Author: Jason
What exactly is a car speed sensor?
A speed sensor is a small electrical device that translates some type of signal from the wheel or transmission gear directly to the speedometer. The signal can also go to the ECU or engine computer and converted to a different signal for the speedometer to use.
What does the speed sensor do on a vehicle?
The speed sensor on a vehicle usually is comprised of a magnetic or metal pickup that detects another metal object spinning. Commonly speed sensors are bolt to the transmission. Sometimes there are more than one depending on the vehicle. The speed sensor will typically make your speedometer needle move and also enable your odometer to accumulate mileage.
How do speed sensors work?
The speed sensor uses electronics and magnetism to detect when the vehicle begins to move. It is precisely calibrated so that it can even tell you the exact (estimated) distance the car moves over a known distance (MPH or KPH). The most common type of speed sensor is bolted to the transmission or transfer case of a truck or 4×4 vehicle. There is either a mechanical connection so the speed sensor has a plastic gear on it and is mated to a gear inside the transmission or an electronic pulse. In the case of a plastic gear, the gear spins consecutively with the transmission gear and once it spins so many times it will in turn spin the speedometer to an accurate value. Many people think the speedometer is accurate and that’s not always true. They will also assume a GPS is an effective way of testing a speedometer. Yes and no. GPS is using a satellite to track the vehicle position and distance against time. Speed sensors don’t really know how far a car is moving, only how fast the parts are spinning. These are two very different ways of determining speed. Speedometers are often calibrated to read slightly higher than your normal speed as a type of error offset. If you change the size of your tires, wheels, final drive ratios, and other gear related modifications that in turn can change your speedometer accuracy by 10-20% or more.
What other types of speed sensors are there for vehicles?
Aside from the transmission mounted gear-type speed sensor or electronic signal, speed sensors can also be mounted at the wheel hubs. In this fashion you will generally have four speed sensors. This style is common with vehicles that have ABS and other vehicle stability features. The ECU needs to know how fast each WHEEL is spinning, not the car as a whole. With this data the ECU can determine if a certain wheel is spinning too fast or too slow and assume you are losing traction. If one speed sensor goes bad in this setup your speedometer value could be cut in half, 1/4, or not work at all depending on the vehicle brand and model. The problem with these types of speed sensors is that over time and mileage the hubs become very rusty and corroded often times seizing the small plastic speed sensors in place. This makes removal quite difficult. These speed sensors are also often much more expensive, sometimes $150 to $200 or more per wheel sensor.
Speed sensor problems and solutions summary
There are quite a few factors that determine if your speedometer display or MPH reading shows up on your gauge cluster. The best course of action is to use diagnostic tools or scan tools to determine if your speed sensors are actually functioning. If you already replaced your entire gauge cluster and the speedometer or odometer still isn’t moving it’s a clear sign that you have a speed sensor problem. If your odometer mileage is accumulating, bur your speedometer needle is not moving then your cluster is probably receiving a good speed signal. This is where we can help and either repair or replace your gauge cluster.
If all else fails contact Tanin Auto Electronix for instrument cluster repair or instrument cluster replacement.