How to Diagnose and Change the Coil

To my lady mechanics, don’t let the coil, or using a meter to check it, intimidate you. These are just big words for a little thing. Pull your hair back, pull the acrylics, and let’s dig in to this.

The coil is the muscle behind the combustion in each cylinder of your engine. Without it the engine will run, but not very fast or efficiently. When your coil starts to go bad the symptoms that present themselves are very similar to many other components symptoms.

Some things you may notice when the coil is going out are:

  • Backfiring
  • The engine is hard to start.
  • When it does start it doesn’t run well.
  • The engine will run rough, and halting, and stalling.

The coil is a simple device. It’s job is to take the 12 V that a battery produces and amp it up to 12,000 V that the spark plug needs to ignite the fuel in the piston chamber. This .gif is a simple example of what the coil does.

To save yourself some money you will want to test the coil to make sure it needs to be replaced. Testing the coil involves using a multi-meter set for continuity.

Continuity Symbol on a Multi-Meter.

I like this video on YouTube. This guy goes straight to the point and clearly explains how to test the coil. He gives you a lot of information in less than 8 minutes.

If you’ve determined that it is the coil that is the problem, it’s a simple part to replace.

  1. Disconnect the negative cable from the negative battery post.
  2. Locate the coil. Usually on top of the engine very near to the distributor.
  3. Remove the spark plug wire from the top of the coil.
  4. Detach the plugs (two) going into the coil. Set aside noting which one went where.
  5. Determine how your particular coil is attached to the engine. On the vehicle I worked on, the coil was attached by a bracket that held down two other components. It was cumbersome and far more difficult than need be, but that’s what it was.
  6. Once the bolts holding the coil in place are removed you can remove the coil from the engine.

As you can see in the photo’s this coil took a few minutes to remove. It was attached to the base plate with a slip-fit sleeve that was bolted to the plate on the underside that could not be reached with a wrench, finger or anything.

To remove the coil the whole plate had to come out which included removing the accelerator bracket which was held down by the fuel line hose holder bracket which was held down by the bolt. Then there were two more bolts to remove.

Once you have the coil out of the engine you will notice there is a sleeve it is setting in. If this is the first time the coil has ever been replaced in this engine the sleeve that holds the coil will have rivets holding it in place on both sides. These rivets need to be drilled out on both ends of the bracket.

When drilling out the rivets use a drill bit as near to the same size, if not exactly the same size, as the rivets. The coil needs to be very secure in this sleeve. Any excess movement when the coil is in the engine IE: Wiggling from engine idling, rattling from bumps on the road, can damage the coil.

The new coil will have screws with nuts in order to replace the rivets you just drill out. Put the new coil into the holder that the old coil was in. Slide the screws through the rivet holes and snug down the nuts.

Reinstall the coil on the engine. Plug in the two wire sets and the spark plug wire in the top.

Attach the negative cable wire back onto the battery.

This job is Done!

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