Wiper Rear Motor Speed Sense Circuit Failure
The ECU can't detect how fast the rear wiper motor is spinning, like a tachometer that suddenly stops working on an engine. The vehicle's computer needs this speed feedback to know if the wiper is actually moving or if there's a motor failure.
The ECU monitors a speed sensor signal from the rear wiper motor that produces a pulse frequency proportional to motor RPM. The controller compares this feedback against commanded speed and detects open circuits, short circuits, or signals that deviate from expected thresholds.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Speed sensor signal frequency | Proportional to motor speed (typically 10-200 Hz depending on wiper speed) | No signal, signal below 5 Hz, or erratic pulses at rest |
| Circuit voltage | 5V reference with modulated signal 0-5V | Constant 0V or 12V, or signal noise above 1V RMS |
Code B1894 is a low-severity fault. Your vehicle is generally safe to drive to a workshop for diagnosis. However, do not ignore it indefinitely — low-severity codes often indicate developing problems that become expensive if neglected. Book a diagnostic appointment within 2–4 weeks. If you notice any additional symptoms (rough running, power loss, unusual smells), treat it as higher priority.
Once the fault is repaired, B1894 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.