A three-wheel electric counterbalance forklift was brought in with a bizarre steering complaint. When driving in a straight line, the truck would violently drift to the left. If the operator let go of the steering wheel, the truck would immediately turn into a wall. The warehouse staff had already replaced the electric power steering (EPS) motor, assuming it was getting a bad signal, but the problem persisted.
Modern electric forklifts don't have a hydraulic power steering pump. They use an EPS system, which is essentially a heavy-duty electric motor mounted directly to the steering column. The controller tells the motor how much torque to apply based on a sensor that measures how hard the operator is twisting the steering wheel.
We hooked up our diagnostic laptop to the steering controller and looked at the live data stream. We found the "Neutral Position" or "Zero-Torque" voltage reading for the steering sensor was sitting at 2.8 volts instead of the factory spec of 2.5 volts. This tiny 0.3-volt drift was enough to trick the controller into thinking the operator was constantly turning the wheel to the left. The EPS motor was actively assisting a turn that the driver wasn't asking for.
What caused this drift? A few weeks prior, the main traction battery had been completely disconnected for a major repair. On this specific forklift model, the EPS torque sensor requires a "Zero-Point Calibration" whenever the 48-volt battery power is completely lost. Without power, the sensor loses its baseline memory. The previous mechanic had replaced the battery, fired up the truck, and driven it away without performing the five-second calibration procedure through the dash keypad. We put the truck on blocks, centered the steering wheel perfectly, and ran the calibration sequence. The voltage snapped back to 2.5 volts, and the forklift drove perfectly straight. It's a perfect example of why you must always check the service manual for required computer re-learns after a power loss on modern electric trucks.
