How does the speedometer work?


A speedometer is a device that measures driveshaft activity to determine your vehicle’s speed. This speed is then displayed on the speedometer so that the driver knows how fast they are going. There are to main types of speedometer; mechanical and electrical / electronic.

Mechanical speedometer:

A mechanical speedometer configuration is more common on older cars, but is often found in newer vehicles too.

Inside your vehicle transmission is a small gear and output shaft that rotates your in direct proportion to your vehicle wheel speed. This shaft is connected to the speedometer (speedo) via a speedometer / speedo cable, which rotates according to the vehicle’s speed. The speedometer magnet inside your dash is connected to the speedo needle that displays your speed on your vehicle dash. As the speedo cable spins the magnet, this causes the speedometer needle to rise or decrease according to your speed.

Electronic Speedometer

An electronic speedometer configuration uses sensors to monitor driveshaft rotations. Each rotation sends a small electronic pulse to the car’s computer. The frequency of these pulses determines how quickly the car is going. The output vehicle speed from the ECU is then either displayed on a speedo needle type system, or on a digital display with LED / LCD segments.


What causes the speedometer to stop working?

Damaged speedometer cable – The speedometer cable is prone to wear over time. If the cable cannot create a connection with the small output shaft from you gearbox or at the other end your speedo head in the dash, then your car’s speed will not be measured correctly.

Other worn components – A mechanical speedometer uses several moving components that can wear, become damaged or malfunction over time. These include the transmission / gearbox speedo output shaft, speed cup, and hairspring.

Faulty speed sensor – In electronic speedo’s the sensors that measure the driveshaft rotations may be working incorrectly. This can cause the speedometer to stop working or become inaccurate.

Blown fuse – Damage to exposed wires can cause the speedometer fuse to blow. This will stop the speedometer from working.

Malfunctioning ECU – The computer inside your vehicle (commonly known as the Electronic Control Unit or ECU) is responsible for measuring the signals from the speed sensors (in an electronic speedometer). If the ECU is malfunctioning, you may receive inaccurate readings or the speedometer may stop working altogether.


Why is a working speedometer important?

A properly working speedometer allows you to see how fast you are going. This helps you to drive within the legal limits and stay safe. A working speedometer is also necessary for your vehicle to pass a Warrant of Fitness (WoF).


Speedometer Fixes & Services in Hamilton

Are you having problems with your car’s speedometer? If so, we can help!

At Grimmer Motors, we can quickly and accurately diagnose and fixproblems in your car’s speedometer / speedo. We can replace malfunctioning parts, allowing the speedometer to work correctly again.

For fast, accurate speedometer replacement and fixes in Hamilton, contact Grimmer Motors today!

Please Note – we don’t generally stock or supply just “parts only” for this service. We are a general automotive repair workshop. If you are able to bring your vehicle to us, we can diagnose the problem, find and fit the necessary parts, or organise the appropriate service for you.


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