What could cause a weak signal from a reluctance type speed sensor?

A weak signal from a reluctance type speed sensor can indeed be attributed to a situation where the air gap is less than specified. Reluctance type sensors operate on the principle of magnetic fields interacting with ferrous materials, and their output signal strength is highly sensitive to the distance between the sensor and the target (often a gear tooth or similar component). If the air gap is too small, it can lead to magnetic saturation, reducing the effectiveness of the sensor in generating a strong signal.

An optimal air gap ensures that the sensor can detect changes in the ferromagnetic target's position effectively, enabling accurate signal output. Therefore, maintaining the correct air gap distance is crucial for the sensor to function properly and minimize any loss in signal strength.