Magneto question

[harryagain]

Change the spark plug. IME 99% of the problems in these systems revolve round the spark plug.

[newshound]

I have an ancient Laser ride-on mower with a Kohler 18 twin cylinder engine. Fine last season, but no sparks now. Checking for continuity, I have 26 kilohms between the two plug leads, but no connection from the HT side to earth. That sounds to me like a failed coil, or am I missing something?

I have already disconnected the "kill wire" as other gremlins have got into the LT side over the winter, but the starter motor and solenoid are fine and I can spin it up nicely with appropriate jumpers. It's a simple "magnet on the flywheel" type, no contacts, no accessible LT side. There is still some magnetism left and, as I say, it ran fine last year.

TIA

[newshound]

Might have answered my own question. Since it obviously gives sparks once per rev, perhaps each end of the HT coil goes to one plug, so that the circuit goes coil - plug1 - ground - plug2 - coil. Would be nice to confirm though. I found one of those "spark gap" plug HT voltage testers the other day, might try hooking it up to see what I am getting on the starter.

[Tim Lamb]

In message , newshound writes

I have a Kohler motor in my Jonserad mower. The manual and cct. diagram are no help to you but I note the raft of safety features, any one of which shuts off the motor! Viz operator presence relay, seat switch not occupied relay, bagger interlock!!

[Bob Minchin]

It does sound like a classic wasted spark type system where there is no connection from the HT side of the coil to chassis.

[Adrian]

The engine isn't earthed? How does the starter motor work...?

[Bob Minchin]

That is not what I said. We are discussing the HT circuit. irrelevant to a starter motor

[Dave Plowman (News)]

If it is wasted spark - ie both plugs fire together - it's common for the plugs to be at either end of the coil winding. In other word, the coil winding is between the two plug central electrodes.

[newshound]

Thanks.

I will try a pair of different plugs as Harry suggests, it's just that I would not expect these to go "suddenly". Ohmeter check shows many megohm "across" the plug but I agree they could be arcing internally. I'll try the Megger, too, when I try new plugs.

Thanks all for the suggestions.

[Dave Plowman (News)]

If one plug had failed to the point where it won't spark, it's very possible the other won't either.

Ford EDIS, which is the wasted spark system I've fitted to my old Rover, is also quite sensitive to the actual plug leads and plugs. They do need to be the correct resistance plug leads and resistor plugs to function properly. I'd measure the resistance of the plug leads. Should be something like 5-10,000 ohms. A burnt out one will be perhaps 100,000 plus. And make sure the new plugs are resistor types if the originals were.

[The Natural Philosopher]

I had a mower that wouldn't start.

I put fresh petrol in it and after a bit it did.

a winter with a badly fitting fuel cap can result in petrol that has lost its ability to ignite

[harryagain]

Yes, good point. I always try to empty the tank on my mower for over-wintering and put in fresh petrol whenI need it next.

[Johnny B Good]

Ordinary plugs usually develop leakage faults which divert the energy away from the spark gap. This is particularly true for inductive discharge HT systems (Kettering (including the transistorised contact breaker type) and magneto where a contact breaker suddenly interrupts the LT energising current in order to generate the 3 or 4 hundred volt back emf which is multiplied up by the HT winding into the 25 to 40 Kilovolt region) but not so much in the case of capacitive discharge systems where such leakage paths can't interfere with the 'inductive kick' process of generating the nominal 3 to 400 volt primary winding impulse voltage, merely in the case of more extreme leakage cause a greater volt drop in the HT windings (the primary still gets hit by the 400v excitation pulse regardless and the reduction of voltage at the spark plug becomes simply a matter of volt drop due to leakage inductance and winding resistance alone).

The resistive plugs use a high temperature ceramic resistor of about

5000 ohms or so and so it's possible for these to go high resistance (open circuit at typical DMM test voltages aren't necessarily a sign of failure - you should test with a Megger to more properly test for 'open circuit resistor').

It's easy enough to test whether the plug is a standard or resistive plug type using a simple test meter. If such a test shows open circuit you'll need to use a high voltage tester to properly prove whether the plug's built in resistor has failed sufficiently enough to compromise the spark performance.

Getting back to the issue of the 'wasted spark' system, it's actually a very necessary requirement that the double ended HT winding (magneto or coil) is floating with no leakage paths to frame (earth). This arrangement effectively places each plug's spark gap in series thus allowing the single HT winding to produce a spark at both plugs simultaneously.

Such double ended HT coil windings need only to generate another 5 to 10 percent more voltage to compensate for the volt loss across the plug which happens to be 'wasting its spark energy' at that part of the cycle.

The waste spark is timed for when the cylinder pressure is low, typically near the end of the exhaust stroke, which reduces the breakdown voltage to a fraction of that required to jump the gap when subjected to the much higher pressure developed towards the end of the compression stroke. IOW, the spark plug that's currently using its sparking energy to ignite a fuel air charge, always gets the Lion's share of the HT voltage (at least under normal working conditions).

Incidentally, this is the reason why you need to use a spare plug to provide an earth return when using the classic "detach each ht lead in turn' to discover which spark plug is failing to ignite the fuel/air mixture on a rough running engine that's misfiring on one or more of its cylinders. You can't just simply detach the HT lead since this will compromise the remaining good plug's sparking performance as well as subject the HT winding to much higher voltage stress than normal.

[Dave Plowman (News)]

There's another interesting feature of wasted spark I don't quite understand. I have EDIS wasted spark on my V-8 Rover. My posh timing light with an inductive coupling and a dial in advance setting doesn't work with it. My older simpler one which had a direct connection to the spark plug does.