FIGURE 36-42
Typical starter solenoid.

A solenoid is an electromechanical device that coverts electrical energy into mechanical linear (back-and-forth) movement. Solenoids can be used to pull or push. In a simple solenoid, insulated wire is wound many times around a hollow cylindrical form. A sliding mild steel core is made to fit inside the hollow form. When the winding is energized, the resulting magnetic field attracts the mild steel, drawing it into the form and producing linear motion. Solenoids can be very strong and produce a lot of mechanical energy. Solenoid design may incorporate return springs, multiple windings, electrical contacts, and mechanical connections.

Fuel injectors and starter motor solenoids are two of the many solenoid-type components used in a motor vehicle Figure 36-42. The operation of a solenoid is similar to a relay, but where a relay uses a magnetic field to close an electrical circuit, a solenoid uses a magnetic field to create lateral movement and, in the case of a starter solenoid, to also close heavy electrical contacts. The metal core, used by the electromagnet to strengthen the magnetic field, is referred to as an armature. It is spring loaded so that it is positioned partially outside the electromagnetic coil and is free to move in and out. When the coil is energized, the magnetic field draws the armature into the center of the coil. If the armature is attached to a lever or plunger, it will be forced to move as well. Stopping current flow causes the electromagnet to de-energize and the spring pushes the armature out again.

Another device that uses a solenoid is a vehicle horn. When the armature is drawn in by the electromagnetic coil, it opens a set of electrical contacts so that current flow through the coil is stopped. This stoppage causes the armature to move out again, closing the contacts, drawing the armature back in. This process occurs at very high speeds. The vibration caused by the rapid movement is transferred to a diaphragm, and the familiar horn sound is produced.

AS-61: Parallel/Series Circuits: The technician can explain current flow and voltage in series and parallel circuits.

The terms “series” and “parallel” describe different ways in which components in a circuit can be connected. In a series circuit, all components are connected along a single path. Electricity can flow only one way, so the flow of electrons, or the current flow, within the circuit is the same at all points. The voltage in the system will change at different points due to voltage drops at various resistors. In a parallel circuit, components are connected so that the circuit divides into two or more paths before recombining. All components are connected to the same voltage supply, so the same voltage is applied to all components. Current flow in different branches of a parallel circuit changes depending on the resistance in each branch; therefore, different branches of the circuit may experience differing amperage if their resistance values are different.

AS-62: Short Circuit: The technician can demonstrate an understanding of the processes used to locate a short circuit in an electrical/electronic system.

Two types of short circuit can occur in electrical systems. In a short to ground, electrical current finds its way to ground before it was intended, usually due to compromised wiring insulation that allows wiring to touch the metal vehicle body. In a short to power, a circuit is exposed to voltage flowing in another circuit, generally also due to broken wiring insulation. The effect of either type of short will depend on the layout of the circuit and where the short occurs in relation to the load.

The most common indicator of a short circuit is a fuse blowing due to excessive current flow. Traditionally, diagnosing the location of the short has required being able to segment the circuit by disconnecting fuses, components, switches, or harness plugs and checking individual sections of the circuit for integrity. Electronic short-circuit finders are now available that send an electrical signal along the circuit. A receiver is run along the circuit until it stops receiving a signal, at which point the location of the short has been found.

AS-69: Fuse: The technician can explain the role of a fuse or fusible link as a protective device in an electrical or electronic circuit.

A fuse, or fusible link, is a form of over current protection device. It consists of a conductive metal strip that melts when subjected to more than a specified level of current. When the fuse “blows,” the circuit is broken and no current flows in the remainder of the circuit. Fuses are installed for two reasons: They can prevent excessive current flow from damaging more expensive components elsewhere in the circuit, and they can prevent overheating within wiring and components, which could potentially cause a fire.