Automotive Ignition System

Your vehicle’s automotive ignition system is electrical, so why would it need maintenance?

Your car or truck’s electrical system includes many mechanical components that need adjusting, wear out, or simply age.  The battery, generator, and accessories are important, but the ignition system must be near-perfect for your vehicle to run properly.  Modern electronic ignition systems last longer and require fewer adjustments, but they still require attention.

The Starter System

Just to be clear we’re not going to be talking about the starter system.  When you turn the key in the automotive ignition switch it trips a relay that powers a solenoid that positions the starter motor then runs that motor to crank your car.  That’s the starter system.  Instead we’re talking about the ignition system proper.  It’s purpose is to ignite the gasoline-air mixture in each cylinder at exactly the right moment to efficiently create power.

Automotive Ignition System

The older more mechanical way of doing things may be easier to understand.  As the crankshaft turns its position indicates the position of all the pistons, and in turn ignition timing.  Here’s the chain of parts and events involved in firing each cylinder at precisely the right time along with their maintenance and replacement needs.

  • Connected to the drive shaft, the points are a pair of contacts that close whenever any cylinder needs to fire.  A condenser (a capacitor) protects the breaker points, but even so they need to be changed every 8,000 to 10,000 miles.  A centrifugal and/or vacuum advance adjusts the timing of the spark at higher engine RPM to fire a tiny bit sooner so that the fuel will be ignited at exactly the right moment.
  • The ignition coil takes pulses from the points at the battery voltage (nominally 12 volts) and steps them up to 20,000 volts or  more.  It’s essentially a transformer.
  • The distributor (also connected to the drive shaft) includes a rotor, contacts, and distributor cap.  It sends the high voltage to the spark plug at the right cylinder.  It needs to be lubricated and checked for wear.
  • Ignition cables and wires have special thick insulation to handle those high voltages.  As they age they dry and crack, allowing sparking that in turn leads to poor performance of misfires.  A high-performance ignition system may reach 50,000 volts, requiring the use of special low-leakage ignition cables.
  • Spark plugs extend into each combustion chamber (cylinder) where the high voltage produces a spark that then ignites the fuel.  Spark plugs must withstand both the spark and high operating temperatures.  Its electrodes do slowly burn off and may be fouled by poor combustion.  They can be cleaned and gapped (the length of the spark gap adjusted to within thousands of an inch accuracy), and they should be replaced every 30,000 to 100,000 miles depending on the vehicle and its type of ignition system.

It’s important to keep up with those maintenance and replacement needs.  Problem symptoms range from poor gas mileage to backfires and misfires that can damage the engine.  Other symptoms include the engine running roughly, stalling, or surging as well as increased emissions and smelly exhaust.  And of course difficult or failure to start when the starter system is working properly.

Recent Advances

Today’s automotive ignition system does away with many of the mechanical parts while still performing the same functions.  That reduces maintenance, increases lifetimes, and in many cases improves performance. Electronic ignition systems actually date all the way back to 1948 (using a vacuum tube!) but weren’t common until the 1970s ( Early designs only eliminated the points, but with that went a tricky adjustment and a frequent problem.  By the 1980s electronic ignition also replaced the distributor by having a separate coil for each cylinder or pair of cylinders.  They now last some 100,000 miles with few or no manual adjustments.

In today’s vehicles spark timing is handled by the ICU — the ignition control unit.  Working along side the ECU (engine control unit) these two microcomputers provide advanced timing control based on a large number of sensors — crankshaft, intake manifold pressure and temperature, throttle, oxygen, knock, and exhaust temperature — to achieve the lowest emissions and highest performance.