As I mentioned in the last post, the speedo has jammed with the needle stuck on 60 kph. It reads over 60 just fine but when the vehicle speed drops below this point, the needle obstinately stays in the middle of the dial.
The most likely cause is that the fly spring on the back of the needle has failed. There are some folks out there repairing period auto speedos so I figured if I can get the fly spring out I can order a replacement of the same rating from the UK… well that was the plan.
I found a very interesting article online by Anthony Rhodes on how to strip and rebuild a Jaeger speedometer. So after some bedtime reading, I was ready to go. This is perhaps the only repair on a Land Rover not requiring the use of a hammer!
Mechanical description from Mr Rhodes…
The speedometer (speed indicator, not odometer) functions in just the same way as a tachometer. The cable spins a thin bar magnet. Just in front of the bar magnet is a disk mounted on a spindle. Also attached to this, on the same spindle, is the pointer that is visible over the dial face. When the bar magnet spins, it causes the disk (drag cup) just in front of it to try to spin as well. The amount of twisting force (torque) imparted by the magnet to the disk is proportional to the rotational speed of the magnet. If the magnet spins twice as fast, the torque is approximately twice as great. The spindle is attached to a flat coiled return spring to resist rotation. The amount the spring winds is proportional to the torque. In this manner, the pointer moves progressively farther as the magnet spins faster. “
Upon assessment of the condition of the fly spring and a bit pf further reading, I don’t possess the mechanical means to establish which weight of fly spring to replace it with. It’s important the spring is of the correct rating otherwise the needle wont read correctly.
I have ordered a new old stock Land Rover speedometer from PA Blanchards.
Although I cant fix it myself, I really enjoyed the mechanical simplicity of the magnetic speedometer. It’s not all that hard to take apart, the only “special” tools required were a optometrists screw driver and a thin bradawl. I tried to avoid magnetised tools too. The face of the speedometer is pressed aluminium and very fragile so this needs some careful handling… as I learnt to my cost.