Speedometer - What The Heck Happened - 1969 Dart
Side shot
Back
Green is the adjuster to correct the speed and the spring that returns the needle to 0
Red is the 0 stop
Functionality
The part (red arrow) spins with the speedo gear and shaft in the speedo cable.
It causes the part it spins around to move. Since it is attached to the needle the needle moves. The spring resists the movement otherwise it would read 120 when ever the car was moving.
Any contact of the outer piece to the inner piece will cause the needle to be dragged farther than it should and when the contact stops the needle rebounds to where it should be till the next contact.
That's one way you get a bouncy needle.
The other is a speedo cable that is sticking so it winds up a bit like a torsion spring then releases causing a speed increase at the needle.
This is the speedo head
Side shot
Back
Green is the adjuster to correct the speed and the spring that returns the needle to 0
Red is the 0 stop
Functionality
The part (red arrow) spins with the speedo gear and shaft in the speedo cable.
It causes the part it spins around to move. Since it is attached to the needle the needle moves. The spring resists the movement otherwise it would read 120 when ever the car was moving.
Any contact of the outer piece to the inner piece will cause the needle to be dragged farther than it should and when the contact stops the needle rebounds to where it should be till the next contact.
That's one way you get a bouncy needle.
The other is a speedo cable that is sticking so it winds up a bit like a torsion spring then releases causing a speed increase at the needle.