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

	LESSON 4: ELECTRICAL CONTROL SYSTEMS (NON-ECM)

	4.2 Normally-Open Electrical Controls	INDEX
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In a normally-open control system, all switches are below their set points and the electrical circuit to the solenoid valve is open.		Learning Objectives: • State how switches are wired in a normally-open control system. • Describe the operation of the solenoid valve if all switches are open. • Describe the operation of the solenoid valve if a single switch closes.
	In such a system, all electrical switches are wired in parallel. Electricity from the 12 volt power supply can take any of the three parallel paths to reach the solenoid valve. All three switches are shown in their normal state (open). When all switches are open, no electricity flows to the solenoid valve and the solenoid remains open. The solenoid valve connects the outlet port to the normally-open port and provides air pressure to the fan drive. The fan drive disengages. When one of the electrical switches closes, electricity flows to energize the solenoid valve. The solenoid valve connects the outlet port to the normally-closed inlet port and blocks air pressure going to the fan drive, engaging the fan drive. EXAMPLE. The engine coolant temperature rises above the thermal switch's set point. The normally-open thermal switch closes causing electricity to flow through the thermal switch energizing the solenoid valve. The solenoid valve plunger moves to connect the outlet port to the normally-closed inlet port. Air pressure exhausts through the valve to allow 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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