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Riding Mower Engine Dies When I Engage the Blades


Why Is My Riding Mower's Engine Dying?

Do you attempt to engage the mowing deck of your riding mower and the engine dies? How about you try to release the brake and the engine dies?

Probably you have a bad seat switch, otherwise known as the operator presence switch. Sears, Snapper, John Deere and all other modern Riding Mowers all utilize this sort of safety switch and it's a common failure point.

Seat switches are something we see in the shop and are misdiagnosed by most people, and a lot of other shops. One of the reasons for confusion is that people expect to see voltage through the wires at the seat switch when in fact there typically should be zero voltage.

On many mowers you can disconnect the kill wire to test if the seat switch may be at fault. For instance, many Briggs and Stratton and Kohler engines will have a simple wire ground that can be disconnected. If you look on speed control bracket, the metal bracket on the engine where the throttle cable connects, you may see a wire that is clipped to a plastic connector. Disconnect that wire and don't let it touch anything and see if the engine will continue to run. Be aware that you may have to ground that wire to metal in order to shut the engine off, depending on what year and make of tractor you have.

On most residential lawn tractors, the switch functions by grounding the coil when the switch believes there is no one in the seat and the parking brake is released and/or the PTO is engaged.

So, if the engine continues to run with the wire unclipped, you can proceed to the seat switch.

In the shop, often we just replace the switch with one that's known to be good. If you don't have that luxury, you can use an ohmmeter or a continuity light to test your switch.

There are many styles of seat switches, some are open contact switches, some are encased in a plastic case, some are large and round, some small and rectangular. All will work in one of two ways, either normally open or normally closed. The basic test is that in one position, there should be continuity through the switch, in the other position there should be no continuity through the switch.

Testing is simple, unhook or unplug the switch, then use one lead from your continuity light or ohmmeter on each wire and test the switch both open and closed. If your switch has four wires going into it, typically they will have either corresponding colors or positions. A schematic is always helpful but you may need to seek the assistance of a shop or a site such as JustAnswer.com to obtain that. Be sure to unhook the switch from the wiring, sending voltage through the kill circuit can easily ruin the ignition coil.

If you find that there is always continuity or there is never continuity in your switch, then replacement is probably the solution. On open contact switches, these are usually copper or other conductive material and can usually just be adjusted to the proper position.

Safety switches can be annoying to troubleshoot and to make matters worse, they often fail intermittently, disguising themselves as other, more expensive problems. But they do serve a purpose so they should never be disconnected or bypassed. In fact, we very often will get a rider in the shop where someone has attempted to jumper or bypass the switch, believing they have eliminated that as a cause. When in fact what they have done is not bypass the switch, but created a constant ground, preventing the coil from firing and the engine from starting. So don't try to shortcut your way around safety switches on your riding mower, go ahead and fix the problem the right way and save yourself the trouble.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


Wes on May 26, 2012:

Good info, second look at hubpages in this category first was a shameful poorly babelfished sales pitch. Something.dk with ads....

Wiring systems on even the most basic push and commercial mowers tend to fall under what's published here regarding lawn tractors. Connectors are my favorite worst componant, doesn't matter if it's a tractor seat or similar operator presense wiring for comm. walk-behinds or self propelled homeowner units. Zip ties and/or electrical tape prior to replacement may reduce science project time. Try tightening connections prior to further testing or replacement. Otherwise I have nothing to add, straight poop on switches and wiring done well.