BTW: when you tested the switch to determine if it was NC or NO, how exactly did you do it? Were the wires connected to the switch when you made the measurement? Was the VOM set to read “voltage” or “measure resistance”? Are you using an analog or digital meter?
Yes, to all.
That is what I thought.
It’s a PFM switch…
I did some google searching. I couldn’t find a wiring diagram for a riding mower, but I did find a service manual for a riding floor scrubber:
http://www.karcher.ca/Service%20Updates/Service%20Manuals/Service%20Manual%20br-bd%201000-e.pdf
It has a seat switch. And the manual provides detailed wiring diagrams and troubleshooting procedures.
The seat switch in the scrubber is normally open. Which only makes sense. Especially from a fail safe point of view. As we have already mentioned.
I know you’ve said over and over that the switch is normally closed. And perhaps it is, and we just don’t have enough information to solve the mystery. But it would only make sense that the designers would use a normally open switch. And it is behaving like a normally open switch. So I would triple check it, just to make sure.
The switch was tested out of the circuit.
The meter was set to read** AC Voltage**.
No, just kidding.
It was set to read ohms.
The n.c. switch will show continuity until it is open.
It is a 44 year old Heathkit analog VOM.
I have owned several digital meters, my first digital meter used nixie tubes for read out. All those meters have bit the dirt but my old Heathkit is still working great. Here I’m talking about Heathkit but it may have been before your time?
Not mine. I 've got an old Heathkit frequency counter that uses a nixie tube display. Still works fine, though it doesn’t compare to my nice handheld RF counter.
Yes, the one on the scrubber is n.o. switch.
No, mine is n.c., I assure you of that.
I know it’s very confusing… ** That is why I posted. **
It’s just seems impossible. 
Well, I’m officially stumped. Desmostylus? David Simmons? Billy? Bbeaty? Paging all EEs and electrical types.
Great! Glad to hear from another Heathkit user.
But My Heathkit VOM is about 8 years older than you.
The first computer that I operated used nixie tubes for read out.
An Ole Blue 1800.
My guess is that you’re looking in the wrong place. My mower, like yours, grounds out when there’s no weight on the seat. It’s not hard to determine how the switch works. One wire in, one wire out that goes to a visible ground screw.
Ones I’ve fooled with will allow you to disconnect the seat wire and get off without killing the engine, provided the blades are not engaged. However, if the seat switch is disconnected, activating the blades will kill the engine. Check to make sure the blade lever in not in the engage position, even if the mower assembly has been removed.
The blade safety switch probably has some cute engineering that involves both sides of the electric system. Once you find it, I’m sure you can figure it out. Been there, done that.
It’s all a matter of Big Brother protecting us from ourselves.
One possibility (which I have used in electronics gear used for human subject experiments, but can’e imagine on a rider mower) is a high impedance bypass (e.g. a 100K-1M resistor in parallel to the switch, inside the case) This is easily rigged for circuits with high impedance sensor lines (IC chips, which doesn’t sound like what you describe)
The reason I designed the circuit this way was to assure proper safety in the event of an external short or open (Or, in my case, misassembly during reconfiguration by my students). My university, desperte for tuition revenues, frowns on electrocuting undergrads or even grad students. Go figure. (I don’t know how they expect me to conduct serious science without toasting students. Sigh.)
I can sketch many simple circuits that might do this using components as simple as the thyristors (basically two SCRs head to tail) used in some AC solid state relays, similar to the DC SCRs used as automobile starter relays in the late 70’s. It might be prudent to use such circuits for a very high impedance trigger input in any high noise, high EM pulse environment (e.g. from the ignition coil), but I wasn’t aware that rider mowers did this.
You don’t want stray pulses from your coil or wires to trigger (vis inductance, leakage, or other parasitic means) the high impedance trigger (gate) input to a solid state relay. In lab gear with a 10Mohm or better input, we saw an occassional false trigger. Moderately high impedance trigger bypasses to ground are not uncommon in human subject gear, esp. if there is capacitance in the filter circuit
It may well be possible to rig the moderately high impedance bypass to ground elsewhere in the circuit (e.g. nearer the engine) but I can’t think of a set-up, off-hand, that would do this and display the behavior you suggest. It’s possible for a high impedance gate to work properly when the switch was in situ, but misbehave due to stray capacitance or a dying capacitor under a true open circuit
I don’t believe this hypothesis, not on a lawn mower, but it might get others thinking in new directions.
Sorry, I been there and done that ** too. **
Hey thanks guys!
My bed time, hope someone has an answer by morning.
What?
No solutions.
Maybe this will help… at least for the ones that didn’t believe it was a NC switch. 
http://mywebpage.netscape.com/arkie3935/Posting/electdrawing.jpg
According to that diagram, disconnecting the switch should allow the engine to run. If it doesn’t, there’s an engine problem someplace. It can’t be anything else, unless that diagram is wildly inaccurate.
The diagram looks good to me. Better than the one that came with my Murray (that I was gonna offer you) in that it shows more detail of switches and engine wiring.
Do I understand correctly that the engine will turn over but not start with the seat unwired? The simple and logical solutions aren’t working. So, what about the far-out and illogical? With the seat harness disconnected, can you hear or feel the fuel solenoid activate when you turn the ignition on? (With the clutch locked down, of course.) [Cute trick if you can do it from a hundred miles away.]
I’m thinking. I’m thinking.
Well… This one is a Murray.
NO, I have never mention anything about start… only mention run.
Fuel solenoid??? No such on this machine.
Hmm. It would appear it is a NC switch. Assuming the diagram is correct.
What’s interesting is that, according to the drawing, it should be possible to start the tractor with the PTO and clutch switches both actuated regardless of whether or not anyone is sitting on the seat.
In other words, it would appear that terminal 4 of the starter switch must be grounded in order to stop the tractor. But if the NC terminals on the PTO switch are open, and if the NC terminals on the clutch switch are open, then the seat switch is basically “out of the circuit” and the tractor should run regardless of the position of the seat switch.
No engine problem.
The drawing shows the ** PTO Switch Disengaged, **with the start circuit switch NO and the other switch NC. If that was true the engine wouldn’t start with the PTO disengaged. Looks like this part of the diagram is in error with the NC and NO switches reversed or the PTO Switch is engaged … but that has no effect on my problem.
I see you still don’t believe the seat switch is NC. 
No drawing error, as I have said I have checked the switch with a VOM.
See my post above about a possible error in the drawing.
True, you are correct, it can be started with no one in the seat… only when the brake pedal is released the engine will die unless the seat switch is open. You should note the wire that bypasses the PTO Switch going to the Brake Switch on to the Seat Switch.
It must be pretty old if it has no starter solenoid(called “afterfire solenoid” in the diagram), unless you’ve bypassed it with a bolt. (More of Big Brother’s rules.) That would be the thing on the bottom of the carburetor with two wires attacted to it.
Since you’ve excluded possibility of the pto handle being in the on position, I can’t think of anything else. It almost has to be going to ground somewhere that it shouldn’t, either in the yellow wire from the various safety switches or in the kill wire from the ignition. I by-passed all the safety cut-outs on one of mine and installed a push-button starter switch, but I don’t normally recommend that to anyone else.
Thanks again for putting us on to the wiring diagram. I’m printing a copy for my workbench since it looks pretty universal. Sorry I can’t help, but I’ll be watching for the solution.