As your car accelerates, the easiest way to determine your precise speed is by simply looking at the speedometer. Believe it or not, this usually reliable device can be compromised and display inaccurate information to the driver; which not only could be a safety issue, but could cause the driver to get a speeding ticket. Most of the time, problems with the speedometer can be traced to a problem with the speedometer cable.
The speedometer cable is connected to the back of the speedometer and runs through the gearbox of today's modern cars, trucks and SUV's. The cable is powered by the drive shaft and turns a magnet that creates an electrical current and sends this information to the onboard computer. Once the ECU receives this data, it calculates the vehicle's speed and sends the information through the cable again and displays the speed on the speedometer.
Because the data has multiple touch points and moves through so many different areas, there are several parts of the speedometer cable that can – and often will go bad over a period of time. Like any other electrical or mechanical component, a bad or failing speedometer cable will display a few warning signs or symptoms of being bad. Noted below are a few of these symptoms that should alert you that a potential problem with the speedometer cable exists.
1. Speedometer needle wavers
The speedometer is supposed to move fluidly as the vehicle accelerates or decelerates. However, there are times when the speedometer will waver or move erratically. When this happens, it is usually due to the speedometer cable or speedometer sensors inside the transmission sending inconsistent data to the speedometer. This symptom is usually noticed when you're driving down the freeway, especially if the cruise control is set. You'll see the speedometer move higher and lower, within 10 miles per hour if the speedometer cable is damaged.
If you notice your speedometer moving rapidly but the vehicle's speed is not changing, it is most likely caused by a problem with the speedometer cable and should be inspected or replaced by a certified mechanic as soon as possible.
2. Squealing noises behind dashboard
A squealing noise is never a good signal. It can be caused by loose belts, or other mechanical systems that control the operation of your vehicle. However, if you notice a squealing sound coming from behind the dashboard, this might indicate a problem with the speedometer cable. This is typically due to a speedometer cable that is going bad and sending sporadic data to the speedometer. If you hear any noise coming from the dashboard, contact a mechanic to pinpoint the precise location of the problem so it can be fixed.
3. Speedometer needle does not move
When the speedometer cable is broken, the needle on the speedometer will not move at all. If you notice this issue, it's important to contact a mechanic as soon as possible. A malfunctioning speedometer is not only a potentially bad safety issue, but it is also a traffic infraction if you're pulled over by police for speeding. Make sure to take this issue seriously to avoid any issues.
4. Check Engine Light comes on
Since the speedometer cable is electronic and sends data to the onboard computer, a problem with this unit will often trigger the Check Engine Light. This light comes on anytime an error code is registered in the vehicle. However, anytime the Check Engine Light is illuminated, it's not a good sign; which is why you should always contact a certified mechanic to diagnose the issue correctly before they repair any damage or replace mechanical parts.
It is extremely rare that a problem with the speedometer cable will ever occur while you own your vehicle; but it can happen. When the issue does happen, it's vital that you have the speedometer cable replaced by a local ASE mechanic that can arrive to your home or office to complete this service.
This article originally appeared on YourMechanic.com as Symptoms of a Bad or Failing Speedometer Cable and was authored by Timothy Charlet.