Electronic – this sealed electrical component

identification

This thing, pictured below, is part of a mower's safety electronics (the mower is a bunton model "forty eight"). All the switches to make sure you can not start the mower with the blades engaged, or in gear, and you are holding the handles run through this thing.
The only markings on it are 1200-18B and 88210 (second picture). Google has been no help with those numbers.

3/4 view of part to be identified

side view of part to be identified with markings visible]

top view of part to be identified]

Here is a crude wiring diagram.

schematic

simulate this circuit – Schematic created using CircuitLab

What is this, and what does it do?

Best Answer

I believe this is an "interlock module". It contains some components and an SCR. The SCR acts as a latching relay that requires no external power (other than from the low voltage side of the magneto) to maintain conduction after the engine starts.

See, for example, Patent US4033311.

By the looks of it, the gear switches must both be closed and the handle switch maybe open for it to start. Or the handle switch may act as a kill switch so it must be closed for it to continue to run, whereas the other switches need only be closed to start it (probably more likely). In the latter case, the thyristor completes the circuit to ground from the red wire through the handle switch.

This is for the kind of engine electrical system that consists of just the magneto, no battery and therefore no starter motor, no electronic ignition and no charging circuit. Like a mechanical kill switch, it must be closed, completing the circuit- typically to ground- before the ignition spark will be generated.

Bypassing it is trivial, but not recommended, for obvious safety reasons.