Ask Me Help Desk - Turning on PTO kills the engine

Craftsman 23hp garden tractor 917.275034

Engine died while cutting the grass. Troubleshooting reveals:

1. Engine runs when PTO swtch is turned off
2. Clutch switch checks good
3. Seat switch checks good
4. Operator Present relay tests good
5. Electric clutch winding reads about 5 ohms

I took apart all connectors and cleaned them - no change. There are no broken or loose wires apparent and no shorts to ground that are apparent. I suspect the problem is either the PTO switch ($29) or the electric clutch assembly ($200+). I'd hate to order both considering that I can't find a difinative fault and the problem could be somewhere else. Bummer!

Anybody out there know the correct resistance that should be measured across the clutch winding? Or maybe you've seen this very problem before?

All help appreciated!

You didn't indicate specifics of how the engine stalls, but all of your troubleshooting assumes an electrical cause. Did you check for mechanical causes such as something stuck in the mower, or a seized clutch/mower bearing, etc. Was there any squeal when the engine stopped while mowing?

I'd also recheck and then check again the operator present relay and seat switch. Those are the only two devices you list that would have any effect on the engine electrics.

Quote:

Originally Posted by Scleros

You didn't indicate specifics of how the engine stalls, but all of your troubleshooting assumes an electrical cause. Did you check for mechanical causes such as something stuck in the mower, or a seized clutch/mower bearing, etc.? Was there any squeal when the engine stopped while mowing?

I'd also recheck and then check again the operator present relay and seat switch. Those are the only two devices you list that would have any effect on the engine electrics.

First, thanks for your good suggestions. The engine stalls without sputter - classic for an electrical interruption just as you surmised. There is no squeal when it dies. The seat switch is normally open so I jumpered it (short) for the purpose of testing. As to the relay, I removed it from the tractor and tested it out of circuit. It's definitely good. One point I forgot to mention - my final step in isolation so far was disconnecting the connector to the clutch assembly. When the PTO switch is engaged, the engine keeps running. With the connector connected, engaging the PTO kills the engine. The mower turns freely as does the clutch pulley. Do you by chance know the correct expected resistance across the clutch? The schematic shows it as a simple primary winding. Heck, the correct reading could be the 5 ohms I'm getting or 400. No way to tell. Your seized bearing suggestion has merit and that of course would lead me back to the $200 solution but that's probably it.

Thanks again!

Quote:

Originally Posted by scnichols

With the connector connected, engaging the PTO kills the engine. Do you by chance know the correct expected resistance across the clutch?

No, but you could sort of tell by the size of the wiring how much amperage should be going through the coil and whether it's 5 or 500 ohms.

Quote:

Originally Posted by scnichols

The schematic shows it as a simple primary winding...

I pulled down the manual and had a look at the schematic. If I'm reading things right, had you a short to ground in the clutch coil, engaging the PTO switch would ground the whole A1 ignition switch rail effectively dropping out the operator presence relay coil by grounding terminals 85-86-30-87A, which would then also ground the M ignition switch rail via the PTO switch terminals D-A thus grounding the ignition unit.

If the clutch coil wiring is light, you could try disconnecting the clutch and jumpering the wiring harness clutch coil connector with a 1/4 watt resistor jumper, and see if engine continues to run when PTO switch is engaged (would also work until resistor blew if wiring was heavy). I'd probably just engage the PTO long enough to see if engine stops too.

Quote:

Originally Posted by Scleros

No, but you could sort of tell by the size of the wiring how much amperage should be going through the coil and whether it's 5 or 500 ohms.

I pulled down the manual and had a look at the schematic. If I'm reading things right, had you a short to ground in the clutch coil, engaging the PTO switch would ground the whole A1 ignition switch rail effectively dropping out the operator presence relay coil by grounding terminals 85-86-30-87A, which would then also ground the M ignition switch rail via the PTO switch terminals D-A thus grounding the ignition unit.

If the clutch coil wiring is light, you could try disconnecting the clutch and jumpering the wiring harness clutch coil connector with a 1/4 watt resistor jumper, and see if engine continues to run when PTO switch is engaged (would also work until resistor blew if wiring was heavy). I'd probably just engage the PTO long enough to see if engine stops too.

You were certainly barking up the right tree there buddy. Inserted a resister bridge as you suggested (I was too frustrated to think of it myself) and it killed the engine when the PTO switch was turned on. That's when it hit me. The clutch primary has such low resistance that it probably has a high in-rush current, requiring a bit of battery to make up the difference from the charging system. Oh yes, I forgot to mention my weak original, 5-year old battery. So, I hooked everything back up, started the engine with jumper cables and turned on the PTO with the jumper cables still attached. You guessed it, the PTO works just fine. Now I have to wait an hour for the local auto parts store to open to buy a new battery. Oh well, it is what it is.

Thanks for taking time out of your day to throw some great ideas my way. I'll post a final comment if the battery does the trick.

Whew, that was a simple fix, but took a lot to get there huh. Good job guys.

Quote:

Originally Posted by scnichols

You were certainly barking up the right tree there buddy. Inserted a resister bridge as you suggested (I was too frustrated to think of it myself) and it killed the engine when the PTO switch was turned on. That's when it hit me. The clutch primary has such low resistance that it probably has a high in-rush current, requiring a bit of battery to make up the difference from the charging system. Oh yes, I forgot to mention my weak original, 5-year old battery. So, I hooked everything back up, started the engine with jumper cables and turned on the PTO with the jumper cables still attached. You guessed it, the PTO works just fine. Now I have to wait an hour for the local auto parts store to open to buy a new battery. Oh well, it is what it is.

Thanks for taking time out of your day to throw some great ideas my way. I'll post a final comment if the battery does the trick.

Well, that was it folks and we can put this frustrating event behind us. Installed the new battery and proceeded to finish the last acre of mowing without incident. What kills me is that the guy at NAPA said he has a similar tractor with a bad voltage regulator so the charger doesn't work. He knows when to put it on the battery charger (are you ready for this?) when the PTO won't engage!

This is one I sure won't forget.

The End!