Engine stalls when blades engaged

Hi all,

I was mowing today, and the engine stalled for no apparent reason. It started again just fine, but if I engage the blades, the motor shuts off instantly (no sputter, just stalls). Even if I just move the lever to engage the blades a tiny bit, the engine immediately dies.

I have read several posts on the internet which have stated that this is a problem with the safety switch on the seat. I suppose I should just order this and replace it, but first I wanted to ask:

This might be unrelated, but I have attached photos of a little box that has wires plugged into it. It has a piece of metal sticking out of the end that looks like it should be screwed into something. It just hangs loose in the engine. Could this cause an issue with the engine? If so, what trouble would it cause? I assume it is no problem, but I wanted to double check.

And back to the seat switch: a few posts mentioned about unplugging a wire in the engine that would prevent the switch from shutting off the engine (in order to confirm that the switch is indeed the problem). I couldn't seem to figure this out. There are several wires that come out from under the gas tank. I assume most of these wires are coming form the switch.

The engine is Briggs & Stratton 28N707, the mower is a John Deere Sabre 15.5/42 HYDR

Thanks for any pointers. I appreciate your time and efforts. I am not too familiar with repair, but I am willing to try to figure these things out.

The little box is a relay and the metal tab is the mounting bracket.
It should be bolted to the mower somewhere to keep it out of the way but the mounting bolt has fallen off.
It will not affect the mower in any way but should be tied up somewhere out of the way to prevent the wires chaffing through and causing a short circuit.

Your problem could be electric or fuel.
Start the mower then apply the brake. get off and manually increase the engine speed by moving the throttle to full open .
It is the butterfly at the back which should have a thin rod attached to it going to a long flat rod.
If the engine dies when the throttle is opened fully and please do not hold it open for more than 20 seconds or so to prevent over reving damage to your engine you have a fuel problem.
If it accelerates happily and will stay wide open for 20 seconds or so then the problem is with your electrics.
Check the seat switch by starting the engine then raising off the seat.
With the parking brake on the engine should stay running.
With the parking brake off the engine should cut out.
Do it both ways several times and try getting off then releasing the brake.
If it works properly every tie then the seat switch is OK.
The wiring between the seat switch and the PTO switch might be faulty but the switch is OK .

Hello, and thank you for your informative reply.

I just started my mower, put the emergency brake on, and the mower continues to run if I get off of the seat.

Also, with the emergency brake off, the mower shuts down if I get off of the seat, so the seat switch seems to be working in that regard.

While I had the engine mower running, I turned the throttle down to the lowest setting, and then back to the highest setting, and it seemed to react normally and run fine at all throttle settings.

My suspicion is that if there was a gas problem to the effect that there is not enough power to turn the blades, then the mower would not shut off so immediately when the lever to engage the blades is just barely moved. The mower shuts off in the same exact way when I touch the blade engage lever as if I were to rise off of the seat to cut the engine off. It doesn't seem that there is more of a load being put on the engine when the leaver is moved less than an inch, but the motor shuts off whenever it is moved just the slightest bit (and I mean not anywhere close enough to actually engage the blades).

You mentioned something about wiring between the seat switch and the PTO switch. Could this be something that got disconnected or corroded?

Also, I am not sure if I understood right about testing the full throttle. Did you want me to use the throttle lever to turn the throttle to full, or did you want met to actually hold open the butterfly flat metal piece on the engine further open than the throttle adjustment moves it?

Thanks again. If anyone has any tips to further diagnose the possibility of the switch causing this issue, please let me know. Otherwise, when I get a chance, I will try to continue researching other people with similar problems to see if I get get any other ideas before ordering a new seat switch.

Good good, we are getting somewhere.

Yes I want to to open up the throttle butterfly , slowly all the way.
When you engage the blades the engine slows and the butterfly opens up to compensated for the slow running engine.
If there is a fuel supply restriction or air leak when the throttle is fully open the engine starves out but from what you have said so far it really sounds electric
And the problem will be between the seat switch and PTO switch.
I will have a dig around to see if I have a wiring diagram for your mower.
JD like to use relays and it could be the relay you had in your hands that is the problem after all.

Hello,

I will attempt to find a wiring diagram at some point this week when I have time. If I find it, I will link it to this thread. Thank you so much for your advice.

Also, I will get back to you once I can test the throttle butterfly as you have suggested. Thanks again.

The best I have been able to come up with are some parts diagrams. I don't know if these will help at all...

The reason your mower shuts off when you start to engage the mower is that a wire under the mower some where has had the insulation rubbed off and either the lever or the linkage contacts the bare wire and grounds out the engine. You will have to look under the mower and move the lever and see what is rubbing any wires under the mower. Any wire that is contacted by the lever or linkage should be inspected. When you find the bare wire tape it and move it away from the lever or linkage. I view those "SAFETY SWITCHES" as a PITA and bypass them, my $.02.
Good luck

Dana

Thank you Dana for your input. I will try to follow some of the wires underneath to see if I can come up with anything.

If I had a better understanding of the wiring, I would bypass the safety switch altogether, but I do not want to cause more issues with the wiring than I already have.

I'd advise all of you to keep what you do in respect to Federal EPA required controls to your selves and hope you are never involved in a personal injury accident with your mower.

Walt Conner

Tinkerer200 said:

I'd advise all of you to keep what you do in respect to Federal EPA required controls to your selves and hope you are never involved in a personal injury accident with your mower.

Walt Conner

Click to expand...

Hi Walt

I have Toro riding mower and I was curious if you would know why they would make it so it slows to a crawl when you turn the steering wheel. It only goes 3 or 4 MPH when going straight and when turning the sharper you turn the slower it goes. It only goes about 1/2 MPH in a sharp turn. 3 or 4 MPH isn't fast enough to upset even in a sharp turn. When mowing I turn a lot and it was so annoying I got rid of that feature too.

Dana