The thermotime switch controls the operation of the cold start injector. It is fitted to the cylinder head casting, with its lower portion immersed in engine coolant. This lower portion houses a bimetallic strip that reacts to coolant temperature. A heater coil is wrapped around the strip. Heating and cooling of the bimetallic strip opens, and closes, a set of contacts. This interrupts, and completes, the circuit for the cold start injector.
When coolant temperature is below the rated value of the switch, the contacts are closed, and so is the circuit for the cold start injector. Then, during cranking the cold start injector will spray fuel into the manifold.
If coolant temperature exceeds the rated value of the switch, the bimetallic strip bends, and opens the contacts. That means that when the engine is above a certain temperature, the cold start injector won’t operate, and this prevents excessive wetting of the spark plugs, and a rich mixture on start-up.
The heater coil also limits the operation time of the cold start injector up to a maximum value, depending upon the application. Typically, this can be about 10 to 12 seconds, at a temperature of minus-20 degrees Celsius.
The heater coil is energized during cranking, and if the engine fails to start within the specified time, the bimetallic strip bends under heating action and opens the circuit. This prevents flooding the engine with excessive fuel.
The time limit is automatically reduced for temperatures up to the rated value of the switch. At that value, the contacts open, under the influence of the coolant temperature.