The ignition switch supplies power from your battery to most of your car’s electronics. Which parts get that power depends on where you turn the switch after you insert the car key.
These are the most common problems associated with a faulty ignition switch:
- Engine dies: This is one of the most common symptoms of a bad ignition switch. Worn switch contacts can momentarily cause a loss of voltage to vital systems, such as the engine controls. This causes the engine to stall.
- Engine won’t start: The ignition switch supplies power to starter motor, ignition system and engine controls. A faulty switch can prevent these systems from getting power, resulting in an engine that won’t start.
- Intermittent loss of lighting and accessories: A worn ignition switch can disrupt the supply of power to vehicle electronics. This can cause symptoms such as flickering dashboard lights and accessories that stop working momentarily.
- Problems when removing the key: If your ignition switch is worn out, it may be difficult to turn or remove the key. On the other hand, if the switch is going bad, the engine may keep running after you take the key out.
How the ignition switch works
Typically, the ignition switch has four positions:
Lock/off: Moving the ignition switch to this position cuts power to all circuits that receive it from the ignition switch and mechanically locks the steering wheel. It also locks the transmission gear selector on cars equipped with an automatic transmission. The ignition switch must be in this position before the driver can remove or insert the key.
Accessory: In this position, the ignition switch supplies battery power to the vehicle’s accessories, such as the radio and lights. But it doesn’t provide power to the engine control circuits.
Run: In this position, the ignition switch provides power to the engine controls and all other circuits fed by the switch, but not the starter motor. It takes about three seconds for the car’s computer to do everything it must do to prepare the engine to start.
Start: When the ignition switch is moved to this position, it supplies power to the starter motor and engine controls. This allows the engine to crank, and does everything else it needs to do if you didn’t give it enough time in the run position
You determine the position of the ignition switch by rotating the ignition lock cylinder. To turn the cylinder, you have to insert the key.
Use the accessory position any time you’re hanging out in or around your vehicle and want to listen to the radio, charge a device or operate accessories without the engine running. Using the run position just to use just the stereo will drain the battery much faster.
The ignition switch also plays a role in keeping your car from getting stolen. Without the key, the switch won’t move to the start or run positions, and will lock the steering wheel and gear selector.
Of course, many cars made in the last 10 years or so have a push button instead of an ignition key and switch. With this design, the key fob sends a signal to the car's computer saying it’s OK to start the vehicle. When the start button is pushed, the computer checks that the transmission gear selector is in park and that the brake pedal is being pressed. If everything looks good, the computer sends a signal for the starter motor to crank the engine.
» MORE: Get an estimate for your ignition switch repair
How to fix the problem
Before spending time and money to replace the ignition switch, a thorough diagnosis should be performed. Some of the symptoms above can be caused by a worn key. Also, the term "ignition switch" commonly refers to the full ignition assembly, which includes the ignition lock cylinder and the ignition switch. Depending on the type of failure, these parts can be replaced individually or as a unit.
Heavy keychains are the top killer of ignition switches. If you carry a lot of keys or have weighty keepsake keychains, strongly consider keeping your car key on a separate chain. Your ignition switch will last a lot longer this way.
A faulty ignition switch should be replaced right away. This is a job best left to a professional, especially if your car is equipped with airbags. Ignition switch replacement usually involves disabling the steering column, which can lead to unintentional airbag deployment if done wrong.
When ignition switch issues arise, it can be dangerous or even impossible to drive the car. The vehicle could leave you stranded, stall in traffic, or even fail to turn off the engine when the key is removed. Have the car towed to your mechanic instead. Driving should only be done once the ignition switch has been repaired.
Image via Flickr/Bill Selak
About the Author
Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.
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