Speedometer Problems

[JGC403]

Finally getting my '76 Duster going again, was driving it back and forth in the driveway to make sure the 4 speed was shifting correctly. I noticed the needle on the speedometer would go all the way up then drop back down and would keep doing this even when holding a steady walking speed. I something sticking inside the speedometer or something?

 

[no poseur]

cable is frayed

 

[elevensies]

Frayed or kinked inner
Try spraying motorcycle chain lube down the outer.

 

[RedFish]

2 components , the cable and the instrument. Most common issue is the cable and in your case most probable. If you accidently crushed the sheathe, or kinked it in rerouting to a different trans, there's your problem.
If the cable is all good and the needle still bounces, the problem is in the instrument itself.

 

[TrailBeast]

We need to know if it suddenly jumps up and then goes down, or slowly climbs up and then suddenly drops.

It makes a difference in the diagnostics.

 

[JGC403]

We need to know if it suddenly jumps up and then goes down, or slowly climbs up and then suddenly drops.

It makes a difference in the diagnostics.

It climbs up then suddenly drops. The car is parked and its sitting at 35mph right now. I'll post a video tomorrow.

 

[RedFish]

It climbs up then suddenly drops. The car is parked and its sitting at 35mph right now. I'll post a video tomorrow.

That points to instrument. I've been seeking parts to repair one with the same symptom for a couple days. In hand testing it would hold the needle up. In the owners car this was wild needle swings.
The needle return spring is as tiny and weak as a watch spring. It cant overcome resistance. There is still some hope though. If you get lucky you'll find debris including metal on the magnet in its gyro is causing the resistance. Good luck

 

[TrailBeast]

That points to instrument. I've been seeking parts to repair one with the same symptom for a couple days. In hand testing it would hold the needle up. In the owners car this was wild needle swings.
The needle return spring is as tiny and weak as a watch spring. It cant overcome resistance. There is still some hope though. If you get lucky you'll find debris including metal on the magnet in its gyro is causing the resistance. Good luck

Yes it does (point to the speedo assy) and not the cable.
They can be found pretty easily, or you could send it to a repair facility.