Symptoms of a Bad or Faulty Speedometer Cable

Common signs include an oscillating or stationary speedometer needle, screeching noises behind the dash, and the Check Engine light coming on.

When your car is accelerating, the easiest way to determine the exact speed is to simply look at the speedometer. Believe it or not, this normally trusted device can be compromised and display incorrect information to the driver; which not only can be a safety issue, but can also result in the driver receiving a speeding ticket. In most cases, speedometer problems are due to a problem with the speedometer cable.

The speedometer cable connects to the back of the speedometer and runs through the gearbox of modern cars, trucks and SUVs. The cable is driven by a drive shaft and rotates a magnet that creates an electrical current and sends this information to the on-board computer. Once the ECU receives this data, it calculates the vehicle's speed and sends the information back over the cable and displays the speed on the speedometer.

Because the data has multiple touchpoints and travels through many different areas, there are several parts of a speedometer cable that can, and often do, fail over a period of time. Like any other electrical or mechanical component, a bad or faulty speedometer cable will display several warning signs or symptoms of a malfunction. The following are some of these symptoms that should alert you to a potential problem with your speedometer cable.

1. The speedometer needle fluctuates

The speedometer should move smoothly when the vehicle is accelerating or decelerating. However, there are times when the speedometer fluctuates or moves erratically. When this happens, it's usually because the speedometer cable or speedometer sensors inside the transmission are sending inconsistent data to the speedometer. This symptom is usually noticed when you are driving on the freeway, especially if the cruise control is on. You will see the speedometer move up and down within 10 mph if the speedometer cable is damaged.

If you notice that your speedometer is moving fast but the vehicle speed is not changing, this is most likely caused by a problem with the speedometer cable and should be checked or replaced by a certified mechanic as soon as possible.

2. Creaking sounds behind the dashboard

A squealing noise is never a good signal. This can be caused by loose belts or other mechanical systems that control your vehicle. However, if you hear a squealing sound from behind the dashboard, this could indicate a problem with the speedometer cable. This usually happens because the speedometer cable fails and sends sporadic data to the speedometer. If you hear any noise coming from the dashboard, see a mechanic to determine the exact location of the problem so it can be fixed.

3. The speedometer needle does not move

When the speedometer cable breaks, the speedometer needle does not move at all. If you notice this problem, it is important to contact a mechanic as soon as possible. A faulty speedometer is not only a potentially serious safety issue, but also a traffic violation if you are pulled over by the police for speeding. Be sure to take this matter seriously to avoid any problems.

4. Check Engine light comes on.

Since the speedometer cable is electronic and sends data to the on-board computer, a problem with this unit will often cause the Check Engine light to come on. This indicator lights up every time an error code is registered in the vehicle. However, every time the Check Engine light comes on, it's a bad sign; this is why you should always go to a certified mechanic to properly diagnose the problem before they fix any damage or replace mechanical parts.

It is extremely rare for a speedometer cable problem to occur while you own the car; but it can happen. When a problem does occur, it is vital to have a local ASE mechanic replace the speedometer cable, who can come to your home or office to perform the service.