The speedometer helps vehicle owners keep within prescribed speed limits and not become a hazard to other commuters. Like all other components, it is not impervious to damage or malfunction. An illuminated CEL, faulty speed readings, and a dead gauge are telltale signs of a speedometer not working. But what causes these symptoms?
Common causes of a speedometer not working include rusted wiring and connectors, defective sensors and cables, a blown fuse, and a faulty computer system. Most of these require repair or replacement.
Across all vehicle types, several factors contribute to a non-operational speedometer. We’ll delve into these triggers in a bit — so sit back and continue reading as this guide explores the common causes of a malfunctioning speedometer and their corresponding fixes.
Types of Speedometers
Before jumping into the causes of a speedometer not working, it is fundamental for vehicle owners to be able to tell what speedometer they have and how it functions.
Cable speedometers are earlier versions of the speedometer found in older vehicle makes and models. According to Car Treatments, they use a flexible cable attached to a speedometer gear and driven by the output shaft.
The cable, in particular, runs from the transmission, through the firewall, and into the cabin. One end has a magnet spinning within a speed cup, while the other plugs into the back of the instrument cluster, where the spinning movement from the transmission is displayed as a readout on the dash.
Electronic or digital speedometers, on the other hand, are commonly found in modern vehicles and motorcycles. Instead of a gear and cable, they use a speed sensor and have a digital display. The sensor feeds rotational speed data to the ECU, which then translates the information into a ground speed readout for the instrument cluster.
Why Is My Speedometer Not Working?
Now that we have the kinds of speedometers out of the way, let’s talk about malfunction triggers that may apply to either or both:
1. Defective Speed Sensor
This minuscule device is the primary reason most speedometers stop working. It measures the transmission rotation and relays that information to the onboard computer or ECU. At any given time it becomes faulty or deteriorates, the speedometer will not function as it should since there is no input to the ECU of how fast the transmission is spinning.
Readings aside, speed sensors also help with ignition timing, fuel flow regulation, ABS, Cruise Control, and Traction Control Management Systems that come standard with the vehicle. More than resulting in erroneous speed readings, defective speed sensors can indirectly disrupt cruise control and other safety functions in a car.
2. Worn or Damaged Speedometer Cable
For a cable-operated speedometer, the condition of its cables significantly affects its operation and should be the 2nd most important item to inspect when a speedometer goes haywire.
After all, it connects the speedo and transmission and makes transmitting data on the rotational speed of driveshafts (view on Amazon) from the gearbox to the speedo unit possible. Without damage-free and functional cables, a speedometer will simply not work.
Several factors can cause the speedometer cable to bind or work sporadically over time. They include normal wear and tear, contaminant buildup, loose connections, and faulty speedometer gear. If you suspect the cables to be flawed, make sure to check these elements.
3. Corroded Electrical Connections
If it isn’t the sensors or cables at fault, start looking into electrical connections linked to your speedometer. These connections are most likely rusted — a reason for the disrupted flow of electrical signals to and from components — and in need of deep cleaning or replacement.
Corrosion of electrical connections can be caused by exposure to moisture, dirt, and debris (and several other factors). Fortunately, preventing its occurrence is relatively easy — regularly inspect and clean the connections, and coat them with dielectric grease or corrosion inhibitors for protection.
4. Faulty Wiring
During an inspection, include speed sensor wirings and those connecting the speedometer to the ECU or onboard computer.
Typically, an interrupted electrical circuit is discovered in these wirings, causing them to become damaged or disconnected. Manufacturing defects, poor installation, rodent infestation, and friction lead to the same outcome.
Whichever the case, you must address faulty wiring immediately before it can cause a signal break between components (subsequently affecting the transmission of crucial speed data). When this happens, intermittent speedometer function and incorrect readouts will be the least of your concerns, as defective sensor wirings can lead to more pressing transmission shifting issues.
5. Inappropriate Electrical Draw
Faulty wiring aside, other systems feeding on electricity can lead to a speedometer not working — especially if these electrical and mechanical components suffer from either power shortage or overload. Case in point, a fast-discharging car battery can cause your speedometer to malfunction in much the same way an inoperative voltage regulator or alternator (view on Amazon) could.
In some instances, electrical draw discrepancies may occur gradually — with sporadic issues only becoming more frequent and severe over time. If you believe this to be true, keep your senses sharp for subtle but obvious indicators like flickering instrument panel lights, a faint burning smell, or a blown fuse.
6. Compromised Instrument Cluster
The instrument cluster is the panel that houses most warning lights and all your gauges — fuel gauge, odometer, and speedometer included. It is usually constructed from plastic, glass, or more durable materials like carbon fiber.
Software issues and physical damage are often to blame for glitches in the instrument cluster. But so are faulty sensors and loose connections.
An unreliable instrument cluster can result from sensor and gauge issues that have dragged on for too long. Or it could be due to the natural wear and tear associated with frequently used, high-mileage vehicles.
A definitive way to confirm both is when multiple gauges act up synchronously (automatically eliminating issues with the speedometer cable or speed sensor).
7. Mechanical Problems
A car with a cable-actuated speedometer relies heavily on mechanical components to accurately display speed readings. It employs a cable connected to a gear that spins as the vehicle moves and rotates the needle on the speedometer dial.
If, for any reason, this gear breaks or has a teeth count incompatible with the tire size of a given vehicle, then the speedometer will likely malfunction (even if the odometer appears to be working).
8. Low Transmission Fluid
Here’s a fact for the newbie — low transmission fluid can result in a speedometer not working and the transmission not shifting. How, you ask? Some of the answers are listed below:
- Inadequate transmission fluid impairs the proper function of the transmission, causing the latter to slip, not engage properly, or shift erratically or to the wrong gear. These mishaps, in turn, result in the speedo unit displaying inaccurate readings.
- Low transmission fluid levels mean insufficient lubrication of moving parts and a higher propensity for overheating, consequently damaging the transmission, the speedometer sensor, and its components.
9. Problematic ECU
If none of the above are to blame for your speedometer not working, then it’s time to check the hub of all engine activities — the ECU.
Some owners include this in their list of priority items to check, while others save it for last. However you go about it, it is crucial to incorporate it into your troubleshooting and diagnostic process.
Personally, I would reserve this step for later since the ECU is a rarer cause of speedometer malfunction than the rest of the items in this list. This isn’t to say that you should pay it lesser attention. It’s just that before you do, ensure all other relevant components have been repaired or replaced. After all, a replacement ECU is expensive. Plus, getting it re-acclimated can be a taxing (if not frustrating) experience.
How to Fix a Malfunctioning Speedometer
The first step in resolving a speedometer not working is to determine which type — cable-driven or electronic — you have in your vehicle before identifying the root cause of the malfunction. Once these factors are established, you can select the best route to fix the problem.
The below procedure encompasses these steps and is followed by DIYers and professional mechanics alike:
1. Determine the Integrity of the Speedometer Cable
This step applies to mechanical or cable-actuated speedometers found in older vehicles. Often, non-functional speedo units of this kind trace back to a deteriorated cable.
Thankfully, the issue requires a relatively simple fix most car owners (even beginners!) can execute. Its only downside is that the cable may be hard to reach in some vehicles as it usually runs underneath, close to the transmission.
2. Inspect the Speed Sensor
If your vehicle is a more recent model, this becomes your first step. Since the functionality of your speedometer hinges on the speed sensor, it is crucial that this component is not plugged and is in good working condition. Once identified as failing, the sensor cannot be cleaned or repaired. Instead, it needs to be immediately replaced.
You may get fault codes like P0500 or P0501 with a high-spec diagnostic scan tool like Foxwell NT624 Elite OBD-II Scanner (view on Amazon) while checking the speed sensor. However, this isn’t always guaranteed.
If the sensor is only partially compromised or the wiring and connections have issues, chances are the error codes you will pull would be unclear — or you won’t generate anything at all.
3. Check Wiring and Electrical Connections
Inspecting electrical connections is a given if the speed sensor is suspected to be at fault. But even if the latter checks out, examining the wiring leading up to the grounds used by the sensor or the speed sensor itself is still necessitated.
Anything that appears corroded or damaged needs to be either cleaned or replaced. Otherwise, proceed to the next step (only if a sensor replacement has yet to suffice resolving the problem).
4. Test for Electrical Draw
If wiring and electrical connections are neither corroded nor loose, consider checking other electrical components and testing them for a potential parasitic or outlier draw. You may have to remove fuses and check on segments individually to determine the source.
Make sure you have the right tools — digital multimeter or ammeter, memory saver (view on Amazon), fuse puller, circuit breaker, and protective gear (to name a few) — for the job.
5. Switch Out Broken Mechanical Components
For cable-actuated units, this means replacing any one of the following:
- Failing speedometer cable
- Wavering speedometer needle
- Damaged speedo gear
- Speedometer head with an inoperative magnet or metal disc
- Calibration unit and variable resistor
- The unit’s gear-driven mechanism
Mechanically-inclined car owners are usually able to pull this off. You should be all set if you belong to this group.
6. Refill the Transmission Fluid
Granted fluid level is just below the requirement, filling the reservoir with an appropriate amount of new transmission fluid will do the trick. This works, too, for fluid that appears slightly dirty when inspected.
But if you are experiencing speedometer malfunction and shifting problems synchronously, a transmission flush would be necessary to remove any debris potentially causing or aggravating the problem.
7. Closely Examine the Instrument Cluster and ECU
I kept these components for last because they are the toughest to address (not to mention the costliest). Replacing the instrument cluster isn’t necessarily as arduous but is equally pricey.
Meanwhile, installing a new ECU is both complex and expensive and is best left in the hands of experts. Resetting the affected ECU can be considered a fix (albeit a temporary one).
(TIP: Rotate the ignition key to the ‘auxiliary’ position after installing the new ECU — doing so will help the unit kick off the operation and start acclimating to other engine systems once your machine is on.)
In performing this process, remember to always revert to your owner’s manual for OEM-specific guidance. Clear persistent fault codes (if any). And finally, test drive your vehicle to ensure the speedometer and all other vehicle systems are working properly.
Speedometer Repair Cost
Repair costs are ultimately contingent on what component you found faulty and the degree of repair you have to do. For instance, a DIY speedometer cable replacement costs approximately $20—$245, and a full instrument cluster replacement falls between $500—$2,000.
Note that these values may increase by as much as $200—$300 if you outsource the task to a local repair shop. The extra cost could be more, depending on the vehicle make and model.
If you are in a bind, visiting an auto salvage yard may yield substantially lower prices. But depending on where you are based, this option may not be readily available to you.
Can You Still Drive if the Speedometer Doesn’t Work?
Technically, you still can. However, it is ill-advised to do so for several important reasons, as follows:
Some states and countries require a working speedometer for road-going vehicles. You will likely incur fines if caught driving a car or motorcycle with a dysfunctional or non-working speedometer.
For configurations where the speedometer and odometer are connected, the other is bound to fail if the speedometer develops a fault. Not only does this result in flawed speed readings but also in inaccurate mileage records on the affected vehicle. You may think it immaterial to the vehicle’s drivability. However, it will adversely impact its resaleability.
The inability to determine your driving speed in real time is a recipe for disaster. The least that could happen is getting a speeding ticket. Meanwhile, the worst-case scenario is an on-road accident that could have been easily averted had a professional mechanic checked the speedometer not working.
Conclusion – Reasons Behind Speedometer Not Working
To recap, below are the causes and fixes for a speedometer not working:
Why Is My Speedometer Not Working?
- Defective Speed Sensor
- Worn or Damaged Speedometer Cable
- Corroded Electrical Connections
- Faulty Wiring
- Inappropriate Electrical Draw
- Compromised Instrument Cluster
- Mechanical Problems
- Low Transmission Fluid
- Problematic ECU
How to Fix a Malfunctioning Speedometer:
- Determine the integrity of the speedometer cable.
- Inspect the speed sensor.
- Check wiring and electrical connections.
- Test for electrical draw.
- Switch out broken mechanical components.
- Refill the transmission fluid.
- Closely examine the instrument cluster and ECU.
If you currently have a speedometer that’s not working, lucky you — this is your chance to put this guide and your skills to the test! Otherwise, seek assistance from a reputable auto shop or mechanic if your speedometer continues to malfunction or to work intermittently.
Kris is an avid off-roader and outdoor enthusiast who loves to brave the elements and take on challenging terrain. He also enjoys sharing his passion and knowledge with others so that they, too, can appreciate the ride.