Repair instructions
What are the Symptoms of a Bad Speed Sensor?
Summary:
A bad vehicle speed sensor (VSS) isn’t just a minor inconvenience it can cause real trouble for your car’s transmission, cruise control, and overall drivability. The VSS plays a critical role in helping your vehicle’s computer systems determine how fast the car is moving and when to shift gears. When this sensor fails, you may notice everything from erratic speedometer readings to harsh transmission shifts. Here’s what you need to know about the symptoms of a bad speed sensor, how it affects your vehicle, and when to seek professional help.
What Does the Speed Sensor Do?
Your vehicle’s speed sensor monitors how fast the transmission output shaft or sometimes the differential or individual wheels is rotating. It sends this data to your car’s powertrain control module (PCM) or transmission control module (TCM), which uses the information to control shifting, fuel delivery, and cruise control functions.
Older systems used analog signals, while modern vehicles typically use digital signals in the form of square wave pulses. No matter the system, one thing’s for sure: when the VSS fails, it can throw a wrench into your entire drivetrain operation.
Top 5 Symptoms of a Failing Speed Sensor
1. Transmission Problems
The most common symptom of a bad VSS is transmission trouble. Because your vehicle relies on speed data to determine shift timing, a faulty sensor can cause:
- Harsh or delayed shifting
- Limited gear operation
- Limp mode activation in some models
The torque converter clutch may also fail to engage, reducing fuel efficiency and causing your transmission to overheat.
2. Erratic or Non-Functional Speedometer
If your speedometer suddenly starts jumping, reading incorrectly, or stops working altogether, a bad VSS could be to blame. Since many modern cars use this sensor as the primary input for the speedometer, you’ll want to get it checked out quickly to avoid unsafe driving conditions.
3. Cruise Control Not Working
Cruise control systems rely on accurate speed readings. If your PCM stops receiving data from the speed sensor, it will likely disable the cruise control system altogether for safety reasons.
4. Check Engine Light or Other Warning Lights
A malfunctioning VSS often triggers the check engine light. In some cases, you might also see ABS or traction control warning lights on your dash. To confirm the issue, we use an OBD-II scanner to check for related diagnostic trouble codes like P0720 (Output Speed Sensor Circuit Malfunction).
5. Poor Fuel Economy and Performance Issues
Without accurate speed readings, the PCM can’t properly regulate shift points or torque converter lockup, which can lead to poor fuel mileage, reduced power delivery, and increased wear on the transmission system.
What Causes Speed Sensor Failure?
– Internal Electrical Faults:
Over time, the internal electronics inside the sensor may degrade, resulting in irregular or no signal output. These can be diagnosed using advanced tools like digital multimeters or oscilloscopes.
– Physical Damage or Deterioration:
The plastic housing or magnetic pickup of the sensor can become brittle, cracked, or worn out—especially after prolonged exposure to heat and moisture.
– Dirty or Contaminated Sensors:
Neglecting regular transmission fluid changes can cause debris and contaminants to build up around the sensor or toothed wheel. This prevents the sensor from picking up accurate signals.
Can You Drive with a Bad Speed Sensor?
Technically, yes but we don’t recommend it.
A faulty speed sensor can cause erratic shifting, incorrect speed readings, and prevent systems like cruise control from functioning. More importantly, it can lead to internal transmission damage if left unaddressed. Getting the sensor diagnosed and replaced early is far less expensive than dealing with a damaged transmission down the line.
Final Thoughts
If you’re experiencing any of these symptoms, don’t ignore them. Whether you’re driving a Jeep Cherokee, a Chevy Silverado, or any other make or model, speed sensor issues can cause serious drivability and safety concerns.
