April 01, 2013

How to replace a lawn mower safety switch

This DIY lawn mower repair guide has step-by-step instructions for how to replace a lawn mower engine safety switch. The safety switch stops the engine when you release the control bar on the handle, by stopping the ignition coil from sparking; it also has a brake pad to slow the flywheel. If the switch isn't working, the ignition coil won't spark the spark plug, and the lawn mower won't start.

To determine if the switch is the problem, remove the kill wire from the ignition coil and check for engine spark. If there's spark, replace the engine safety switch with the manufacturer-approved walk behind mower part.

A broken safety switch can also make the ignition coil keep sparking after you release the handle lever. In that case, remove the spark plug until you can replace the mower engine safety switch.

This procedure works for Briggs & Stratton engines that power Craftsman, Poulan, MTD, Troybilt, Toro, Husqvarna, Murray, Yard-Man and Snapper walk-behind lawn mowers.

Quick links

Instructions

Tools required

Phillips screwdriver

Socket wrench

5/16-inch socket

3/8-inch socket

Needle-nose pliers

Slot screwdriver

Work gloves

Repair difficulty

Time required

30 minutes or less

Instructions

01.
Disconnect the spark plug wire
Let the engine cool.
Wear gloves to protect your hands.
Disconnect the spark wire so there's no chance that the mower could accidentally start.
PHOTO: Disconnect the lawn mower spark plug wire.
02.
Remove the engine cover
Remove the starter rope from the rope guide. If the rope won't slip past the guide and the handle, loosen the guide.
Remove the screws securing the engine cover to the engine. Slide the rope through the hole in the engine cover.
PHOTO: Remove the starter rope from the guide.
PHOTO: Remove the engine cover.
03.
Remove the fuel tank
Remove the screws that secure the fuel tank to the engine.
Lift the tank off, being careful to keep the fuel tank upright so it doesn't leak.
PHOTO: Remove the fuel tank.
04.
Remove the dipstick tube
Remove the screw that secures the dipstick tube to the housing.
Remove the dipstick tube from the engine and plug the hole with a paper towel to keep debris from falling into the engine.
PHOTO: Remove the dipstick tube.
05.
Remove engine blower housing
Remove the bolts securing the blower housing to the engine and remove the housing from the engine.
PHOTO: Remove the blower housing.
06.
Remove the safety switch
Take a digital photo of how the zone control cable connects to the engine safety switch.
Use a pair of needle-nose pliers to squeeze the tabs on the cable housing. Pull the cable housing to remove it from the safety switch bracket.
Press down on the tab that attaches the kill wire to the safety switch and disconnect the wire from the safety switch. Remove the 2 mounting screws that secure the engine safety switch to the engine.
Remove the safety switch.
07.
Install the new safety switch
Position the new safety switch on the engine and secure it with the mounting screws.
Reinstall the kill wire by pushing down the locking tab and inserting the wire into the safety switch. Release the tab. Reconnect the zone control cable and work the engine control bail on the handle to make sure it works smoothly.
08.
Reassemble the mower
Position the engine blower housing on the engine and bolt it into place.
Remove the paper towel from the dipstick opening, slide the dipstick into the engine and secure it with the screw.
Feed the starter rope handle through the engine cover up to the rope guide.
Position the engine cover on the engine and screw it into place.
09.
Reconnect the spark plug wire
Attach the spark plug wire and make sure the mower works properly.

Warning: Undertaking repairs to appliances can be hazardous. Use the proper tools and safety equipment noted in the guide and follow all instructions. Do not proceed until you are confident that you understand all of the steps and are capable of completing the repair. Some repairs should only be performed by a qualified technician.