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We have a Craftsman lawn mower LT1000 20HP model 917.271920. It had been running and cutting great.

The operator was riding the mower through a gate from the back yard to behind the fence when he ran into a big flat rock that was sometimes used to hold the gate open.

As he recalls, the blade was not engaged, and only the outside of the deck hit the rock. The blades look fine. However, the engine shut down immediately, and we have not been able to start it again.

When we turn the key, we just hear and feel a single loud thunk -- not just a click. With the transmission engaged (via rod in the rear), we feel the thunk under the hood and under the seat; when disengaged, only under the hood. The engine does not crank.

We tried to jump with a battery booster, but still only the single thunk.

The following sums up what we know:

Battery is new and has well over 12V
We also see 12V at the solenoid and at the starter
Cable connections look good
The fuse by the solenoid is good
We jumped the seat switch to make sure it was not the issue
The mower deck moves freely up and down
The mower freewheels, we can push it around

What would you suggest we check next? Thanks in advance!

The thunk sounds like possibly the starter pinion gear engaging the flywheel. Remove the cover and watch it when key turned to start.

The safety key on the fly wheel did exactly what it is designed to do and prevented damage to you & the mower by shearing off.
Pull off the engine cover and the flywheel nut/bolt.
You will see there is a square hole, 1/2 in the engine shaft, 1/2 in the flywheel.
On your mower they will not be lined up because the soft ally key that goes in there will have broken.
Pull off the fly wheel & replace the key.
Depending upon the model you will usually have to remove the coil (s) as well

Fly wheels camn be a bugger to get off but I find they resond best to a very big 2 jaw puller and ratle gun.
Remember to leave the nut / bolt in the end of the crank, undone about 1 full turn to prevent damage to the end of the shaft.

Thanks to both of you for your comments. Would the key only shear if the blades were turning? It's certainly possible that they were. Also thanks for the detailed instructions on how to check the key. We'll probably have to wait until the weekend to check.

The key shears because a large load suddenly is applied to the engine and the engine ie what was pushing you forward when you hit the rock.
Every time you banged into some thing, the blades jambed or any other shock load was put on the engine the key takes a bit of a hit and eventually it gives up the ghost.

It might not be the key but sounds very much like it.

Agreed with Bert. Sounds like a sheered flywheel key.

Thought I would report on the solution to the cranking thunk. Before I could make it down to take a look at the mower, a neighbor stopped by and 吐ixed it.

He lifted the deflector shield on the mower deck and noticed that the vortex baffle (159770) was folded back over the discharge. After using a hack saw to cut off the bent section, the mower cranked and started with no problem.

I don稚 see why a bent baffle would prevent the mower from cranking, since the blade was not engaged; but it did. The rock must have bent the baffle when the mower hit it.

Thanks for all your suggestions. We saved them for the next incident!

The baffle is what suspends the clippings for mulching. If it was bent in such a manner as to prevent blade rotation, that may have been enough to keep the engine from spinning. If you don't mulch I'd probably just chunk it.

Yes, I see how a bent baffle could prevent the blade from rotating; but the blade was not engaged while we tried to crank it.

wponder said:

Yes, I see how a bent baffle could prevent the blade from rotating; but the blade was not engaged while we tried to crank it.

Click to expand...

Ok. Then the only thing I can think of is the hit must have had such force as to bind the drive belt /pulley/guard to the point where the engine would not rotate upon starter engagement. When your buddy started messing with it, the bind was freed and the engine was a happy camper.

It's one of those "Unsolved Mysteries From the Twilight Zone" where mowers act strangely,
won't start when everything is in working order. I've seen such mystery machines in my
day I tell ya. Get everything the way it's supposed to be and it still won't run or run right.
I've seen weedeaters throw tantrums and test the patience of a Saint! No wonder people
throw them out on the curb to get rid of them. Riding mowers are not immune to this type
of behavior either. We should start another category on here called "Mowers From Hell" and
listen to the horror stories, the mysteries of why they won't run as if they were in the Bermuda
Triangle or some such place. LOL Is your machine like that? :thumbdown:

DaveTN said:

It's one of those "Unsolved Mysteries From the Twilight Zone" where mowers act strangely,
won't start when everything is in working order. I've seen such mystery machines in my
day I tell ya. Get everything the way it's supposed to be and it still won't run or run right.
I've seen weedeaters throw tantrums and test the patience of a Saint! No wonder people
throw them out on the curb to get rid of them. Riding mowers are not immune to this type
of behavior either. We should start another category on here called "Mowers From Hell" and
listen to the horror stories, the mysteries of why they won't run as if they were in the Bermuda
Triangle or some such place. LOL Is your machine like that? :thumbdown:

Click to expand...

That would be a cool category.:thumbsup:

logan01 said:

That would be a cool category.:thumbsup:

Click to expand...

I have an LT 1000 2001, while mowing my daughters lawn 30 miles from home, I hit a rock with corner of the deck, not the blades. Would not start, no lights , fuse not blown, took home to repair. Found bad wire from feed on starter to positive side of fuse. Wire was in loom , no reason that the jolt of the rock could even damage the wire on the other side of mower but that was the problem. Fixed 2 months ago no problems since.