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Discussion Starter · #1 ·
I just replaced 2 neutral interlock switches, and also the 6-prong PTO switch but I'm still unable to engage blades. Motor dies as soon as I engage blade clutch.

18 hp Vanguard engine is fairly new, was R&R-ed a few years ago, starts up on first or second pull of rope.

I'm guessing it's shorting out in the wiring harness somewhere. The only way I can move the mower is to keep the interlock switches pressed down by tying them down with string to imitate parking brake engaged and gear is in start position. I can move mower but still can't engage blade clutch without the engine dying out.

Any suggestions before I take the mower to shop? Local shop has raised labor to $100/hr. now.
 

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I am a homeowner, and I don't have your machine. I just bought a 2022 Scag SFC30.
Despite all that, maybe a couple of questions breaking down the problem might help.
I just noticed with my SFC30, that if my engine speed/throttle was 1/3 or less, my engagement of PTO would conk the motor out.
Is your engine speed high enough when engaging the blades and pto? That's first that comes to my mind.
Again, I am not familiar with all the particulars of the machine, and probably should "put a sock in it".

OK, in the case that question is a "red herring" (meaning false lead), maybe there could be something electrical with the pto switch interacting with some of the abnormalities of the interlock switches that you have to "jerry-rig" to get the mower to move.
My next guess would be that after you've moved the mower with your "tying down the interlock switches", you probably have to undo that "jerry-rig" thing, to then engage the pto blades(?). Maybe the logic built into the wiring, says, if those interlocks are down, the mower is in a moving state where pto cannot be engaged. Any attempt will shut the engine down as an emergency fail safe. Try undoing your "interlock tie down work" and seeing if you can engage the pto.
 

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Discussion Starter · #7 ·
I am a homeowner, and I don't have your machine. I just bought a 2022 Scag SFC30.
Despite all that, maybe a couple of questions breaking down the problem might help.
I just noticed with my SFC30, that if my engine speed/throttle was 1/3 or less, my engagement of PTO would conk the motor out.
Is your engine speed high enough when engaging the blades and pto? That's first that comes to my mind.
Again, I am not familiar with all the particulars of the machine, and probably should "put a sock in it".

OK, in the case that question is a "red herring" (meaning false lead), maybe there could be something electrical with the pto switch interacting with some of the abnormalities of the interlock switches that you have to "jerry-rig" to get the mower to move.
My next guess would be that after you've moved the mower with your "tying down the interlock switches", you probably have to undo that "jerry-rig" thing, to then engage the pto blades(?). Maybe the logic built into the wiring, says, if those interlocks are down, the mower is in a moving state where pto cannot be engaged. Any attempt will shut the engine down as an emergency fail safe. Try undoing your "interlock tie down work" and seeing if you can engage the pto.
If I have to drive the mower to the local repair shop, neutral switches will have to stay rigged in "normally open". Otherwise it only moves by pushing brute force.

If I connect the (2) new neutral interlock switches, the engine shuts down as soon as parking brake is released, switch goes from normally open to normally closed which causes a short somewhere. Same for gear shift neutral switch. So that's why I have to bypass the 2 switches and rig up the old switches so they stay "normally open" and don't ground spark to engine block when brake is released and/or gear is engaged forward.

Is there a third neutral/interlock safety switch I'm missing somewhere? This is a 48in walk behind Scag hydro, old frame but new engine. Something is obviously grounding spark when system is ready to run and mow, which suggest OEM wiring harness may be shorting something, unless it's another hidden 3rd neutral switch I missed somewhere. My blade clutch is fairly new, replaced prior to new engine swap.

Engine is idling okay and running fine since it's basically a new block, $1000 engine direct from the Briggs Vanguard factory in Georgia. This electrical problem just started, I've been cutting yards with the mower fine and dandy for the last year and now this is the new headache.
 

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Check the belt and all the pulleys, if anything isn't spinning (like a frozen bearing somewhere) or the belt has got pinched in a tight spot so it can't rotate that would likely result in an engine dying as soon as the pto is engaged. Blades too, are they able to spin freely?

Might not solve your problem...
 

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Discussion Starter · #9 ·
Check the belt and all the pulleys, if anything isn't spinning (like a frozen bearing somewhere) or the belt has got pinched in a tight spot so it can't rotate that would likely result in an engine dying as soon as the pto is engaged. Blades too, are they able to spin freely?

Might not solve your problem...
Again, I'm leaning toward the problem being most likely an electrical short in wiring, since I can bypass circuit with the 2 neutral switches tied down in "normally open".

All three blades, all three spindles and the two belts (drive belt and blade belt) appear to function normally which would NOT suggest a mechanical fault. If a belt was off the pulley or something was binding related to blade clutch or spindle, engine should still run when parking brake is released and PTO switch isn't being engaged. But that isn't happening.

I'll probably be loading up mower for a ride to see mechanics next week. Sometimes you got to let the local Scag experts take it in for inspection.
 
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Again, I'm leaning toward the problem being most likely an electrical short in wiring, since I can bypass circuit with the 2 neutral switches tied down in "normally open".

All three blades, all three spindles and the two belts (drive belt and blade belt) appear to function normally which would NOT suggest a mechanical fault. If a belt was off the pulley or something was binding related to blade clutch or spindle, engine should still run when parking brake is released and PTO switch isn't being engaged. But that isn't happening.

I'll probably be loading up mower for a ride to see mechanics next week. Sometimes you got to let the local Scag experts take it in for inspection.
The reason I mentioned belt and path and pulleys is because when unsure of the cause we start down a list of possibles, then we start with the easiest and cheapest... It came up while doing a google search, so I thought I'd mention it. Easy to overlook, it's not always the obvious, the mower dies when the PTO is engaged...

Because the next thing is to go through the wiring piece by piece... Starting anywhere, go through the whole thing until you find what is wrong. Last time I checked shop labor rates were in excess of $100 an hour, I hope your dealer is reputable and knowledgeable.
 

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So, a couple of things as I read.
You say that pushing by brute force, with taking out your rigged neutral switches, you can mow? Meaning the pto works, but only works then?
That's significant. So confirm that.
Secondly, is this an electrically engaged pto clutch? It has an electrical switch and electrical motor behind it to engage the clutch?
So, perhaps, if the pto is electrical, and when you disable drive by undoing your neutral switch rigging, you're somehow able to engage the blades with the electric pto, but just not at the same time as having the drive(rigged)? Granted that's not a workable solution.

I doubt it's worth exposing and thinking about yanking out a wiring harness that may have been replaced with the new motor.
Especially where it worked fine and normal post replacement for quite a while.

You sound like you're somewhat knowledgeable about electrical troubleshooting. Maybe it's worth getting a hold of a wiring diagram for your model?
It could be, and speculating, guessing here, that the pto circuit needs to sense some sort of load/resistance in the neutral interlock switches circuit, otherwise it declares an abnormal condition and it trips a fail-safe shut the motor down.

Thiose neutral interlock switches are obviously abnormal, because you had to :"jerry rigg" them, tie-ing them down. That miight close a circuit to drive forward, :"getting the job done, ALL's normal", but those "failed" switches could be presenting some other anomaly to that pto circuit.
Maybe there's a shop manual procedure to test the neutral interlock switches. They're probably more sophisticated then just a knife-balde, open-close switch.
AND,my guess is those neutral interlock switches are probably still abnormal and affecting things, THOUGH, they allow you to drive forward when tied down.
 

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Discussion Starter · #15 ·
So, a couple of things as I read.
You say that pushing by brute force, with taking out your rigged neutral switches, you can mow? Meaning the pto works, but only works then?
That's significant. So confirm that.
Secondly, is this an electrically engaged pto clutch? It has an electrical switch and electrical motor behind it to engage the clutch?
So, perhaps, if the pto is electrical, and when you disable drive by undoing your neutral switch rigging, you're somehow able to engage the blades with the electric pto, but just not at the same time as having the drive(rigged)? Granted that's not a workable solution.

I doubt it's worth exposing and thinking about yanking out a wiring harness that may have been replaced with the new motor.
Especially where it worked fine and normal post replacement for quite a while.

You sound like you're somewhat knowledgeable about electrical troubleshooting. Maybe it's worth getting a hold of a wiring diagram for your model?
It could be, and speculating, guessing here, that the pto circuit needs to sense some sort of load/resistance in the neutral interlock switches circuit, otherwise it declares an abnormal condition and it trips a fail-safe shut the motor down.

Thiose neutral interlock switches are obviously abnormal, because you had to :"jerry rigg" them, tie-ing them down. That miight close a circuit to drive forward, :"getting the job done, ALL's normal", but those "failed" switches could be presenting some other anomaly to that pto circuit.
Maybe there's a shop manual procedure to test the neutral interlock switches. They're probably more sophisticated then just a knife-balde, open-close switch.
AND,my guess is those neutral interlock switches are probably still abnormal and affecting things, THOUGH, they allow you to drive forward when tied down.
"...You say that pushing by brute force, with taking out your rigged neutral switches, you can mow? Meaning the pto works, but only works then?
That's significant. So confirm that."

With the Scag 48in hydro WB mower, clutch is powered by magneto energy, so yes it's electric.

Also you disengage each of the 2 hydraulic pumps by pulling down on a lever located on each side of pump. Allows for zero resistance when pushing a dead mower or pulling it up on a trailer with a winch. I can not get blade clutch to engage at all, as soon as electric PTO switch is flipped on, something grounds out the circuit -- no spark to fire plugs and no current to operate clutch and turn the blades.

I would no longer assume the old neutral switches are bad, since I already R&R'ed with brand new ones and problem didn't go away. Likewise for replacement PTO switch.

BTW I installed new 18hp Vanguard engine myself, fairly simple. OEM wiring harness. To R&R engine, the trick is properly torquing down the electric clutch to the Briggs vertical crank, i.e. the ole "Indian rope" trick, you snake a rope down into a cylinder wall to lock up engine to torque it down. But I diverge here.

As long as I can drive the mower onto my trailer, it's heading to mower shop ASAP. My backup 36in Exmark slows me down on the big yards.
 

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Ok. Interesting. So the wire branch from the magneto to the pto switch is likely fine. The magneto supplies current always , but it won’t flow through to the clutch engage motor, because switch is open until you press it to engage the blades. So wire branch from the switch to the clutch motor could have been chafed or cut, and when current is supplied via switch action, you get your short and overload.
But I suppose the clutch motor could be fused and fuse blown or it’s wiring is burnt and shorted.
An interesting test might be attach an ohmmeter lead to the magneto wire input blade of the pto switch and the other lead to an exposed nut on the frame. Operate the switch and see if you get continuity or a very low resistance. Not sure if the switch is simply mechanical and you could get a valid reading with the engine off. It might need to be powered to move it’s mechanical plunger- solenoid like.
Anyhow, good luck.
 

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Ok. Interesting. So the wire branch from the magneto to the pto switch is likely fine. The magneto supplies current always , but it won’t flow through to the clutch engage motor, because switch is open until you press it to engage the blades. So wire branch from the switch to the clutch motor could have been chafed or cut, and when current is supplied via switch action, you get your short and overload.
But I suppose the clutch motor could be fused and fuse blown or it’s wiring is burnt and shorted.
An interesting test might be attach an ohmmeter lead to the magneto wire input blade of the pto switch and the other lead to an exposed nut on the frame. Operate the switch and see if you get continuity or a very low resistance. Not sure if the switch is simply mechanical and you could get a valid reading with the engine off. It might need to be powered to move it’s mechanical plunger- solenoid like.
Anyhow, good luck.
Wow. You have absolutely no idea what you’re talking about. Why are you trying to help?

A magneto doesn’t supply any voltage to the electrical system whatsoever.
An electric PTO does not have an engagement motor.
An electric switch is just a contact switch, not a solenoid.
The reason the engine is dying is because the safety interlock system isn’t working correctly. There is no problem with the PTO. The safety interlock kills the engine if proper conditions are not met. That’s the system that isn’t working.

Duuuuuude. Wow. Just stop. 😵😵
 

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As long as I can drive the mower onto my trailer, it's heading to mower shop ASAP. My backup 36in Exmark slows me down on the big yards.
Yes! Do this! All the parts you changed out likely were not bad, and you probably got an incorrect switch along the way. Or even plugged a wire onto one backwards. Point is, it sounds like you had one problem, and made a couple more trying to “fix” it.
 

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Discussion Starter · #20 ·
Wow. You have absolutely no idea what you’re talking about. Why are you trying to help?

A magneto doesn’t supply any voltage to the electrical system whatsoever.
An electric PTO does not have an engagement motor.
An electric switch is just a contact switch, not a solenoid.
The reason the engine is dying is because the safety interlock system isn’t working correctly. There is no problem with the PTO. The safety interlock kills the engine if proper conditions are not met. That’s the system that isn’t working.

Duuuuuude. Wow. Just stop. 😵😵
Clue me in on this one. Having "no idea"? FYI voltage is induced by magnet attached to flywheel. Where does electrical energy come from, to activate blade clutch, if it isn't from the energy created by the magneto? With no battery, no alternator? Pull start motor only. Just so you know, a magneto DOES create voltage, technically it supplies pulses of high voltage. Other electronic parts in the circuit are using that magneto voltage to supply current to the electric clutch so it can move the blades. The original Briggs points-style magnetos had about 6 -8 volts on the primary when pulling the starter rope and around 300 - 400 volts on primary when actually running.

WRT the electric PTO, I never stated anything about an "engagement" motor, Also I never used the word "solenoid" in any of my original threads here. You're projecting words on me, which I never stated.

Yeah, I agree that "safety interlock system isn't working correctly". That statement and 99 cents might get you a cup of coffee. Although all comments are appreciated, yours are no better than any of the other posts, basically which are as good as a needle in a haystack right now. I'll let the Scag mechanic confirm that I have a bad wiring harness somewhere. I'll let you know what the professional Scag mechanics find out for me this week. I'm 95% confident that I've already ruled out neutral switches and PTO switch at this point, so far spending about $100 in parts that I didn't actually need but sometimes you get lucky with the shotgun approach.
 
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