Riding lawn mower not cutting? Let’s talk about some of the common reasons why this can happen, and more importantly, how you can fix it.
For the most part, if your riding lawn mower is not cutting, the diagnosis is pretty simple.
Tools Needed (Commissions Earned)
How is the Cutting Deck Supposed to Work?
Before we can get started with the diagnosis, let’s talk about how the system is supposed to work so we can find the failure point.
Keep in mind that this is for most riding lawn mowers; yours might differ. For the most part, they’re relatively similar.
Typically, the engine is a vertical shaft. This means that the crankshaft is vertical and exits through the bottom of the block.
Beneath the frame, you’ll see the crankshaft sticking out and there the PTO is attached.
When you pull the PTO switch, an electrical current energizes large magnets in the PTO to grab onto the internal ring of the PTO. This internal component is held onto the crankshaft with a metal key.
As the PTO is energized, it then spins the belt which runs to the mower deck. When that belt turns, it turns the mower blades and voila! you’re cutting grass.
So when diagnosing a cutting failure, we need to isolate the individual components.
Riding Lawn Mower Not Cutting Diagnosis
PTO Electrical
I like to start with the PTO because it’s a simple-ish test, and oftentimes a rather expensive component to replace.
With the key turned to the accessory position (not cranking), you should be able to pull the PTO switch and hear an audible click come from the PTO. This is the sound of the magnets grabbing.
If you hear the PTO click, then you can immediately cross off the electrical side including the PTO switch and the electrical part of the PTO because you know that they’re functioning.
If there is no click, then we need to look further and try to isolate some more. Trace the wires back from the PTO, they’ll often come to a relay which then runs to the PTO switch.
We need to ensure that electric power is making its way to the PTO. So with your multimeter set to DC, simply probe the PTO leads with the PTO switch pulled and the key in the accessory position. You should be reading around 12 volts DC as the path should be connected.
If you do, but there’s still no click, then there’s a good bet that the PTO itself is bad. More likely, you won’t get voltage at the PTO so you know that the fault lies in the path.
Further Electrical
Inspect the wiring for frayed or damaged wires. Remove all terminals and make sure that they’re clean and making a good connection.
Use the multimeter to test both sides of the relay, if there is one. Lastly, test the PTO switch itself. You should have 12 volts coming into the switch when the key is in the accessory position and 12 volts leaving the switch when the path is connected (i.e. when the PTO switch is engaged). If it is faulty, replace it.
Lastly, if the PTO checks out well and there’s voltage coming across the PTO switch and relay all the way to the PTO, but still no power, inspect the safety switches.
Safety Switches
These little buggers are there for a reason – safety. They’re meant to be smarter than the average human, so they prevent the machine from running the mower deck if reverse is engaged or when there isn’t someone in the operator’s seat.
However, if the switch is on the fritz, the machine might be incorrectly reading one of these conditions and preventing the riding mower from cutting.
PTO Mechanical
Along with the electrical side of the PTO, we have to consider the mechanical aspect as well. If you get up close and look at the PTO, you’ll see a small metal shaft that extends into a plate on the PTO itself. The best way to do so is to jack it up off the ground.
This is to prevent the whole PTO from spinning; we only want the pulley side to spin in order to provide power to the belt. If the entire PTO were to spin, it would simply tear the wires, make an awful noise, and then we’d have further issues to fix.
This is somewhat more uncommon, but it does happen. That metal rod gets worn down over time, and there’s not a ton of “extra” length to it, so the PTO plate can simply jump over that stub and spin freely. Again, this will result in torn wires which need to be repaired.
PTO Key
The PTO key connects the crankshaft to the internal side of the PTO. This part is always spinning when the crankshaft is spinning. It only grabs onto the outer part when the magnets are energized.
Unfortunately, if the key is sheared, then the crankshaft will spin without turning the inner portion of the PTO. Even if the magnets are energized, the outer half will grab the inner half, but there will be no power transfer to the belt. Thus, no cutting.
We need to remove the PTO to inspect the key. The easiest way is to jack the mower up, remove the deck drive belt, and remove the PTO.
It’s held on by one bolt, and the easiest method is to use an impact driver to remove this bolt. Without the impact, you’ll simply be turning the crankshaft over, unless you have the means to hold the flywheel in place. I’ve done this, but it’s much simpler to use an impact driver.
With the bolt out, grab your least cumbersome (if that’s a thing) set of pullers and pull the PTO off. You should see the key, and you can quickly tell if it’s missing or sheared.
Mower Deck Drive Belt
This is the belt that transfers power from the PTO to the cutting deck. Perhaps the belt is missing altogether, and that would be a simple diagnosis.
It’s also quite possible that it’s just plain worn. Over time they can stretch and then there’s not enough tension to adequately transfer the power. Or, if you spilled too much oil the last time you did an oil change, perhaps there’s not enough friction and the belt is just slipping.
Mower Deck Spindles/Bearings
It’s possible that the deck spindles are seized from rust and grass debris. This could prevent the blades from spinning, and the belt might just be slipping past them.
Recap
There you have it, a number of things to inspect if you find your riding lawn mower not cutting. Did I forget something? Let me know in the comments below.
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