Switches exist in many forms but have common char-acteristics. They all depend upon physical movement for operation. A simple switch contains one or more sets of contact points, with half of the points station-ary and the other half movable. When the switch is operated, the movable points change position.
Switches can be designed so that the points are normally open and switch operation closes them to allow current flow. Normally closed switches allow the operator to open the points and stop current flow. For example, in an automobile with a seatbelt warning buzzer, the switch points are opened when
Figure 6-23. These symbols for normally open switches are used on electrical system diagrams.
the seatbelt is buckled; this stops current flow to the buzzer. Figure 6-23 shows the electrical symbols for some simple normally open switches.
A switch may lock in the desired position, or it may be spring-loaded so that a constant pres-sure is required to keep the points out of their normal position. Switches with more than one set of contact points can control more than one circuit. For example, a windshield wiper switch might control a low, medium, and high wiper speed, as well as a windshield washer device (Figure 6-24).
Switches are shown in simplified form on elec-trical diagrams so that current flow through them can easily be traced (Figure 6-25). Triangular con-tact points generally indicate a spring-loaded return, with circular contacts indicating a locking-position switch. A dashed line between the mov-able parts of a switch means that they are mechan-ically connected and operate in unison, as shown in Figure 6-26.
In addition to manual switches, automotive electrical circuits use a variety of other switch
designs. Switches may be operated by temperature or pressure. Switches designed to sense engine coolant temperature contain a bimetal arm that flexes as it heats and cools, opening or closing the switch contacts (Figure 6-26). Oil pressure and vacuum switches respond to changes in pressure.
Mercury and inertia switches are motion-detector switches, that is, they open and close circuits automatically when their position is dis-turbed. A mercury switch uses a capsule contain-ing two electrical contacts at one end. The other end is partially filled with mercury, which is a good conductor (Figure 6-27).
When the capsule moves a specified amount in a given direction, the mercury flows to the opposite end of the capsule and makes a circuit between the contacts. This type of switch often is used to turn on engine compartment or trunk lamps. It can also be used as a rollover switch to open an electric fuel pump or other circuit in an accident.
An inertia switch is generally a normally closed switch with a calibrated amount of spring pressure or friction holding the contacts together.
Any sharp physical movement (a sudden change in inertia) sufficient to overcome the spring pres-sure or friction will open the contacts and break the circuit. This type of switch is used to open the fuel pump circuit in an impact collision. After the switch has opened, it must be reset manually to its normally closed position.
Figure 6-22. Many different types of switches are used in the complete electrical system of a modern automobile.
Figure 6-25. This starting and ignition switch has two sets of contacts linked together by the dashed line.
Triangular terminals in the start (ST) position indicate that this position is spring-Ioaded and that the switch will return to RUN when the key is released.
(DaimlerChrysler Corporation)
Figure 6-24. The instrument panel switch in this two-speed windshield wiper circuit has two sets of con-tacts linked together, as shown by the broken line. The Park switch is operated by mechanical linkage from the wiper motor armature.(DaimlerChrysler Corporation)
Figure 6-26. A coolant temperature switch in its normally open position.
Figure 6-27. A mercury switch is activated by motion.
Relays
A relay is a switch that uses electromagnetism to physically move the contacts. It allows a small current to control a much larger one. As you remember from our introduction to relays in Chapter 2, a small amount of current flow through the relay coil moves an armature to open or close a set of contact points. This is called the control circuit because the points control the flow of a much larger amount of current through
Figure 6-28. A relay contains a control circuit and a power circuit.
Figure 6-29. When the horn button is pressed, low current through the relay coil magnetizes the core. This pulls the armature down and closes the contacts to complete the high-current circuit from the battery to the horn.
Figure 6-30. Energizing a solenoid moves its core, converting current flow into mechanical movement.
(GM Service and Parts Operations)
Figure 6-31. A starter solenoid mounted on the starter motor. Solenoid movement engages the starter drive with the engine flywheel gear.
a separate circuit, called the power circuit (Figure 6-28).
A relay with a single control winding is gener-ally used for a short duration, as in a horn circuit (Figure 6-29). Relays designed for longer periods or continuous use require two control windings.
A heavy winding creates the magnetic field nec-essary to move the armature; a lighter second winding breaks the circuit on the heavy winding and maintains the magnetic field to hold the arma-ture in place with less current drain.
Solenoids
Asolenoidis similar to a relay in the way it oper-ates. The major difference is that the solenoid core moves instead of the armature, as in a relay.
This allows the solenoid to change current flow into mechanical movement.
Solenoids consist of a coil winding around a spring-loaded metal plunger (Figure 6-30).
When the switch is closed and current flows through the windings, the magnetic field of the coil attracts the movable plunger, pulling it
against spring pressure into the center of the coil toward the plate. Once current flow stops, the magnetic field collapses and spring pressure moves the plunger out of the coil. This type of solenoid is used to operate remote door locks and to control vacuum valves in emission control and air conditioning systems.
The most common automotive use of a solenoid is in the starter motor circuit. In many systems, the starter solenoid is designed to do two jobs. The movement of the plunger engages the starter motor drive gear with the engine flywheel ring gear so that the motor can crank the engine (Figure 6-31). The starter motor requires high current, so the solenoid also acts as a relay. When the plunger moves into
Figure 6-32. A starter solenoid also acts as a relay.
the coil, a large contact point on the plunger meets a large stationary contact point (Figure 6-32).
Current flow across these contact points completes the battery-to-starter motor circuit. The plunger must remain inside the coil for as long as the starter motor needs to run.
A large amount of current is required to draw the plunger into the coil, and the starter motor also requires a large amount of current. To conserve
bat-Figure 6-33. A starter solenoid, showing the pull-in and hold-in windings.(Delphi Corporation)
tery energy, starting circuit solenoids have two coil windings, the primary or pull-in winding and the secondary or hold-in winding (Figure 6-33). The pull-in winding is made of very large diameter wire, which creates a magnetic field strong enough to pull the plunger into the coil. The hold-in wind-ing is made of much smaller diameter wire. Once the plunger is inside the coil, it is close enough to the hold-in winding that a weak magnetic field will hold it there. The large current flow through the pull-in winding is stopped when the plunger is completely inside the coil, and only the smaller hold-in winding draws current from the battery. The pull-in winding on a starter solenoid may draw from 25 to 45 amperes. The hold-in winding may draw only 7 to 15 amperes. Some starter motors do not need the solenoid movement to engage gears;
circuits for these motors use a solenoid primarily as a current switch. The physical movement of the plunger brings it into contact with the battery and starter terminals of the motor (Figure 6-34).