1990 BMW 735i Ignition Coil Replacement Cost

There are two sets of windings inside each ignition coil. The primary windings have hundreds of turns of heavy wire, while the secondary side contains thousands of turns of fine wire.  Power supplied by the battery flows through the primary windings. When your car’s computer decides it’s time to fire that cylinder’s spark plug (or plugs), it cuts the current to the primary windings, causing the energy in them to collapse into the secondary windings. This multiplies the voltage, which is then sent to the spark plug in order to make it fire.

Oil or coolant leaks can damage ignition coils—the leaks need to be repaired along with the ignition coil.

Failing ignition coils can cause the engine to run roughly or a lack of power upon acceleration.The Check Engine Light will often flash when the engine is misfiring—continuing to drive under these conditions can damage the catalytic converter.

A failing ignition coil can create several symptoms:

Check engine light comes on

Engine runs poorly or stall

Increased emissions

Reduced fuel economy

Car won’t start

If the coil is failing, you may hear arcing from under the hood, which sounds like a rubber band snapping. The ignition coil may look burned or white where the terminal connects to the insulated wire.

If the ignition coil goes bad, the car won't be much fun to drive — it might stall, and will definitely run poorly. Also, the amount of unburned fuel going into the catalytic converter could cause it to fail, adding an expensive repair to your bill. With severe ignition problems, the vehicle should be towed to a repair facility.

Some carmakers recommend replacing the ignition coils at scheduled intervals; this schedule should be followed to reduce the likelihood of failure. Some coils will last the lifetime of the vehicle, and other may only last 5,000 miles. Most will make it 60,000 miles or more.  

For vehicles that use one coil per spark plug: If the engine misfire is only on one cylinder, the components for only that cylinder should be examined. If the coil is suspected, it will be tested and, if faulty, will be replaced. For older cars, with distributors: One coil will fire multiple spark plugs. If all these plugs are failing, the ignition coil will be suspected and tested.

A failed ignition coil can be the result of failures elsewhere in the ignition system. The ignition wires and spark plugs should be inspected and replaced if necessary.

Proper testing is key — spark plugs, spark plug wires and other components can cause similar symptoms. Some ignition coils are better than others at generating high-voltage electricity for the spark plugs. Purchasing ignition parts that are built to OEM standards or better is recommended. Using inferior parts may lead to a repeated repair.