The first time this happened to me, I was preparing my yard for a family gathering, time was not on my side and I needed the yard done yesterday. I found a quick-fix for my problem and I’ll share it here in this post.
So what’s up when a Husqvarna mower dies, when the mower blades are engaged? The mower grass box is not fully closed, which causes the safety sensor to remain open.
Other possible causes include:
- Grass Box Misaligned
- Grass Box Sensor Wiring Loose
- Grass Box Sensor Faulty
- Bad Gas
- Carburetor Dirty
- Carburetor Faulty
That’s a list of all the most likely reasons that the engine dies as you apply the blade button or lever. Now lets take a look at how we can quickly figure out what’s going on and how to fix it.
Grass Box May Not Be Fully Closed
As you already know, your mower is designed with several safety features built in. Various sensors are attached to components of your mower which relay their position to a main control panel.
Only if a set procedure is followed will the control panel allow the engine to start or stay running. The grass bag is one of the components that’s monitored by the control panel, and its sensor lives at the rear of the mower, between the mower and the grass bag.
As the grass bag/box closes, the frame of the grass bag presses on the sensor and closes it. This tells the engine control panel that the box is closed and it’s safe to engage the blades. Check that the box is closing fully, clear any grass that might be jamming it.
Box Open – This grass box is not fully closed and will cause the engine to shut down as you engage the blade lever or button.
Grass Box Misaligned
This often happens when you accidentally reverse into something and the grass bag support arms get bent out of shape. This as you’ve probably guessed, causes the grass bag frame to no longer press on the gasbag sensor.
The fix here is to straighten out the arms, so that the grass bag fits squarely. This can be a tedious job, but worth doing because a misaligned grass bag will be a right pain in the jacksie. You’ll have grass trailing’s and clumping – Horrible!
Quick Fix – If your grass box won’t close properly, removing the wiring from the back of the sensor and connecting them together will by pass the sensor.
Sensor wiring is accessed from behind the grass box at the rear wheel.
Use a piece of wire to jump the sensor, a car fuse works the best.
Loose Grass Bag Sensor
I know you’re already up to speed on what a grass bag sensor does and where it lives, now lets take a look at the wiring. This isn’t at all technical, so stay with me here. At the rear of the sensor you’ll hopefully find the wiring connector, depending on the model, it may be a single block or two push on connectors.
Check that they are fully connected, sometimes they can come loose or the wiring may be damaged. Remove the connector and check for corrosion, this as you know will prevent a proper signal being sent to the control panel. If the wiring terminals are dirty, try cleaning them with electrical contact cleaner, and use a small amount of silicone grease, that prevents moisture getting in.
If the wiring terminals are corroded, then we’ll need to replace them. You can buy a handy wiring repair kit at the auto parts store, it will have all you need, new push on connectors, joiners and tape or if you want to go the pro route then buy a soldering iron.
Grass Bag Sensor Faulty
This is not very common, the Husqvarna sensors are well made and tend to go the distance, never the less, we’ll need to check it’s function and output. For that we’ll use a volt meter. We can test the grass bag sensor in place, first remove remove the block connector and if your using a volt meter, set it to continuity, that’s the symbol.
With a test lead on each of the terminals of the sensor, press the sensor closed by hand, no change in the reading indicates a faulty sensor. Replacing the sensor is a simple two minute job, to release the old one push in the plastic wings and push the sensor out from the wiring side.
Grass Bag Sensor – The Husqvarna grass bag sensor is a four pin, although some mowers will only use two of them. The same sensor is fitted to other components that utilize the four pins.
Testing The Sensor – Set the volt meter to ohms, and check for continuity when the sensor is pressed. The meter should read close to zero ohms, if not replace it.
Bad Gas Causes Poor Running
Stale gas is a common problem for all small engines, the problem with stale gas – it’s less combustible than fresh. Although your mower may seem to run OK, stale gas will likely cause it to hesitate and stall when extra demands like run the blades or climb a steep hill are placed on the engine.
The fix here is simple enough, drain out the old gas and fill with fresh, use a gas stabilizer if your going to store your gas either in the mower or in a gas can for more than a month. Check out the “Carburetor troubleshooting guide”(internal link).
A lawnmower engine is very simple and forgiving. The carburetor that’s bolted to it, is neither of those things. It’s a finely tuned piece of kit, any slight change in the air/fuel balance will cause the mower engine to run poorly. A carburetor has two jobs, mix the gas to air at a ratio of 14.7:1 and to supply the correct amount of this mixed gas to the engine for any given engine load demand.
Dirt in any of these components, will cause fuel starvation, which in turn causes the mower to not start, lack power or stall when the engine is loaded.
Common causes of fuel starvation are:
- Gas tank outlet blocked
- Gas filter blocked
- Gas valve faulty
- Gas lines blocked
- Needle valve dirty
- Fuel bowl dirt
- Fuel jet blocked
- Failed fuel pump
Check out the complete “Carburetor troubleshooting” guide or check out the “Carburetor cleaning video” here. The videos are walk behind mowers, but the tractor mower carburetor cleaning process is identical. (internal links).
Carb Cleaning – Cleaning the carburetor and fresh gas will have it running sweet.
John Deere mower dies when blades are engaged? Your grass box is not fully closed, the safety sensor is shutting down the engine.
What causes a 4 stroke engine to backfire? A bad plug is a common cause of backfiring, but other possible causes include:
- Wrong plug type
- Faulty carburetor
- Broken shear-key
- Faulty shut down valve
- Stale gas / wrong type gas
- Intake vacuum leak
- Cylinder carbon build-up