Transmission sensors are critical electronic components that ensure your vehicle shifts smoothly and operates efficiently. When these sensors malfunction, you may experience harsh shifting, cruise control failure, or an illuminated check engine light. Understanding how to diagnose and fix transmission sensor issues can save you from costly repairs and keep your vehicle running reliably.
Understanding Transmission Sensors and Their Function
Transmission sensors monitor and control critical functions, allowing for smooth and accurate gear shifting. Transmission speed sensors are used to calculate the actual gear ratio of the transmission while in use. There are generally two speed sensors that work in conjunction to provide accurate transmission data to the vehicle’s powertrain control module.
The first is known as the input shaft speed (ISS) sensor. As described, this sensor is used to monitor the speed of the transmission’s input shaft. The other sensor is the output shaft speed (OSS) sensor. After registering data, the two transmission speed sensors, also commonly referred to as vehicle speed sensors (VSS), send data to the powertrain control module (PCM), which compares these two inputs and calculates which gear the transmission should engage for efficient driving.
Key Types of Transmission Sensors
Modern vehicles rely on several types of transmission sensors, each serving a specific purpose:
Transmission Range Sensor (TRS): Also known as the neutral safety switch, it detects the gear position and ensures the correct gear is engaged. Turbine Speed Sensor: Monitors the rotational speed of the torque converter or input shaft to regulate shifting. Additionally, Transmission Fluid Temperature Sensor: Monitors the temperature of the transmission fluid to ensure it is within the optimal range. Vehicle Speed Sensor (VSS): Measures the speed of the vehicle and helps control shifting patterns.
Common Symptoms of Faulty Transmission Sensors
Recognizing the warning signs of transmission sensor failure is essential for timely intervention. Here are the most common symptoms:
Harsh or Erratic Shifting
The first symptom of a bad transmission speed sensor is harsh or improper shifting. Without a valid speed signal from these sensors, the PCM will not be able to correctly control the shifting of gears within the transmission. The transmission can display symptoms such as delayed shifts, harsh shifts, and limited gear operation if the VSS is malfunctioning.
An automatic transmission is hydraulically controlled and designed to shift smoothly. When the transmission shifts hard, it can damage internal components including valve bodies, hydraulic lines, and in some cases mechanical gears.
Cruise Control Malfunction
Another symptom of a bad speed sensor is when the cruise control stops working. Since the transmission speed sensors monitor the input and output shaft speed, it also plays a part in monitoring the cruise control. As a safety precaution, the ECU will shut down the cruise control and render it inactive.
Check Engine Light Illumination
If the signals are lost from the transmission speed sensors, the PCM will set a DTC and illuminate the Check Engine Light on the vehicle dashboard. If the signals are lost from the transmission speed sensors, the PCM will set a DTC and illuminate the Check Engine Light on the vehicle dashboard.
Erratic Speedometer Readings
The erratic or inaccurate speedometer reading is one of the common signs of a bad speed sensor. If the reading of your speedometer is constantly going up and down, then there is a chance of a failed vehicle speed sensor.
Reduced Fuel Economy
A notable yet often overlooked indicator of speed sensor complications is diminished fuel efficiency. Top Flight Automotive highlights that incorrect readings from a malfunctioning speed sensor can increase fuel consumption. This inefficiency stems from the vehicle’s computer system relying on the speed sensor to manage engine timing and fuel utilization.
Diagnosing Transmission Sensor Problems
Proper diagnosis is the first step toward effective repair. Here’s how to identify transmission sensor issues:
Using an OBD-II Scanner
An OBD-II scanner is an essential diagnostic tool that reads trouble codes from your vehicle’s computer system. Sensor failures can trigger error codes like P0715 or P0720 to indicate input or output speed sensor issues. The NT510 Elite provides detailed information regarding these codes, helping users diagnose and repair them efficiently.
P0700: Transmission Control System Malfunction – This code indicates a general transmission problem, necessitating further diagnostics. P0715: Input/Turbine Speed Sensor Circuit Malfunction The transmission’s input speed sensor signals the TCM to enable it to shift the transmission to the proper gear needed for changes in speed. P0720: Output Speed Sensor Circuit Malfunction This code is generated when the PCM does not receive an adequate signal from the output speed sensor circuit or if there is an abnormal RPM difference between the input RPM sensor and the output speed RPM sensor.
Physical Inspection
Beyond scanning for codes, a visual inspection can reveal important clues. Check the sensor wiring for damage, corrosion, or loose connections. Check the connector wiring to insure there are no breaks or exposed wire. It is best to replace a broken connector but not absolutely necessary as long as the connector fits tightly onto the sensor.
Testing with a Multimeter
Turn the key to the ignition on, but engine off, position (one key click past Accessory) Using the multimeter’s voltmeter connect the probes to the ground and reference wires in the connector. If no voltage is received, the sensor is faulty. If voltage is received, you may need to have the car diagnosed by a qualified tech.
Common Causes of Transmission Sensor Failure
Understanding what causes sensor failure can help you prevent future problems:
Wear and Tear
Over time, sensors can degrade due to constant use and exposure to harsh conditions. The lifetime of a transmission speed sensor can be up to 120,000 miles, but can fail at any time because of wiring issues, or simply from wear and tear if you drive your vehicle a lot.
Electrical Issues
Faulty wiring, loose connections, or electrical surges can damage sensors. Electrical Connection Problems: Faulty wiring, corroded terminals, or damaged connectors can disrupt the signal between the sensor and the control module, leading to inaccurate or lost data transmission.
Contamination
Dirt, debris, and transmission fluid contaminants can interfere with sensor operation. The magnetic probe of a sensor of this type can easily become covered in sludge if a vehicle’s transmission fluid is neglected.
Extreme Temperatures
High or low temperatures can affect sensor accuracy and functionality. Excessive Heat or Vibration: Continuous exposure to high temperatures and vibrations from the drivetrain can degrade the internal components of the sensor, reducing its efficiency or causing complete failure.
Physical Damage
The transmission speed sensors may sustain damage due to debris, road hazards, or improper handling during maintenance, compromising theeir ability to function properly.
Step-by-Step Guide to Replacing Transmission Speed Sensors
Replacing a faulty transmission speed sensor is a manageable repair for many vehicle owners. Here’s a comprehensive guide:
Tools and Materials Needed
- Socket wrench set (typically 10mm)
- Flat-tip screwdriver
- Drip pan
- Replacement transmission speed sensor
- Transmission fluid (approximately 1 quart)
- Jack and jack stands
- Safety glasses and gloves
- OBD-II scanner
Safety Precautions
Before you start any repairs on your car, it’s important to take safety precautions. This includes wearing protective gear such as gloves and safety glasses, as well as securing your vehicle on a level surface with the parking brake engaged. Additionally, disconnect the negative battery cable to avoid any electrical shocks or accidental activation of the airbag system.
Locating the Sensor
The transmission speed sensor is located in the rear of a longitudinal mount transmission. For transaxles, the speed sensor is located on the long side of the output shaft. This would be the end of the transaxle that faces the front of the engine.
Removal Process
Slide under the vehicle and locate the transmission speed sensor. Place a drip pan below the transmission. Get a drip pan and place it underneath the transmission output shaft housing where the speed sensor resides.
Remove the mounting bolts. Remove the mounting bolts to the speed sensor. Unplug the harness from the sensor. Use a flat tip screwdriver and pop out the sensor. Keep in mind that about one quart of transmission fluid will leak out when you remove it.
Installation of New Sensor
Use a rag and clean out the threads of the sensor housing. Set the new vehicle speed sensor into the threaded area and make sure it is straight. Start to tighten it by hand. Once it gets snug, use the wrench again to tighten it a few more turns.
CAUTION: When replacing the new speed sensor be sure the O-ring on the new sensor is correctly fitted on the sensor before installation. Also, I’d recommend you apply a dab of ATF to the o-ring before inserting it into the trans so it slides and does not tear the O-ring.
Use the rag to wipe off the end of the electrical cord so that a good connection will be made with the sensor. Press it onto the sensor until it snaps in place.
Post-Installation Steps
Any fluid captured during the removal process should be put back into the transmission before running the vehicle. After replacing the vehicle speed sensor, clear any DTCs from ECM memory, then test drive the vehicle. First, make a short run around the parking lot or just a short distance, and check for leaks. Then, on a longer test drive, make sure the check engine light doesn’t come back on and speed-related systems are working properly again.
Cost Considerations for Transmission Sensor Replacement
You can take your car to the repair shop or dealership and spend between $100 and $250 or more to have one speed sensor replaced or you can replace the sensor yourself for a fraction of the cost. The DIY cost for replacing a speed sensor is the cost of the sensor. Depending on the vehicle year, make and model, a speed sensor can cost as little as $7.92 or as much as $100. For most vehicles, the cost for a speed sensor is between $25 and $45.
The amount you’ll spend on transmission speed sensor replacement often varies from one model of vehicle to the next, due to discrepancies in parts and labor costs. Fortunately, in most cases, this repair does not tend to be exceedingly expensive, and can often be completed in less than two hours.
Preventative Maintenance to Avoid Sensor Failures
Regular maintenance is the best defense against transmission sensor problems. Here are essential preventative measures:
Regular Transmission Fluid Changes
One of the most critical maintenance activities is changing the transmission fluid according to the manufacturer’s recommended schedule. Transmission fluid acts as both a lubricant and a coolant. Without regular changes, the fluid can degrade, becoming contaminated with metal shavings and debris that can harm the transmission’s internal components.
Routine Inspections
Regular Maintenance: Routine maintenance, including fluid changes and inspections, can help prevent sensor-related issues. Have your transmission system inspected regularly to catch potential sensor issues early. Keep the Transmission Clean: Ensure the transmission fluid is clean and at the correct level to avoid contamination that can affect sensors.
Avoid Extreme Driving Conditions
Try to avoid driving in extreme temperatures or conditions that can put additional strain on the sensors. This includes avoiding excessive towing beyond your vehicle’s capacity and minimizing stop-and-go traffic when possible.
Address Warning Signs Promptly
When this sensor malfunctions, it can lead to erratic shifting, dashboard errors, and potentially severe transmission damage if ignored. Timely diagnosis and repair can prevent costly long-term transmission problems and ensure a much smoother drive.
When to Seek Professional Help
While many transmission sensor replacements can be DIY projects, certain situations require professional expertise:
Diagnosing a faulty transmission speed sensor is pretty straightforward, as is replacing it, but to diagnose a problem fully, you’ll need to be comfortable using a multimeter to check voltage. If not, take your car to a qualified mechanic for a proper once-over.
Repairing a faulty transmission range sensor is usually straightforward, but in some cases, it may need to be replaced entirely. Skilled mechanics play a crucial role in fixing cars by diagnosing and addressing transmission range sensor issues effectively. A damaged spline or bad o-ring is easier to replace than the whole sensor, but older models may require a new transmission range sensor if replacement parts are hard to find.
Internal transmission range sensors require more extensive work. To replace an internal TRS, you need to essentially rebuild the transmission. An external TRS, on the other hand, is accessible through the hood of the car, next to the transmission shift linkage, sometimes behind the coolant line near the battery of the car.
Understanding the Consequences of Ignoring Sensor Problems
Delaying transmission sensor repairs can lead to serious complications:
When these sensors fail, it can lead to erratic shifting, reduced performance, and potential transmission damage. This may cause the transmission to shift roughly, slip, or take longer than normal to engage the next gear. Over time, these harsh shifts can damage internal transmission components, such as hydraulic lines, valve bodies, or even mechanical gears, further escalating repair costs.
Increased Wear and Tear: Erratic shifting can cause additional stress on transmission components, leading to premature wear. Transmission Damage: Prolonged operation with a faulty sensor can result in significant damage to the transmission. Reduced Performance: A malfunctioning sensor can negatively impact overall vehicle performance and drivability. Expensive Repairs: Ignoring sensor issues can lead to more severe problems that require costly repairs or even a full transmission replacement.
Advanced Diagnostic Techniques
For more complex sensor issues, advanced diagnostic methods may be necessary:
Live Data Monitoring
Modern OBD-II scanners can display live data streams from transmission sensors, allowing you to observe sensor behavior in real-time. This can help identify intermittent problems that don’t always trigger fault codes.
Resistance Testing
Might want to measure the two terminals for: A) open, infinite ohms B) short, zero ohms C) Resistance from either lead to body = infinite ohms <unlikely unless catrostrphic failure IIRC it doesn't list the actual ohms but similar sensors read 560 to 680 ohms, but don't quote me on this.
Oscilloscope Testing
The final check is by means of oscilloscope output expecting a square wave. This goes into another circuit for clean up before hitting the ECM.
Transmission Sensor Maintenance Best Practices
To maximize the lifespan of your transmission sensors:
- Follow manufacturer-recommended service intervals for transmission fluid changes
- Use only the specified transmission fluid type for your vehicle
- Inspect sensor wiring during routine maintenance
- Keep the transmission area clean and free from debris
- Address fluid leaks immediately to prevent sensor contamination
- Avoid aggressive driving habits that stress the transmission
- Monitor transmission temperature, especially during towing or heavy loads
Regular maintenance includes checking sensor connections during transmission service to prevent failures.
Conclusion
Transmission sensors play a vital role in the smooth and efficient operation of your vehicle’s transmission. When these sensors fail, it can lead to erratic shifting, reduced performance, and potential transmission damage. By understanding the signs of sensor failure and taking proactive steps to address issues, you can maintain the health of your transmission and avoid costly repairs.
Whether you choose to replace transmission sensors yourself or seek professional assistance, prompt action is essential. Regular maintenance, careful monitoring of symptoms, and proper diagnostic procedures will ensure your transmission continues to operate smoothly for years to come. Remember that a small investment in sensor replacement today can prevent thousands of dollars in transmission rebuild costs tomorrow.
For more information on transmission maintenance and repair, consult resources from reputable automotive organizations such as the Automatic Transmission Rebuilders Association (ATRA) at https://www.atra.com, the National Institute for Automotive Service Excellence (ASE) at https://www.ase.com, and manufacturer-specific service manuals available through Alldata at https://www.alldata.com.