Crankshaft Position Sensor A Circuit Low Input
The PCM has detected a voltage signal from the CPS that exceeds the maximum allowable variation from manufacturer’s specifications.
Code Set Parameters
Differences in system wave form or voltage that exceed 10-percent of a predetermined reference voltage will usually cause a code to be stored in the PCM and a malfunction indicator lamp to be illuminated. Some vehicles require more than one failure (typically three) to trigger service engine soon lamp illumination. Nevertheless, the initial failure/s may cause a “pending code” to be stored with no illumination of the service engine soon lamp.
Symptoms may include an illuminated service engine soon lamp, a no start condition (see diagnostic strategy), an engine misfire, a rough idle, choppy acceleration, reduced engine performance, substandard fuel economy.
Common failures in the CPS system include a damaged crankshaft position sensor, sensor connector, or electrical wiring. Technicians have reported that the presence of oil leaking on the outside of the engine often causes deteriorated insulation on wiring and may lead to shorted and loose connectors. Broken crankshaft reluctor rings or missing teeth may also cause this code to be set. Broken timing belts that are wound around crankshaft and camshaft timing gears can often result in damage to CPS system components and wiring. Sensor and wiring failure often reveals itself as an open or shorted (power or ground) circuit. The PCM could also be the culprit but this scenario occurs rarely.
Failure to inspect timing components properly during replacement of a broken timing belt may lead to a perceived misdiagnosis, if the engine will still not start afterwards due to a damaged gear, reluctor ring, sensor, connector, or wiring. Technicians report that intermittent failures occur frequently due to loose or corroded electrical wiring or connectors.
- Manufacturers utilize the electro-magnetic crankshaft position sensor in two different manners
- Both designs use either a reluctor ring or teeth (attached to the crankshaft) to interrupt the field of the stationary electro-magnetic CSP sensor; creating what is interpreted by the PCM as a square wave form pattern
- These interruptions provide the PCM with the precise crankshaft position
- In the first design, the PCM uses the crankshaft position only for misfire detection and is not critical to spark timing or ignition
- Engine control systems that use this type of system may allow the engine to start and run, despite a crankshaft position circuit failure, but engine performance and fuel economy will likely suffer
- Malfunction indicator lamp illumination may not occur until multiple failures are documented in this type of system
- In the second type of system the PCM uses crankshaft position to calculate spark timing and ignition control
- A crankshaft position sensor failure in this system design will normally lead to a no-start condition, an immediate stored trouble code, and an illuminated service engine soon lamp (on the first failure).
Several specialty tools will be required to diagnose this code successfully
- They include a scanner, a digital volt/ohmmeter, and possibly an oscilloscope.
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
- In the event of an intermittent condition, you may also utilize the oscilloscope to monitor waveforms created by the distributor, camshaft, and/or crankshaft sensor/s, while looking for glitches or other inconsistencies.
A viable starting point for obtaining a successful diagnosis is checking for an engine RPM signal when the engine is cranked or running
- This is accomplished by using a scanner or observing the vehicle tachometer as the engine is being cranked (using the starter) or is running, depending upon CPS system design
- If no RPM signal is detected, then visually inspect the crankshaft gear, crankshaft position sensor, and sensor connector for damage and repair as necessary
- If no obvious signs of damage are found, then test the CPS system for a reference voltage signal (typically 5-volts, but check manufacturer’s specifications)
- If you have access to an oscilloscope, then test the CPS signal wire for the presence of a square 5-volt waveform pattern for each engine revolution
- If no pattern is detected then test the resistance of the (disconnected) CSP sensor and compare the values with manufacturer’s specifications
- If the CSP sensor checks out, then test the system circuitry for the proper voltage and resistance
- Repair open or shorted wiring as necessary
- While PCM failure is certainly a possibility, it is rare and all other possibilities should be exhausted prior to condemning the PCM.