Rough Road Sensor A Signal Circuit
The PCM has detected a variation in crankshaft position that it contributes to a rough or uneven driving condition, a code of this type may be stored. “Sensor A” indicates an area of the rough road sensor system circuitry, rather than a particular component.
Code Set Parameters
When the PCM detects an unexpected crankshaft position fluctuation while driving on rough or uneven surfaces, and there are no relayed antilock brake or traction control codes present, a code of this type may be stored and the service engine soon lamp could be illuminated. Some models will require multiple failure cycles in order for a service engine soon lamp to be illuminated.
Symptoms may include an engine misfire or hesitation accompanied by a stored trouble code and possibly an illuminated malfunction indicator lamp. Traction control and/or antilock brake systems may also be affected by the conditions which contribute to this code being stored.
Common causes include missing, disconnected, or defective rough road sensors and/or other rough road detection hardware.
Techs report that common misdiagnosis include unnecessary replacement of wheel speed sensors, camshaft, and crankshaft position sensors.
- Modern OBD-II systems utilize rough road condition sensors to offset small variations in crankshaft and camshaft position caused by rough or uneven driving surfaces
- These sensors are typically mounted near the wheels and can range from a single sensor to four sensors
- The sensors are usually of the variable resistance variety and work in conjunction with a stationary rheostat and a fulcrum arm
- One end of the arm is attached to the sensor and the other is attached to the lower control arm or some other component that is reactive to road conditions
- As road conditions change, the fulcrum arm causes a integrated contact to move up or down a circuit of resistors arranged in a curved line
- As the individual resistors are contacted, circuit resistance changes causing voltage reading at the control module to change
- These variations in sensor voltage are translated by the control module as bumps in the road and relayed to the PCM via the communication bus
- If the PCM detects a variation in crankshaft or camshaft position, or an isolated engine misfire, without an input from the respective control module that indicates a rough road condition is present, a code may be stored and a service engine soon lamp illuminated
- Should the PCM detect a misfire or sudden crankshaft/camshaft inconsistency, with an input from a respective controller that indicates a rough road condition is present, the misfire or cam/crank position variation may be overlooked.
A scanner, or code reader, and a digital volt/ohmmeter will be instrumental in a successful diagnosis
- An oscilloscope can also be helpful in certain cases
Begin your diagnosis with a thorough visual inspection of all rough road sensors, wiring harnesses, electrical connectors, and hardware
- Replace or repair defective components, damaged, or disconnected wiring or connectors as required
If the wiring harnesses, connectors, and hardware of the rough road sensor system are in order, connect the scanner or code reader and record all stored trouble codes and freeze frame data
- Diagnose and repair codes in the order in which they were stored
- After recording trouble codes, clear the codes and test drive the vehicle to see if any codes return
- If no codes return, you have an intermittent condition and it can be very difficult to diagnose such a problem
- You may need to allow the condition to worsen in order to successfully diagnose this condition.
If the code is reset, begin with a test of the rough road sensor connectors
- Inspect connector faces for dirt, debris, and corrosion and repair or replace components, wiring, and connectors as necessary
- Typical voltage readings at the sensor connectors will be a 5-volt reference, a ground, and a signal wire but check manufacturer’s wiring diagrams and specifications for exact model information
- Using the digital volt/ohmmeter, test the voltage and ground signals at the sensor connector
- If both the voltage and ground signals are present, reconnect the sensor and test the signal wire at the connector while simulating a rough road condition
- If the sensor has power and ground, but provides no signal voltage or the signal voltage fails to change with changing conditions, replace the sensor
- Use the digital volt/ohmmeter to test the sensor using the manufacturer’s specified reference voltage throughout varying rough road conditions
- If the sensor fails to comply with manufacturer’s specifications, replace the sensor and retest the system for proper operation.
If the sensor/s and system circuitry are intact and check out, disconnect the wiring harness from the respective controller and check for system continuity and resistance on all circuits
- Repair or replace circuitry as required.
If all of the items listed above pass the test according to manufacturer’s specs, suspect a faulty control module, The antilock brake control module and the traction control module are chief among the controllers which interact with the PCM in completing this system circuitry.
The problem could also be located in the controller are network
- CAN is the abbreviation for “controller area network.” The CAN represents a communication bus that allows multiple microcontrollers to communicate with one another without the need for a host computer
- It is a message based protocol originally designed for automotive use
- The CAN bus network is actually a complex conglomeration of wiring harnesses and connectors used as a pipeline of information shared between two or more automotive control modules
- These controllers control virtually every electrical function of the vehicle, with the PCM being the primary controller
- Control modules receive input data from various sensors and emit output signals to system components and other control modules
- For example: The PCM receives an input signal from the vehicle speed sensor
- This signal is systematically forwarded to affected control modules that utilize the data to accomplish various other tasks
- The cruise control module uses the data for speed control purposes, the traction control system uses the data to regulate vehicle traction control strategy, and the anti lock brake controller compares vehicle speed from the PCM with data inputs from individual wheel speed sensors to operate the anti lock braking system.
Unlike other diagnostic codes, this type of code can sometimes be best left to a professional simply because of the bulk of circuitry involved
- An experienced technician with a specialized scanner (Autohex or Tech II) may be able to determine the general area of the malfunction much more rapidly and easily than someone using a code reader and a digital volt ohmmeter
- Disconnecting and testing every single pin of the CAN bus could prove to be extremely time and cost prohibitive
- Additionally, some type of memory saving device must be installed, lest the PCM and other controllers lose their memory and require reprogramming
- A specialized diagnostic CAN scanner will show pin values and control module operation without risking a meltdown
- It can accurately diagnose computer and circuitry problems by monitoring vehicle operation while the vehicle is being operated
- To diagnose this type of code using a digital volt ohmmeter would entail probing thousands of circuits, independently
- One misplaced probe could destroy expensive control modules and require that the vehicle be totally reprogrammed.
At the most, you may attempt to perform a continuity test after all control modules are disconnected, and this could literally require 40-hours or more, depending upon the vehicle
- Some applications are equipped with up to 18 separate control modules.
If you choose to tackle this monumental task, begin with a careful visual inspection of all system circuitry, connectors, and fuses
- Control module ground circuits should be tested for continuity with battery ground
- These types of codes are frequently caused by defective or disconnected system grounds
- An auxiliary ground cable can be helpful in diagnosing system ground discrepancies
- Engine and transmission ground cables, straps, and wires are sometimes left dangling after repairs are performed
- Look for loose or corroded electrical connectors that may increase circuit resistance and cause these types of codes to be stored.
Obtain a CAN bus system wiring diagram and/or pin out value chart, then use the digital volt ohmmeter to test continuity between individual controller connectors
- Compare your findings with the manufacturer’s referenced values and repair open or shorted circuits as required
- It is often much more frugal to replace defective wiring rather than attempting to remove it from the complex web of wiring harnesses