The starting system in a modern vehicle consists of a several components: a starter motor that rotates the engine at a rate sufficient for the ignition to fire, the necessary wiring, a battery to supply the current to the starter and a switch inside the passenger compartment that when closed, supplies power to the energising circuit of the starter relay. The starter relay is the subject of this article.
The starter relay, usually called a solenoid in automotive applications, is simply an electronic switch. Relays can be electromechanical or solid-state. The theory of operation is the same for both. Two circuits are necessary for the relay to operate. An energising, or control, circuit applies a small amount of current to an electromagnet, closing the main contacts in the contact circuit that carries high current to the starter motor.
Solenoid: An electronic device that converts electricity into mechanical motion. A relay is a solenoid that uses a small current to close contacts, controlling a much larger current.
Armature: The component within the solenoid that moves. The stem is placed within the electromagnet; the contacts that close the main circuit are at the opposite end.
Electromagnet: A temporary magnet produced when current is applied to a coil of wire wrapped around a core made of magnetic (ferrous) material.
The starter relay is used in an automobile application to facilitate engine starting, without running the very high current needed through the passenger compartment. This reduces the wire size and length required to deliver high current to the starter motor. When the driver engages the ignition switch, the energising circuit of the relay is closed and the electromagnet energises. The magnet pulls a contact into the closed position and high current is available to the starter. Release the ignition switch to de-energise the electromagnet and stop current flow to the starter.
Common starter relay problems
- Failure of the energising circuit. The relay is effectively "dead" when this circuit fails. Attempting to start the engine will be unsuccessful. Replace the starter relay.
• Failure of the contact circuit. The usual reason for this failure is a burnt-out contact caused by the high current carried through the unit over time. The energising circuit may still function. A "clicking" sound is the indication of this failure, caused by the contacts attempting to close.
• Another possible problem, although quite rare, is the "welding" of the main contacts into the closed position. This would result in the starter being continuously powered, which could be a dangerous situation.
• External failure, such as corrosion of the external contacts. The studs on the outside of the unit may break or become corroded if subjected to moisture. Broken studs necessitate replacement of the unit, while light corrosion may be removed with fine sandpaper. If good electrical contact is re-established, and the unit is not damaged internally, it should function.
Replacement of a starter relay is not a difficult task for someone with basic mechanical skills. Safety first: always disconnect the battery cable before touching the relay. Tools or wet fingers that happen to touch both sides of the contact circuit may cause sparking and could turn over the engine, creating a very dangerous situation. Gain access to the relay. Disconnect the wires, noting their positions. Remove the relay from its mount. Install the new unit in the reverse order.