Technical Description of OBD-II Trouble Code
The input voltage for the Crankshaft Position Sensor B Circuit is too low.
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to OBD-II equipped vehicles such as Jeep, Nissan, Dodge, Ram, BMW, Ford, GM, etc. Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
When I have previously diagnosed a stored code P0387, it was because the powertrain control module (PCM) detected a low input voltage signal from the secondary crankshaft position (CKP) sensor circuit. Sensor B typically refers to a secondary CKP sensor in a system that uses multiple CKP sensors.
CKP sensors are responsible for monitoring the engine speed (RPM) and crankshaft position. These factors are used in various engine management scenarios, but they are especially important for ignition timing and fuel delivery.
The PCM uses the position of the crankshaft, particularly in relation to the camshaft/s, to calculate 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. The CKP sensor and each camshaft position (CMP) sensor have circuits dedicated to providing the PCM with an input signal, a 5-volt reference signal, and a ground.
CKP sensors are typically of the electromagnetic, hall-effect variety. They are mounted on the exterior of the engine in close proximity to a circuit completing engine ground, usually a reluctor ring with precisely machined teeth affixed to one end of the crankshaft or integrated into the crankshaft itself.
The CKP sensor is positioned so that the crankshaft reluctor wheel passes very closely to its magnetic tip. As the crankshaft rotates, the raised areas on the reluctor wheel complete an electromagnetic circuit with the sensor. When the gaps between the teeth pass the CKP sensor, the circuit is briefly interrupted, creating voltage fluctuations that the PCM interprets as a waveform pattern.
While the engine is running, the PCM constantly compares input signals from the CKP and CMP sensors. If the CKP input signal voltage is lower than expected under certain circumstances, a P0387 code will be stored and the malfunction indicator lamp (MIL) may illuminate.
Other trouble codes related to crankshaft position sensor B include P0385, P0386, P0388, and P0389.
Code Severity & Symptoms
When a P0387 code is stored, the engine is unlikely to start, and this code should be considered severe. Even if the engine does start and run, there is a high risk of a no-start condition and drivability issues.
Symptoms of this code may include:
Causes
Possible causes for this code to be set are:
Diagnostic and Repair Procedures
Prior to diagnosing a code P0387, it is necessary to have access to a suitable diagnostic scanner with an integrated digital volt/ohmmeter (DVOM) and oscilloscope. It is also important to have access to All Data DIY as a reliable source of vehicle information.
A good starting point for code diagnosis is a visual inspection of system-related wiring harnesses and connectors. Wiring and connectors that have been contaminated with engine oil, coolant, or power steering fluid should be carefully examined, as petroleum-based fluids can compromise the protective insulation on wiring and lead to shorted or open circuits (resulting in a stored P0387 code).
If everything appears to be in good condition, connect the scanner to the vehicle’s diagnostic port and retrieve all stored trouble codes and freeze frame data. It is helpful to write down this information, as it may be useful if the P0387 code proves to be intermittent.
Test the voltage at the CKP sensor in question. 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. There should also be a ground signal and one or more output circuits. If reference voltage and ground circuits are detected at the CKP sensor connector, proceed to the next step.
After disconnecting the electrical connector from the CKP sensor in question, test it according to the manufacturer’s specifications using the DVOM. If the resistance levels in the CKP sensor do not comply with the manufacturer’s specifications, it should be replaced. If the CKP sensor does meet the specs, continue to the next step. Reconnect 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 sensor ground circuit. Power up the oscilloscope and select the appropriate voltage setting. With the engine running, observe the waveform pattern on the oscilloscope, paying attention to any unexpected spikes or glitches. If spikes or glitches are observed, 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 missing voltage blocks are noticed in the waveform pattern, suspect either a broken or worn reluctor ring or excessive metallic debris on the magnetic tip of the CKP sensor. If no problems are found in the waveform pattern, proceed to the next step.
Connect the test leads of the oscilloscope to the CKP sensor signal input and ground circuits near the PCM connector and observe the waveform pattern. If the waveform pattern near the PCM connector is different from 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 begin testing individual circuits with the DVOM. Any shorted or open circuits must be repaired or replaced. If the waveform pattern is identical to what was seen when the test leads were connected near the CKP sensor, suspect a defective PCM or a PCM programming error.
Additional diagnostic notes:
