Table 3 - 1. - Replacement Switches and Their Characteristics

SOLENOIDS

A SOLENOID is a control device that uses electromagnetism to convert electrical energy into mechanical motion. The movement of the solenoid may be used to close a set of electrical contacts, cause the movement of a mechanical device, or both at the same time.

Figure 3-17 is a cutaway view of a solenoid showing the solenoid action.

A solenoid is an electromagnet formed by a conductor wound in a series of loops in the shape of a spiral. Inserted within this coil is a soft-iron core and a movable plunger. The soft-iron core is pinned or held in an immovable position. The movable plunger (also soft iron) is held away from the core by a spring when the solenoid is deenergized. When current flows through the conductor, it produces a magnetic field. The magnetic flux produced by the coil results in establishing north and south poles in both the core and the plunger. The plunger is attracted along the lines of force to a position at the center of the coil. As shown in figure 3-17, the deenergized position of the plunger is partially out of the coil due to the action of the spring. When voltage is applied, the current through the coil produces a magnetic field. This magnetic field draws the plunger within the coil, resulting in mechanical motion. When the coil is deenergized, the plunger returns to its normal position because of spring action. The effective strength of the magnetic field on the plunger varies according to the distance between the plunger and the core. For short distances, the strength of the field is strong; and as distances increase, the strength of the field drops off quite rapidly.

Figure 3-17. - Solenoid action.

While a solenoid is a control device, the solenoid itself is energized by some other control device such as a switch or a relay. One of the distinct advantages in the use of solenoids is that a mechanical movement can be accomplished at a considerable distance from the control device. The only link necessary between the control device and the solenoid is the electrical wiring for the coil current. The solenoid can have large contacts for the control of high current. Therefore, the solenoid also provides a means of controlling high current with a low current switch. For example, the ignition switch on an automobile controls the large current of a starter motor by the use of a solenoid.

Figure 3-18 shows a cutaway view of a starter motor-solenoid combination and a section of the wiring for the solenoid. Notice that the solenoid provides all electrical contact for current to the starter motor as well as a mechanical movement of the shift lever.

Figure 3-18. - Starter motor and solenoid.

MAINTENANCE OF SOLENOIDS

If you suspect that a solenoid is not working properly, the first step in troubleshooting it is a good visual inspection. Check the connections for poor soldering, loose connections, or broken wires. The plunger should be checked for cleanliness, binding, mechanical failure, and improper alignment.

The mechanism that the solenoid is connected to (actuates) should also be checked for proper operation.

The second step is to check the energizing voltage with a voltmeter.

If the voltage is too low, the result is less current flowing through the coil and a weak magnetic field. A weak magnetic field can result in slow or poor operation. Low voltage could also result in chatter or no operation at all. If the energizing voltage is too high, it could damage the solenoid by causing overheating or arcing. In either case, the voltage should be reset to the proper value so that further damage or failure of the solenoid will not result.

The solenoid coil should then be checked for opens, shorts, and proper resistance with an ohmmeter. If the solenoid coil is open, current cannot flow through it and the magnetic field is lost. A short results in fewer turns and higher current in the coil. The net result of a short is a weak magnetic field. A high-resistance coil will reduce coil current and also result in a weak magnetic field. A weak magnetic field will cause less attraction between the plunger and the core of the coil. This will result in improper operation similar to that caused by low voltage. If the coil is open, shorted, or has changed in resistance, the solenoid should be replaced.

Finally, you should check the solenoid to determine if the coil is shorted to ground. If a short to ground is found, the short should be removed to restore the solenoid to proper operation.