How to splice a lawnmower spark plug wire, step by step


A lawnmower is one of the machines that help you keep your lawn looking beautiful. Maybe it is the garden tool that you will use the most. But even when you maintain your lawnmower well, there is always the possibility that something breaks. You want to use your mower, but it fails to start. Even after several pulls to the cord, it just does not work. Or, in the case of a riding lawn mower, you try to start. You hear the starter motor, but the engine just doesn’t run. You might have various reasons popping in your mind. You check the battery and sparkplug and quickly see that both are fine. But on further investigation, you find that a damaged sparkplug wire might be causing the issue. To fix this, you do not have to buy a new one in most cases, as the old wire can still be used by splicing it.

How to splice a lawnmower spark plug wire, step by step:

  • Step 1: Check the condition of the sparkplug wire
  • Step 2: Remove the top cover of the lawnmower
  • Step 3: Notice the type of coil and sparkplug wire
  • Step 4: Repair the wire
  • Step 5: Replace the boot if loose
  • Step 6: Clean dirt and corrosion present
  • Step 7: Check the continuity using a Digital Multimeter
  • Step 8: Reinstall the components and start the engine
How to splice a lawnmower spark plug wire, step by step 1

As suggested by the name, the sparkplug wire is the connection between the ignition coil and the sparkplug. If the wire is damaged, the engine will not get a spark and hence will not function. Before getting into the details of the reason for the required procedure, it is important to understand the working of the lawnmower’s ignition system.

Diagnosing a Spark Plug Wire Problem

Finding a spark plug cable problem can be tricky. When the sparkplug cable is fully broken, it is reasonably easy. The lawnmower will just stop working. The first step will be to check the spark plug itself. But when it is working fine, the spark plug cable will be the next logical part to check. But when the wire is just damaged, it is less obvious. It can cause a voltage drop. The sparkplug will not receive all the energy it needs. The spark will give a less powerful spark that can lead to the following effects:

  • High fuel consumption: The less optimal spark can cause the lawnmower to use more gas than usual.
  • The engine will not start: Even there is a spark. It is so weak that the lawnmower has great difficulty starting. Or it will not start at all.
  • Erratic operation: The engine feels very heavy and tends to run very irregularly.
  • Misfiring: This effect is not solidly observable, but all others are generally the aftereffect of this. Engine stroke timing suffers, and power losses are massive.

You would like to know more about the full operating cycle of a spark ignition combustion engine with a curious building. More awareness will also allow you to learn more of the methods you use to help detail, preserve, and restore. If you are still not as involved in technical information, you can skip to the next segment, where a step-by-step guide for splicing a lawnmower sparkplug wire.

How does the lawnmower ignition system work

The spark plug has a tip with no insulation on both heads. When the high voltage travels through the spark plug, the slight distance between the two main terminals causes the current to leap and create a spark. The high voltage is obtained from the ignition coil, which raises the 12V current of the battery. The spark plug is an integral feature of ignition engines. The sparkplug wire carries the high voltage across it and is heavily insulated.

Splicing a lawnmower spark plug wire, step by step:

Equipment required:

For splicing the lawnmower sparkplug wire, you will require:

  • Socket wrench
  • Screwdriver
  • Insulation tape
  • Digital Multimeter
  • Feeler gauge
  • Soldering iron (if you have one)

•Step 1: Check the condition of the sparkplug wire

  • Checking the sparkplug boot: After locating the sparkplug, check whether the sparkplug is snuggly fit over the sparkplug. The boot is present on one end of the wire. If the fitting is not tight, the grip is faulty and needs repair.
  • Visual observation: Check for damages to the wire’s insulation. Move the cable around and check for cracks. If you find cracks, this could be the location where the cable is broken. If you can not find any indications on the outside of the cable, and the check with the multimeter shows a broken wire. You will need to replace the whole cable.
  • Resistance and continuity check: Using a digital multimeter (DMM) in the continuity setting or not having this the ohms setting. Check the resistance and compare it to the one provided by the manufacturer. Make sure you move the cable around so you will notice it if there is a broken wire inside. If you do not do this, the broken wire’s two ends may be touching each other during the check. If you use the continuity setting on the multimeter, a beep will mean that it is fine. If you move the cable around and notice that the beep is not continuous, there is a broken wire. If you use the multimeter resistance setting, the result should be low, close to 0 ohms. If the resistance is higher, the cable is broken. Take care that if you hold the probes in your hand during the test, you do not mix the body’s resistance as a good result.

•Step 2: Remove the top cover of the lawnmower

  • Remove the cover/ hood: For a riding lawnmower, lift the hood. In the case of a push mower, remove the engine cover if it has one.
  • Remove the start mechanism: Depending on the lawnmower, you have to remove the starting mechanism as well. If this is the case, use a socket wrench to do this.
  • Lift the cover off: Pull the cover off, making sure that any fragile components are not damaged.
  • Place the nut back in place: To secure the metallic spacers present on the studs, screw the nuts back on.

•Step 3: Notice the type of coil and sparkplug wire

The mounting of the coil and the sparkplug wire may vary from engine to engine. If the machine consists of a single-cylinder, the type of coil is different.

  • Type 1: In the most single-cylinder engine, the sparkplug wire is attached permanently attached to the coil
  • Type 2: In a multi-cylinder setup, the wire gets plugged in a distributor box due to more wire. And these are easy to access and remove. 

•Step 4: Repair the wire

If you have found during the check of the cable an indication where the cable is broken, you can try to repair it. If you did not find any indication, there is no other option than to replace the cable.

  • Remove the insulation: Cut off about 3 inches of the hard black insulation from the wire at the location you think the problem is. Fold back the braided insulation
  • Cut the cable: The core should be visible now. Remove the white dielectric from both ends of the copper cable.
  • Join the broken wire: Check again if you can find the place where the cable is broken. If you find it, add the two sides together. If you have a soldering iron, you could use this to join the two sides even better. If you do not have it, try to join the two sides as well as you can.
  • Properly insulate: Apply a lot of electric tape on the spliced wire to ensure that the insulation is regained. Check the continuity again with the digital multimeter.

•Step 5: Replace the boot if loose

If the boot from the cable is loose, it needs to be replaced. Buy a new coil (for a single cylinder) or wire for (multi-cylinder). Replace the wire and properly gap the coil for a good operative condition of the lawnmower.

•Step 6: Clean any dirt and corrosion present on the different components

Use sandpaper and a metal wire brush to clean the flywheel, the different electrical connections, and all related parts. Make sure to remove all visible dust and rust. Especially the brown corroded flywheel should be cleaned. The magnets should be wiped off properly as well.

•Step 7: Check the continuity using a Digital Multimeter

  • Use a Digital Multimeter to check the wire for continuity: Set the multimeter to continuity mode. Touch both ends of the wire with the probes from the multimeter. The meter will beep, indicating the wire is good to use. If your multimeter does not have a continuity setting, you can also use the resistive setting. Use the lowest setting and measure the resistance of the cable. It should be a low value, close to zero.
  • Use the correct sparkplug gap: Use a feeler gauge of the nominal value suggested in the lawnmower’s manual. Push the feeler gauge of advised thickness between the gap. If the gap is greater, gently press the tip down. Suppose you can not find the needed gap in your user manual and can not find it anywhere else. Use .030″ as the gap distance. This is the most seen gap on lawnmowers.

•Step 8: Reinstall all component and check the start

Reinstall all the parts, stater mechanism, and lower the hood. Start the mower and check if you have solved the issue.

Preventive Measures:

  • Safety: Depending on what you are doing, use safety gloves, goggles, and insulation boot
  • Disconnect the battery: Do not forget to remove the battery’s ground connection (if the lawnmower has a battery).
  • Hot engine: Keep spliced wire away from high temperature such as exhaust manifold

Related Questions

1. How to air gap the coil?

The air gap in the coil is done according to the distance provided by the manufacturer. Turn the flywheels magnet towards the coil and use a feeler gauge to check or set the required thickness.

2. What is the right spark plug gap for my lawnmower engine?

The exact gap depends on the spark plug and the lawnmower brand and type. For most lawnmowers, that gap will be .030″ or .020″ (less often). Check the lawnmower user manual for the actual gap number that should be present between the electrode tips. If you can not find it, set it to .030″ and try how it works. Use a feeler gauge to check the actual size.

3. What is a Spark-Ignition Engine

Most of the engines used in lawnmower use gas. If your lawnmower uses diesel, it does not have a sparkplug, as it uses compression to ignite the diesel.

The term spark-ignition internal combustion engine is used because of the spark it uses to start the combustion cycle.

  • Spark means the jump of current from one electrode of the sparkplug to the other with air as a medium to travel
  • Ignition means the start of the combustion process of the air-fuel mixture
  • Internal means processes take place in an enclosed space
  • Combustion means the burning of fuel that generates power to drive the rotary power

The engine converts the chemical energy of fuel into the mechanical strength of the crankshaft. The operating cycle is the Otto cycle, which has four processes going on.

  • Intake of the air-fuel mixture
  • Compression of the mixture
  • A spark from the sparkplug
  • Combustion of fuel
  • Power stroke on piston head driving it down
  • The exhaust of burnt gases

The piston moves linearly and transfers power as rotational energy via a connecting rod.

Final Remarks

Although small yet essential, the sparkplug wire needs repair and replacement for good running and lawnmower’s extended life. Replacement of sparkplug wire is necessary as a maintenance process and if the engine fails to start. This article has summarized the process for readers’ help.

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