Tacho shaft for different automobiles
Your Speedometer cable worn out? » Read more about common malfunctions • Repair • Replacement manual
Mechanical speed indication thanks to the speedometer cable
The speedometer cable is part of a car's speedometer. It is a purely mechanical component that even works reliably if the electrical system fails or the engine is shut off. The speedometer cable is also sometimes referred to as "speedo cable" or "speedometer shaft". The speedometer cable belongs to the assembly of the "eddy-current speedometer"
Operating principle of an eddy-current speedometer
An eddy-current speedometer realizes the speed indication by purely mechanical means. The speedometer cable is attached to a small additional gear provided on the vehicle drive. It consists of a flexible shaft that runs inside a sleeve. There is a layer of graphite between the shaft and sleeve. It is intended to prevent the inner shaft from seizing up. A permanent magnet is located at the other end of the speedometer cable. This is put into rotation when the vehicle is in motion. The faster the magnet rotates, the higher its magnetic pull. This drags along another shaft, which is however kept from turning by a spiral spring. The faster the vehicle moves now, the stronger the magnetic field, consequently pushing the spring shaft further. A pointer that indicates the vehicle's current speed by means of a scale is located on the other end of the spring shaft.
Speedometer cable defects
A speedometer cable is permanently being rotated around its own axis when driving. The result are constant torsional forces on the inside. If the graphite lubrication inside the sleeve declines, the friction generates a high counter force. This may result in material fatigue inside the speedometer cable, eventually resulting in breakage. In this case, the speedometer fails abruptly and no longer moves smoothly. Consequently, a proper reading of the speed is no longer possible. A speedometer shaft can not be repaired, only replaced. It is mounted to the drive and also the inside of the dashboard at defined points by means of split pins. Using an appropriate tool, typically split pin pliers or bent-nose pliers, the speedometer cable can be removed and replaced by a new part with just a few hand movements. This doesn't usually involve replacing the entire speedometer cable, only the part between the drive and the permanent magnet. The remaining components of the speedometer cable are usually maintenance-free and survive a car's life.
Purchasing a speedometer cable from the accessories trade
Speedometer shafts are normal wear and tear parts. Their price starts at around 10 euro and they typically survive 100,000 kilometers. Although they are very simple components from a technical viewpoint, their manufacture requires skill and precision. Low-cost carriers are known for saving cost when it comes to the lubrication. This is not visible from the outside, but evident from a short life of the component. Here, it is highly advisable to trust in the quality provided by well-known brands. Although a speedometer cable is very inexpensive, they are installed deep inside the engine compartment. Thus, their replacement can be quite complex. A high-quality speedometer cable guarantees reliable speed indication for many tens of thousands of kilometers.
False readings from the speedometer
Yet, if the speedometer shows incorrect values, the issue is usually not caused by the speedometer itself. In most cases it is the result of using an incorrect tire size with the vehicle. The scale at the speedometer is based on a defined conversion of the drive shaft's speed into the vehicle speed. If the tire size strongly differs from the permissible sizes, this arithmetical conversion from the drive to the speedometer is no longer correct.
The speedometer cable also acts as a torsion bar that always has a certain inner elasticity. It needs this elasticity, so that it can be laid along tight radii. However, this inner twist can also result in a deviation at the speedometer. The legislator permits speedometers to show a speed that is faster by 10% + 4 km/h. Yet, the speed must not be displayed as slower than it really is. That's why most vehicles are traveling at a slightly slower speed than is displayed to the driver.