Vehicle Speed SensorA / B Correlation
The PCM has detected a vehicle speed sensor reading between the A and B speed sensors. Vehicle speed sensors A and B are sometimes referred to as input and output speed sensors. Usually these data readings do not coincide with the wheel speed sensor readings of the ABS system. This malfunction could include a higher than normal reading, lower reading, or an erratic reading.
Code Set Parameters
Variations in vehicle speed and wheel speed that exceed 3 or 4 mph for a predetermined period of time (usually 5-seconds) will cause a trouble code to be stored and a malfunction indicator lamp to be illuminated.
Symptoms typically include speedometer and odometer malfunctions, erratic or harsh automatic transmission shift patterns (limp-in mode), the tachometer may behave erratically, the antilock brake or brake lamp may be illuminated, ABS braking may be deactivated, the vehicle rev limiter could operate with a lower threshold (limit RPMs to a lower figure than normal), a stored trouble code, and an illuminated service engine soon lamp.
The most common cause of this malfunction is related to a faulty vehicle speed sensor. Other causes may include open or shorted wiring or electrical connectors in the vehicle speed sensor circuit, aftermarket or non-OEM size tires and wheels that the PCM is not calibrated to accommodate, faulty hub bearings, a bad reluctor ring, or a faulty PCM (PCM failure is rare).
Antilock brake sensors are frequently replaced in error when attempting to diagnose a vehicle speed sensor code. Follow manufacturer’s guidelines to ensure that you are testing the correct sensor before replacing any components. If antilock brake codes are present, the related condition could be a contributing factor in this vehicle speed sensor code. Diagnose codes in the order in which they are stored. If this code is not the first code stored, suspect that it is a reaction to the conditions of another code being stored. While vehicle speed sensor replacement repairs this condition most of the time, a thorough diagnosis must be performed before condemning any component.
- Late model OBD-II systems require the vehicle speed sensor to perform many different duties
- Of course, its primary purpose is to operate the speedometer and odometer, for driver convenience and vehicle maintenance functions
- It is also used in modern variable assist power steering systems
- Vehicle speed is used to help the PCM regulate the degree of power assist applied and provide a seamless maneuvering experience at different speeds
- Vehicle speed is used by the antilock braking system for comparison purposes but the data is no less critical to operator safety then antilock brakes themselves
- High-end and high performance vehicles use vehicle speed to regulate vehicle ride height and to activate automated aerodynamic and downforce components
- Cruise controls systems, engine drivability settings, and transmission shift strategy are all affected by vehicle speed
- If this sounds complicated - it is - but you may be able to use it in your favor
- For instance, if your cruise control stopped working about the same time that your speedometer starting bouncing around like crazy and this code was stored, you probably have a faulty vehicle speed sensor.
Vehicle speed sensors are normally of the fixed electro-magnetic variety
- Some models are equipped with multiple vehicle speed sensors but many use one sensor to provide inputs to multiple control modules
- The vehicle speed sensor is usually located near the transmission output shaft, however some are in the rear differential and others use wheel speed sensors
- A steel or aluminum ring, known as a reluctor, passes in very close proximity to the sensor and the teeth on the ring interrupt the voltage signal emitted by the electro-magnetic sensor
- These interruptions are read by the PCM as waveforms of voltage and these voltage waveforms are translated into mph.
Several tools will be instrumental in successfully diagnosing this code
- A suitable OBD-II scanner (or code reader), a digital volt/ohmmeter, an oscilloscope will be most helpful in performing a successful diagnosis
Begin with a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by clearing the code and operating the vehicle to see if it returns
- This will help to determine whether or not the malfunction is intermittent
After the codes are cleared, test drive the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
Connect the scanner and observe vehicle speed data as the vehicle is driven
- If vehicle speed data fails to increase with vehicle speed, is erratic, or seems incorrect, suspect a vehicle speed sensor malfunction
- Begin by making a visual inspection of system wiring and connectors
- If no abnormalities are detected, then remove the vehicle speed sensor and check for the presence of metal particles from the transmission, differential, or hub bearing (use caution when removing the sensor as some models may leak fluid after it is removed)
- Remove metal shavings or particles as required
- If the sensor is cracked or worn, it should be replaced at this time
- Re-install the vehicle speed sensor, reset the code, and test drive the vehicle
- If the code returns, continue with the diagnostic procedure
- If the code does not return, then you may have repaired the condition by removing the metal fragments from the sensor
- The code setting condition may also be intermittent.
If the code returns, check voltage and ground at the vehicle speed sensor and compare your findings with the manufacturer’s specifications
- If no voltage and ground signal are present at the sensor, disconnect the sensor and check the sensor resistance level
- Compare your findings with the manufacturer’s specifications
- If the readings fail to coincide, replace the sensor, reset the code, and test drive the vehicle
- If no voltage and/or ground signal are detected at the vehicle speed sensor, then disconnect the PCM connector and perform a resistance test on the system circuitry
- Repair open or shorted circuits as required
- If your findings coincide with manufacturer’s specifications, use the oscilloscope to observe the voltage wave patterns as they are being input to the PCM
- Inconsistencies or abnormalities in the voltage wave form pattern may indicate a broken or damaged reluctor ring
- You may attempt to inspect the reluctor ring by removing the vehicle speed sensor and using the opening as an access hole