How Does a Speedometer Work?

A vehicles speed is determined by the speedometer cable which attaches in the transmission to one end of the drive gear. This rotates at a speed corresponding to the vehicles speed. A wire transmits this information up to the speedometer on the dashboard.
The cable is protected in the cable housing. This instrument prevents damages and kinks to the cable. Over time, the speedometer cable and housing can fail due to general wear and tear.

3 Signs Your Speedometer is Broken or Failing

Speedometer Fluctuates Erratically and Gives Inaccurate Readings

A manual gauge or LED-powered speedometer both have a smooth transition, meaning when you accelerate or decelerate the gauge will climb progressively, not jump erratically. For example: your speedometer will climb 25, 26, 27 and so on, not 25 – 35 straight away. If you notice the speedometer needle jump suddenly, the cable could be damaged or the sensors are not accurately sending a signal.

Speedometer Does Not Register Any Speed at All

If your speedometer needle has stopped moving altogether when in motion, this is another warning sign. The speedometer cable and housing have most likely failed or it could be caused by a faulty electrical connection to the dashboard. Either way, you’ll need to talk to your local mechanic to fix the problem.

Squealing or Whirring Noises from The Dashboard or Underneath of Car

If the speedometer cable and housing is starting to fail, you may hear squealing, grinding or whirring noises from your car. Noise may stem from the speedometer needle jumping about, but it can come from the transmission under the car. If you notice these strange noises, contact your mechanic immediately, who’ll be able to diagnose the exact problem.

How Is a Broken or Failing Speedometer Cable and Housing Fixed?

While the speedometer itself doesn’t normally break, the cable and housing may fail over time. If you notice the symptoms above, a mechanic will need to replace the speedometer cable and housing. Here’s how it’s done:

Car jacked up on support stands and battery disconnected
Broken speedometer cable and housing is removed
A new cable and housing is installed and the vehicle removed from jack stands
Battery is reconnected and the vehicle is road tested

How Important is Speedometer Cable and Housing Replacement?

A broken speedometer cable or housing puts you at greater risk of speeding and traffic violations. If you’re going too fast, the chance of an accident increases. Once you notice any of the symptoms described above, get your local mechanic to check your vehicle at the earliest convenience.

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