Technical Description of OBD-II Trouble Code
High Input in Crankshaft Position Sensor B Circuit
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to OBD-II equipped vehicles (Jeep, Nissan, Dodge, Ram, BMW, Ford, GM, etc.). While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
In my previous experience diagnosing a stored code P0388, it was due to the powertrain control module (PCM) detecting a high input voltage signal from the secondary crankshaft position (CKP) sensor. In a system that uses multiple CKP sensors, sensor B typically refers to a secondary CKP sensor.
CKP sensors monitor the engine speed (RPM) and crankshaft position. The PCM uses the position of the crankshaft to calculate the ignition spark timing. Since the camshafts turn at half the speed of the crankshaft, it is crucial for the PCM to distinguish between engine intake and exhaust strokes (rotations). The CKP sensor and each camshaft position (CMP) sensor have dedicated circuits for providing the PCM with an input signal, a 5-volt reference signal, and a ground.
CKP sensors are usually of the electromagnetic, hall-effect type and are mounted on the exterior of the engine. They are placed very close to a circuit completing engine ground, typically a reluctor ring with precisely machined teeth affixed to either end of the crankshaft or integrated into the crankshaft itself. Systems with multiple CMP sensors may have a reluctor ring on one end of the crankshaft and another one machined into the center of the crankshaft.
Typically, the CKP sensor is mounted so that the crankshaft reluctor ring passes within a few thousandths of an inch of its magnetic tip. As the crankshaft rotates, the raised areas of the reluctor ring complete an electromagnetic circuit with the sensor. The indentations between the raised areas briefly interrupt the circuit. The PCM interprets these rapid circuit completions and interruptions as a waveform pattern representing voltage fluctuations.
The PCM constantly compares input signals from the crankshaft position and camshaft position sensor circuits. If the CKP input voltage remains too high for a set period of time, a P0388 code will be stored and the malfunction indicator lamp (MIL) may illuminate.
Other trouble codes related to crankshaft position sensor B include P0385, P0386, P0387, and P0389.
Code Severity & Symptoms
When a P0388 code is stored, it is likely to be accompanied by a no-start condition. Therefore, this code should be considered severe. Even if the engine starts and runs, there is still a high risk of a no-start condition in the near future.
The symptoms of this code may include:
Causes
The potential causes for this code to be set are:
Diagnostic and Repair Procedures
Before diagnosing a code P0388, it is necessary to have access to a suitable diagnostic scanner with an integrated digital volt/ohmmeter (DVOM) and oscilloscope. Additionally, access to All Data DIY as a reliable vehicle information source is recommended.
Most successful code diagnoses begin with a visual inspection of all system-related wiring harnesses and connectors. Wiring that has been contaminated with engine oil, coolant, or power steering fluid can compromise the protective insulation and lead to shorted or open circuits (resulting in a stored P0388 code). Therefore, circuits, electrical sensors, and/or connectors that have been contaminated should be carefully inspected.
Connect the scanner to the vehicle’s diagnostic port to retrieve stored trouble codes and freeze frame data. It is important to write down this information as it can be helpful if the P0388 code proves to be intermittent. If possible, test-drive the vehicle to see if the code is reset. If it is:
Refer to the vehicle information source to locate a system wiring diagram and check for voltage at the CKP sensor. Typically, a five-volt reference is used for CKP sensor operation, but it is important to check the manufacturer’s specifications for the specific vehicle. A ground signal and one or more output circuits should also be present. Proceed to the next step if reference voltage and ground signals are detected at the CKP sensor connector.
Using the DVOM, disconnect the electrical connector from the CKP sensor in question and test it according to the manufacturer’s specifications. If the resistance levels of the CKP sensor fail to comply with the manufacturer’s specifications, replace the CKP sensor. If the CKP sensor resistance values align with the manufacturer’s specifications, continue to the next step.
After reconnecting the CKP sensor in question, attach the positive test lead of the oscilloscope to the signal output wire and connect the negative lead to the CKP sensor ground circuit. Power up the oscilloscope and select the appropriate voltage setting. Start the engine and observe the waveform pattern on the oscilloscope. Pay attention to any unexpected spikes or glitches. If spikes or glitches are detected, carefully wiggle the wiring harness and connector for the CKP sensor in question to determine whether the problem is a loose connection or a defective sensor. If there is a broken or worn reluctor ring, or if the magnetic tip of the CKP sensor has excessive metallic debris attached, it may cause missing voltage blocks in the waveform pattern. Proceed to the next step if no problems are found in the waveform pattern.
Locate the PCM connector, insert the test leads of the oscilloscope into the CKP sensor signal input and ground circuits, and observe the waveform pattern. If the waveform pattern near the PCM connector is dissimilar to what was seen when the test leads were connected near the CKP sensor, suspect an open or shorted circuit between the CKP sensor connector and the PCM connector. If this is the case, disconnect all related controllers and test individual circuits with the DVOM. Shorted or open circuits will need to be repaired or replaced. If the waveform pattern is identical to what was seen when the test leads were connected near the CKP sensor, there may be a defective PCM or a PCM programming error.
Additional diagnostic notes:
