly, then the problem is still within the cable.

Did you replace just the inner cable core, or the entire cable assembly including the outer jacket?

Great idea. Didn’t replace the inner cable

 

[BOBS 2 68S]

I pull the drivers side floor vent when I work on the speed-o unit. Also mark the wires where they came off, make reassembly easier.

 

[anoblefox]

I second pulling the drivers vent especially IF you are replacing the speedo head. I have owned an EB since 73 and the two most difficult jobs were replacing the heater core and the speedo cluster. looking down the drivers vent hole you could see the electrical connections OR you could feel them; but not both at the same time. I guess i'm just getting too old to work on the tough jobs anymore! PS, bifocals are very cumbersome doing jobs like these!

 

[tampabronco]

If we had access to the back of the speedometer more easily than we do in a bronco, you could attach a drill and run it up to a steady speed.
If the needle still bounces, then there’s a problem in the instrument. If it works smoothly, then the problem is still within the cable.

Did you replace just the inner cable core, or the entire cable assembly including the outer jacket?

Before I remove the cluster….. how do you connect a drill to it to test?

 

[Madgyver]

replace cable as a complete unit. inner and outer.

 

[Juiceman]

this is an older post and wondering what the outcome/solution was? My speedometer works fine until I reach 60 mph and then goes "limp" and the needle falls all the way right to 100. When I slow back down below 50 mph, the speedometer starts working again. I am hoping it is just the cable or the plastic gear and not the cluster.