Wiring circuit for starter interlock switches on flail mower

Hi and apologies if this is not an appropriate post for this NG.
A friend has an ancient flail mower without any functioning interlocks. This means that it can be started in gear with the clutch engaged. As it has an 11Hp engine this is not to be tried more than once. There are switches which detect the position of the clutch handle and whether neutral has been selected on the gearbox but they are not connected and there is no loom as such with obvious gaps that they could fill. There is only a (working) kill-switch which earths part of the ignition circuit. However, this is supposed to incorporate the gearbox and clutch switches.
He has asked me to come up with a circuit that does just that. Any thoughts? I've asked for details of the operation of the two unused switches. I think that these beasts usually require the clutch to be held in when starting and this is achieved with a clip rather like those on the old auto-stop fuel filling nozzles that drops out when you squeeze the clutch handle.
TIA and happy thinking
Richard

On Thu, 14 Jun 2012 15:00:59 -0700 (PDT), Richard Savage
Yea, this isn't. An electronics NG such as sci.electronics.repair would be more appropriate (though the name would imply integrated circuits and resistors and such, it'll do nicely).
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Define "Ancient". Some people think anything over 10 or even 20 years old is "ancient".
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At 11hp, is this an electric start, or a pull start (I pull start my "ancient" 16HP tractor, but only because I need to replace the toothed drive on the electric starter, and it's easy enough to pull start, so I haven't). If there's a battery, you have the benefit of being able to use a relay or otherwise actuate one that is present. If not, a lot more clarity is needed for the electrical schematic.
I've got to think it's electric start, because if it were pull start, you'd have a hell of a time managing to pull it fast enough under load.
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You need to use a DMM and take measurements from those switches to determine if they're open (no continuity / infinite resistance) or closed (continuity / zero or so resistance) when their respective controls are in the position where they should't start. Measure both switches, and run through the various positions for them (which also ensures the switches DO something - perhaps the wiring was gutted because one ceased functioning). From that, you can make a "truth table" to figure out whether you're interrupting ground or making ground, running in series, or in parallel to the solenoid on a relay.
There's also the matter of whether the switches have two vacant terminals, or if one is tied say to ground.
I'm guessing they're open circuit when it's a no-start situation - in which case, run them in series inline with the starter input from the ignition switch. If they're closed circuit when it's no-start, it'll require a bit more doing (unless the starter relay has a disable terminal).
Get a fix on what's what and then post to an electronics NG to continue your fix.
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I wonder - has anyone considered contacting the manufacturer (or the company which may have purchased them, as often happens), or looked for sites for such implements (search for the manufacturer and model). You may be surprised.
Last year, some fella drove about 230 miles round trip to pick up a garden tractor carcass (no engine) from me. There's some people really into some older gear.

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Thanks Sean. Have asked for clarification of switch operation, age/ make/model and starting method and will post that here and to an electronics NG.
Meanwhile this is the first reply from Robin:
"They are push to make switches. NC when clutch engaged or gear engaged, NO when not engaged or no gear selected. At least I'm pretty sure that's the case. I did wire them up originally much as you'd first think of wiring them up, only to have the engine die when you engaged *both* clutch and gears. "
Richard

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More:
It's a 1970s or more likely early 80s Saxon pedestrian flail mower with a Robin EY40B pull start engine fitted with the separate coil and KTR unit. Both the switches are push to make, the clutch switch makes on clutch engagement, the gearbox switch makes on reverse gear selection only. As far as I can work out, they were originally wired in series as that's what the cable remnants seem to determine.

On Fri, 15 Jun 2012 01:02:31 -0700 (PDT), Richard Savage
[snip - my original reply]
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Pullstart complicates things, because that means you don't have 12V to drive a relay. Also means there isn't a starter relay (or start inhibit relay). Since the switches are closed when the selectors are in the "shouldn't start" position, it's probable they're intended to provide an alternate ground path for the igntion (kill). If simply wired to the ignition kill, as your friend found, it'll still kill it when the engine is running and you go to engage the implements as you'd expect to be able to in order to utilize the tool.
Does the engine have an alternator winding? Might be a little bullet-like connector on the side of the flywheel assembly that may have a fuse in it (I'm not familiar with the Robin engine, though I could go look it up in a reference book here) - that'd provide 12V when the engine is running, and could be used to actuate a relay.
Here's a basic primer on relays: http://www.the12volt.com/relays/relays.asp
Check out Figure 1 - that's the terminal diagram for a very common SPDT relay type used for automotive applications (you could pop the bonnet on amost any car at a wreckers and pull several of these out). An SPST relay would serve as well, since you don't need to switch a different signal in - just disable the interlock once started.
You'd wire 85 to chassis ground, 86 to the alternator output from the engine (all of this assumes it is present), 30 to the ignition kill line, 87a to the control switches (in parallel, so either one provides a ground path), and 87 needs no connection.
Now, when the engine isn't spinning (or not not spinning fast enough to really generate 12V, such as when you very first pullstart it), you have no 12V, so the solenoid isn't energized, and 87a (normally closed) has continuity through to 30 (common). As a result, if either lever is set to a no-start, the ignition system is grounded, just as if the kill were active. Take them out of their no-start positions, and there's no ground, and the igntion can fire. Once you've started the engine, the alternator produces 12V, and the solenoid engages, switching from 87a to terminal 87 (which has nothing wired to it) - now, the position of the two levers won't kill the engine, though the existing kill switch still will (which isn't wired through the relay). Kill the engine with the kill switch (or because it runs out of petrol, or whatever), and the 12V goes away, solenoid disengages, and terminal 87a is once again selected, and the levers will inhibit starting.
Absent a relay configuration as described (and a necessary alternator tab), it makes me wonder if perhaps the switches are in place for an electric start model/option (though electric start would need some way of charging the battery, which would be the alternator tab). On a small engine, the alternator can be wired inside the flywheel - it doesn't need to be a separate belt-driven apparatus like you see on cars. The original engine on my tractor had a generator - when the engine was spinning it, it generated voltage, and when you applied voltage to it, it spun to start the engine.
You're really capable of getting the thing pullstarted when the implement is engaged? I wouldn't dream of expecting to get my tractor pullstarted with it either in gear or the PTO tiller engaged.

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<lots of useful stuff>
Thanks Sean. Info passed onto Robin who should come back with answers (and thanks!) soonish.
Rgds
Richard

Thanks to all who replied.
This seems to be the solution:
From my reading of the thread and a little thought I suspect the
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Reply from Robin:
That would be a logical assumption and quite possibly the answer, but the layout of the belts and clutch levers doesn't lend itself to this being the logical layout: There are two hand levers for the two clutches<*1> (calling it a clutch is glorifying it, it's just a belt tensioner arrangement), one for the gearbox drive, and one for the flail drive. The clutch lever on the left handlebar is set up to operate the left belt / clutch arrangement which operates the gearbox drive, and the clutch lever on the right handlebar is set up to operate the right belt / clutch arrangement which operates the flail drive. A simple swapping of the lever control cables will give the 'no reverse with flails engaged' scenario detailed below - and as I said above, quite possible the answer - but the controls / cables / belts layout doesn't lend itself to this and would explain why the previous owner(s) had installed it this way.
<*1> Of the two clutch levers, only one has the cut out switch facility.
Many thanks for the AFL bod who came up with this probable solution. When back at work on Monday I will see if the cables will fit the other way around.
cheers,
Robin
Cheers
Richard

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Hello and thank you Sean.
First reply from Robin:
"They are push to make switches. NC when clutch engaged or gear engaged, NO when not engaged or no gear selected. At least I'm pretty sure that's the case. I did wire them up originally much as you'd first think of wiring them up, only to have the engine die when you engaged *both* clutch and gears."
I have asked for confirmation of the switch data and age/make/model and starting method.
When I have that I'll post it here and to an electronics NG.
Ta
Richard