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Small but indispensable: the starter motor magnetic switch
The magnetic switch is a small car part on the engine's starter motor. Although starter and magnetic switch look like a single part, they are two separate components which can be separately replaced or repaired.
Task of the starter magnetic switch
The starter is a small but very powerful electric motor. It powers up the moving parts of an engine sufficiently long for the ignition to take over. In order to reduce wear and consumption, the starter is only switched on for the duration of the starting process. During driving it is mechanically separated from the engine. To this end the starter has a magnetic switch: the switch pulls the starter shaft into the matching starter motor pinion.
Construction of a magnetic switch
The magnetic switch consists of a casing with an electro-magnet, a tension spring, a cylinder and a claw. Starting the car engages the magnetic switch. As soon as the ignition key is released, the magnetic field in the magnetic switch collapses and a tension spring pulls the steel cylinder back into its original position.
Magnetic switch defects
The starter motor magnetic switch is a mechanical-electrical part exposed to high voltage and temperatures. Although its construction is robust, its life span is limited to 7 – 10 years. Indications of wear can be:
- jammed cylinder due to intrusion of dirt
- jammed cylinder due to lacking lubrication
- jammed cylinder due to burr formation
- insufficient magnetic effect
- mechanical wear on the claw
In case of very cold temperatures it is possible for a new magnetic switch to jam. With careful hammer blows during starting the jamming can be fixed. Older starters with a tendency to jam are generally defective and should be repaired or replaced.
Especially jamming is not ideal for a starter. Although the circuitry is safeguarded, wires might get charred. In that case replacement of the magnetic switch is the only option.
Magnetic switch repair
Check the magnetic switch' functioning is very simple. If you hear a click when turning the ignition key, the part is functional. If you hear nothing, it is defective. This can have very simple causes:
- blown fuse
- corroded contacts
- broken cable / marten's bite (rodent damage)
Only when these error sources can be eliminated, the starter motor should be removed. The starter is connected to the engine with only a few bolts. The starter contacts are screwed on as well. With a charging unit or jump lead the starter can be tested separately from the engine. The magnetic switch release function and the starter motor rotation must function properly. Is this the case, and still the starter motor fails to start the engine, a ground fault is probably the cause. Generally this is caused by the earth cable which runs from the battery to the bodywork. It should be checked and if necessary cleaned and sanded at the contact points. Prior to installation of the starter motor it is useful to clean it with brake cleaner. Especially the contact points should be totally free from corrosion for the current to flow into the starter module circuitry. The electric wiring must be properly fixed as well in order to prevent its loosening by the engine's vibration. With a bit of battery terminal grease on the bolts, further corrosion of the contacts can be prevented.
If the magnetic switch is definitely defective, it can be released from the starter motor very easily. Generally it is attached to the starter with 2 – 3 bolts.
Replacing a magnetic switch
A new magnetic switch for a starter motor costs ca. 10 euro (£8). Nevertheless, too cheap spare parts are not recommended. A magnetic switch is a part of the electric circuitry and must be able to withstand high voltage. To prevent a defect occurring soon, in case of starter motor magnetic switch replacement we recommend the purchase of a high-grade product from an established brand. These are available at ca. 40 euro (£34).