No Spark on crank, but spark on run.. help??

Hi All,

Need some help here. Very familiar with automobiles, not so much with small engines. Just fixed needle and seat in carburetor of briggs 28Q777 single piston 15.5 (?) HP engine. Was severely leaking fuel into oil. Also did a head gasket, valve cover gasket, adjusted lash on the valves and air gap on the armature. When running, the engine runs VERY good, strong and smooth, not burning anything, sounds good...excellent.

However, this engine has ALWAYS been a bit difficult to start. I noticed something very interesting tonight. When the engine is cranking (electric start), there is NO spark that's occurring. I have the spark plug out, gapped to 0.030" and grounded against the head. However, immediately after releasing the key (so the key position is in "run") there is spark visible. I tried this many times, and always the same, no spark when the engine is cranking, but spark when I release the key. Thinking back, this has always been how the engine was (inherited the mower about 6 years ago). It will start when I release the key and the engine catches, and fires right up and runs like a top.

Curious, I disconnected the electrical connector to the magneto, the one I think is responsible for grounding it out. Now I get spark when I'm cranking, but the engine won't fire. I suspect it's because this is also shutting off fuel through the solenoid in the fuel bowl.

So, Briggs and stratton geniuses out there...what do you think is going on? What would be causing the magneto to ground in the crank position? If you'd like me to test something, please indicate what you need and how you'd like me to test it.

Many thanks,
Brian

I suspect the ignition wire on the switch is in the wrong location. Or the switch is bad. The other guys that work on them regularly can help you more than me. Me I'd have to chase wires they know what wires go where.

Well, let's start simple. The magneto has one wire, and the spark plug that comes off it. What should that wire to the magneto do? Does it ground the magneto? I need to understand how the function of these components work to properly test it. I assume it's not a power wire since it's just working on electromagnetism and doesn't need additional power to function.


Thanks,
Brian

I think RDA may be on the right track, but to me it sounds like a pinched or cut wire in the start circuit. Please post the model and serial numbers for the unit, so I can get a wiring diagram.

Rivets said:

I think RDA may be on the right track, but to me it sounds like a pinched or cut wire in the start circuit. Please post the model and serial numbers for the unit, so I can get a wiring diagram.

Click to expand...

HI Rivets, thanks for your interest.

Briggs model number 28Q777
Type: 0690-E1
Code: 9903292E


Mower is a Toro/Wheelhorse 15-38 HXL

Found a wiring diagram on partstree.com http://www.partstree.com/parts/toro...tor-sn-9900001-9999999-1999/wiring-schematic/

Not sure if this is the definitive source for this type of stuff.

If you can give me some of the theory behind the wiring to the magneto and kill switch/ignition switch, I can start testing from my end. I've attached a photo of the back of the ignition switch. If you need other shots, let me know.

Thanks again,
Brian

Download this manual and use the section for 15-44HXL or XL, which will be the same wiring as your tractor. I know it is big and might look overwhelming, but if you have electrical troubleshooting experience, you will understand it. You are looking for a short to ground in the cranking circuit, which is linked to the run circuit. I'll check back when I can, so if you don't see me for a couple of days, it's just that I am working 50 hour weeks. Not much time to give to the forum. Hope you understand, I hope some of the other Techs will chime in if I'm not around. There are a couple of good electrical troubleshooters out there.

https://lookup3.toro.com/request/gogetpub.cfm?vSvcDir=/manuals/&vPubNum=trpdemys.pdf&vKey=6327

Rivets,

Thanks again for your interest and that info. I'll definitely download that manual for future use. Tonight, I went back outside and tested some more, and I believe that RDA is correct. I'm finding that in the crank position, the "kill" relay is being triggered. I found the connection at the ignition switch that makes this happen. So I believe internally the switch has failed, and is triggering this kill relay when its not supposed to. To solve this, I disconnected the kill wire to the magneto and cranked it. It started almost immediately. When I return the switch to the off, it runs for about a second or two, and then dies because the fuel solenoid is working properly, and stops the flow of fuel. So I think I'm going to just run like this for a while. I've left the kill wire easily accessible, so if I decided to put a second switch in, I can do that. Or, if the fuel solenoid dies, I can just ground it to the frame to shut the motor off. I've got other projects to do with my time, so this is a solution that works for me.

Thanks again for your time, and don't work too hard!

Brian

Briggs engines ground in the switch to stop the engine,shorted switch or wrong switch. Diode open won't shut off. Closed diode no spark. Your diode is ok ,it will run,and stop . Switch???????????????? Diode in harness to coil.

Single cylinder engines don't need a diode in the shut down harness.
Key switch is my suggestion as others have mentioned.

Switch has short, in crank position. I am old retired mechanic. 50 years on auto, and inboard boats.