Engine Speed Sensor: Testing and Inspection

Diagnosis of Engine Speed Sensor

The electrical load of the crankshaft position sensor consists of two serial resistors inside the ECU. The connecting line between the two resistors is also connected to a voltage divider, biasing the crankshaft signal to 2.5 Volts. With a perfectly symmetrical circuit of the crankshaft sensor, the voltage at that connection (bias node) is supposed to be 2.5 V DC at all engine speed conditions.

Electrical check
The voltage at the bias node is sampled at a high frequency, and the minimum and maximum voltage sampled are stored. Each time when a number of samples have been taken, the minimum and maximum voltage of that sampling period are compared against thresholds to detect a short circuit to ground or to battery voltage. Furthermore, the A/C portion of the voltage sampled is calculated and compared against a third threshold. This allows to detect a strongly asymmetric circuit, indicating one open signal line of the crankshaft sensor.

Exceeds the error counter a calibrated threshold, the fault code management will be triggered.






Signal activity and rationality check
At engine start, signal edges are counted on one selected camshaft while crankshaft synchronization is not yet established. Synchronization is established as soon as one valid crankshaft reference gap is detected. A malfunction of the crankshaft sensor is detected when the counter of the camshaft edges reaches a calibrated threshold. A fault code for no signal is set if no crankshaft teeth have been detected, otherwise a fault code for implausible signal is set.