1998 Mercury Grand Marquis Vehicle Speed Sensor Replacement Cost

In order for systems like anti lock brakes, traction control, and the speedometer to function properly, the ABS control module and Powertrain control module must know the speed the wheels should be spinning, as well as the speed the wheels are actually spinning. To create the data the computer needs for the speed the vehicle should be spinning, the transmission output shaft speed, for most vehicles, is measured by the vehicle speed sensor. The vehicle speed sensor will detect the speed of the output shaft many times per second, and send that data to the necessary control units for further use.

As the output shaft for the transmission turns, a ring on the output shaft spins very closely to the vehicle speed sensor, allowing the sensor to read how fast the ring is spinning by features on the ring. The speed of the output shaft, then, is reported back to various control modules by a digital or analog signal, whichever the control unit is designed to interpret. This is used to show the vehicle speed on the speedometer, but also has applications for the anti-lock brake system and traction control system since the information can be compared to wheel and engine speed.

When the vehicle speed sensor fails or the wiring harness prevents signal transmission, the speedometer is the first thing that will stop functioning properly. This can be an intermittent issue where the speedometer only works occasionally, the speedometer may cease all function, or the speedometer may read erratically or incorrectly. Also, the traction control and anti lock brake warning lights specific to each vehicle may be illuminated, depending on the design of those systems. You may notice the vehicle hesitate to accelerate, cruise control will not function properly, and the vehicle may not shift properly through all of the gears.

Testing and replacement of the speed sensor should be done as soon as possible, but in the case that the speedometer is the only thing affected, normally older vehicles, the vehicle can be driven until the sensor is tested and replaced. It should be mentioned that driving without a speedometer is illegal in most locations, and if the anti lock brakes and traction control are disabled, the vehicle will be more prone to sliding under heavy braking or acceleration.

The vehicles that will experience the highest rate of failure for the vehicle speed sensor are those that are left unmaintained. As transmission fluid ages and becomes contaminated, those contaminates will become stuck to the speed sensor or ring, and prevent accurate readings. If the powertrain control module cannot read the vehicle speed, even though the sensor is working properly, the result will be the same as if the sensor had failed. Generally, a vehicle speed sensor should last well over 100,000 miles, and the average replacement frequency is closer to 150,000 miles, with exceptions. To prolong the life of your vehicle speed sensor, as well as all other components in your transmission, ensure you are maintaining the transmission in accordance with the owners manual service and maintenance section.

Diagnosing a failed speed sensor will begin with reading diagnostic trouble codes, checking the fuses for the circuit, and testing the circuit for presence and accuracy of battery voltage, signal voltage, and ground. If the circuit tests ok, there are a few safe ways to test the speed sensor, all of them involving a computerized diagnostic scan tool. It is not recommended to test the speed sensor. If the speed sensor is found to read incorrectly, a transmission flush may be recommended, but if the speed sensor is not functioning at all, it will need to be replaced.

Replacing a vehicle speed sensor is straight-forward, simple to do, and typically requires removing one or two fasteners, and one electrical connection. Once those are disconnected, the speed sensor should easily slide out of the exterior of the transmission housing. The electrical connector for the vehicle speed sensor harness is then cleaned, inspected, and attached to the new sensor once fastened in place.

RepairPal recommends that testing is completed prior to replacing the vehicle speed sensor. Though it is likely that the vehicle speed sensor is at fault when the speedometer stops working, it is not guaranteed, and the cost and frustration of replacing a good speed sensor can be avoided easily.

The battery should always be disconnected when replacing electrical components. You will need the battery connected to test the circuit, but leaving the battery connected when it is unnecessary can or will result in damage to the vehicle, or possibly electric shock.

Testing the vehicle speed sensor circuit is typically something anyone with a multimeter can accomplish, but testing the vehicle speed sensor safely will likely require professional scan tools to measure the signal from the vehicle speed sensor at different vehicle speeds. If the vehicle speed sensor is diagnosed as faulty, changing the sensor can be done by most DIYers once the sensor location is found.