DriveMaster® - Lesson 4: Electrical Control Systems (NON-ECM), © 2001 Horton, Inc.

	LESSON 4: ELECTRICAL CONTROL SYSTEMS (NON-ECM)

	4.3 Normally-Closed Electrical Controls	INDEX
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The following schematic diagram is of a normally-closed electrical control system. When all switches are below their set points, the electrical circuit to the solenoid valve is closed.		Learning Objectives: • State how switches are wired in a normally-closed control system. • State how to determine if a solenoid valve is normally-open or normally-closed. • Describe the operation of the solenoid valve if a switch opens.
	In such a system, all electrical switches are wired in series. Electricity from the 12 volt power supply must flow through all three switches before it reaches the solenoid valve. All three switches are shown in their normal state (closed). Electricity flows to the solenoid valve and the solenoid is energized. The solenoid valve connects the outlet port to the normally-closed inlet port and provides air pressure to the fan drive. The fan drive disengages. When one of the electrical switches opens, the electrical current is broken and the solenoid valve de-energizes. The solenoid valve connects the outlet port to the normally-open inlet port and blocks air pressure to the fan drive, engaging the fan drive. EXAMPLE. The engine coolant temperature rises above the thermal switch's set point. The normally-closed thermal switch opens, causing a break in the electrical current and de-energizing the solenoid valve. The solenoid valve plunger moves to connect the outlet port to the normally-open inlet port. Air pressure exhausts through the valve and allows engagement of the fan drive, turning the fan.	NOTE: A quick, easy way to check if a solenoid is normally-open or normally-closed is to blow in the top. If the air escapes from the outlet port going to the fan drive, the solenoid is normally-open. If the air is blocked from the outlet port going to the fan drive and escapes through the exhaust port, the solenoid is normally-closed.
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