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Replacing magneto with electronic ignition


Ken G
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My Rover is out of commission unless or until I can repair the magneto, which has gone open-circuit between the end of the high-tension coil and the output slip-ring (seemed highly improbably but it is so). Meanwhile I am wondering about electronic ignition, either as a permanent replacement or an emergency alternative. I'm sure people have done this. Has anyone any experience?

I should point out that the ML CG-4 magneto armature runs at engine speed, and with the four-cylinder engine therefore delivers two sparks per revolution; the contact-breaker rotates with the armature, with a cam follower rubbing on the inside of a circular (well obviously not literally) cam. There is a brush connecting to the non-ground side of the contacts, that goes off to the ignition switch to be shorted to ground to stop the engine. Hence in theory if the magneto low-tension winding is disconnected internally (quite easy), the switching of the contacts is accessible via that brush, and could be used to trigger a capacitor-discharge electronic ignition. It would have to be a system where the triggering occurred with the opening of the contacts, one of which is inherently negative ground.

The distributor is off to the side, driven by a 1:2 step-down gearing, and its input is accessible so that's no problem.

Have others cracked this one?

Ken G, 1925 Rover 16/50 (San Francisco)

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Guest imported_JPIndusi

If you can get the primary disconnected and make a connection to the contact points, then in theory you should be able to use an ordinary 6 volt or 12 volt coil (with dropping resistor) to give you the high voltage to fire the spark plugs.

You should use a condenser (capacitor) that goes with the coil to prevent the points from burning out. The condenser is connected in parallel across the points (one side usually to ground as is one side of the contacts). The high voltage wire from the coil should go into the distributor cap as usual.

Be sure to ground the coil case. You can probably use a 6 volt lantern cell to power this ignition unless the car already has a storage battery and generator.

Joe

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JPIndusi

Yes, perhaps! There are two potential problems, perhaps three.

a) The contact assembly rotates with the armature (at engine speed), so the only access is via the brush at the top that was designed to pass low-tension current only for long enough to stop the engine, when the ignition switch short-circuits the LT winding. I doubt that the brush would stand passing coil current indefinitely.

B) The cam profile was obviously designed to short the winding for a predetermined number of degrees of rotation; I think this is called dwell in conventional distributor make-and-break contact cams. It isn?t at all obvious that this is an appropriate dwell for a coil.

c) I think typical coil currents are many amps, and I doubt that the contacts would tolerate that much current for long.

Against that, I could probably use a lower current through a coil than intended by its manufacturer, in that the car runs with a much less enthusiastic spark than in modern engines. But of course, it might run better with a bigger spark, or soot up the plugs less. Anyway, my thought is to ?amplify? the contacts externally so they only have to pass a few milliamps or tens of milliamps, and probably to use a capacitor discharge system so only the moment of contact opening matters and dwell is irrelevant. Alternatively, by molding a new magneto top cap, I could install some form of optical or magnetic detector, with perhaps a double-sided mirror or long permanent magnet replacing the rotating contact assembly (remember, two sparks at 180 degree intervals). But that?s getting into building stuff, and before I do that I hope I can get specific advice from someone who has gone through this exercise. (By the way, there is a 12 volt battery which operates the electric starter and the lights, but of course in normal daytime driving there is no load except brake lights).

BillP

The obvious symptom (apart from no spark!) is that there is no continuity between the high voltage output slip-ring and the body of the magneto. Between those points you should measure the resistance of the winding, around 5 k ohm. When I first got the armature out, I found that if I pushed on the wire leaving the winding where it entered the insulated molding that carries the slip-ring, I could restore continuity momentarily. I then resorted to the old wireman?s trick of pushing a pin into the insulation where the wire enters the insulator, and found much to my surprise that I had continuity through the winding to the body, but none over the inch or so from there to the slip-ring.

Then the task was to remove the slip-ring to see how the wire is supposed to be connected. Unfortunately that involves removing a bearing (the inner part on which a ball-race runs) which is an interference fit. Clearly the designer did not expect this to be dismantled by anyone else. It requires a special tool, essentially a puller than can grip the groove where the balls run without damaging it, and so far my attempts to improvise have failed. By email, the specialist in England has confirmed this; he has built a tool. If and when I succeed, I don?t quite know what to expect, but I hope it will be obvious where the break lies and that I can repair it.

Meanwhile I would still like to plan a backup system in case I cannot repair the magneto.

Ken G, 1925 Rover 16/50 (San Francisco)

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Guest BillP

Hi Ken~

Well, you're here asking opinions...

That the problem has been encountered and a solution crafted shows that the mag is repairable. Can your correspondent in the UK either loan you the tool or send drawings from which you may have another made? Not to minimize the repair, but a few evenings in the workshop repairing a seventy-five year old magneto to as-new working condition would, for me, be vastly more rewarding than adapting, even successfully, a modern ignition suystem with mirrors and such to this great old car. I understand I'm not there staring at this problem and it's easy to have an opinion from a thousand miles distance.

Of course, no matter what path you choose, nothing will be much different and the car will run the same; it'll still be a great old car.

Do you know any old machine shop or tool & die guys in the Bay Area that might have a look at the mag? Also maybe old recip airplane engine mechanics; those engines often had magnetos.

Sorry more opinion than help.

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Ken,

I wonder if you could adapt something simlar to a Ford EDIS to your Rover. It uses a toothed wheel which is attached to the crankshaft, with one missing tooth. It uses this to determine timing. I'm no expert, but the concept shouldn't be too hard to apply. On the Fords I believe this is a fairly substantial toothed wheel, with a sensor to detect the teeth and an electronic control module to fire the coils. In the case of the Fords I've seen each plug has its own coil, completely bypassing the distributor.

Probably not practical to mount in your vehicle, but an interesting concept in itself... particularly in that the dwell and timing are completely electronically controlled.

I ran across this information looking at the Megqsquirt FI project, there is a spin off project with EDIS control. Interesting stuff...

Rich

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Just such a problem arose with the 1933 Studebaker President racecar I had experience with when owned by Brooks Stevens Museum. The present owner had a Chrysler (1960's vintage?} fabricated to adapt to the engines timing case (This is a 337 CID inline 8 cylinder) to replace the exotic 8 cyl Scintilla Mag. They found a talented mechanic/machinist to do the adapter which used a small cogged drive belt (1/2" width about 4" C/L to C/L) that allowed to adjust timing the distributor relative to the gear driven cam and crank. It works very well and so well that they have delayed restoring the magneto. I can provide the owners email if you want better details. Stude8

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Guest imported_JPIndusi

Ken:

I agree that you will want to limit the current through the points and brush assembly so using an electronic igniton (capacitive discharge, for example) would be best. From my knowledge of magnetos, the spark is fired when the points suddenly open the primary coil. This sudden interruption of the current in the primary induces an electromagnetic pulse in the secondary which produces a high voltage spike and fires the plug. The primary current is not feable but may not be as high as the current in a coil ignition so the electronic ignition makes sense. At high speeds a 6 or 8 cylinder car has very little dwell time before the next plug fires so I don't think this is a problem.

The rest of your reply is confusing but I can't physically see what you have. I assumed that there was a device similar to a distributor but perhaps this is not the case.

Joe

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Thanks everyone. Good thoughts. But don?t get me wrong; I shall repair the magneto if possible, but I want a back-up position, and in fact wouldn?t mind having an alternative that can be installed fairly easily on the road. Also, if I cannot repair the magneto myself, I shall be without the Rover until after my trip to England, returning early June, unless I can put in something else. Since the weather is nice, I would like to be able to drive! This is twice I have returned home on a flat-bed truck because of failure of the magneto, so I would like a way to get home under my own steam.

Yes, the magneto contacts work just like those in a conventional coil system, opening at the moment of spark. In fact, the whole thing is exactly like a coil ignition except that instead of the battery, the ?coil? (really a transformer) rotates in the field of a permanent magnet, generating its own power. The output goes via a few inches of cable to a perfectly normal looking distributor on the side of the magneto (driven by a 1:2 gear inside the magneto case). I attach a very poor diagram. It?s not visible, but at the right-hand end of the cable loop is a brush bearing on a slip-ring. The high tension coil has somehow lost continuity to that slip-ring, although the coil itself is intact. The bearing I have to remove to get at the slip-ring is on the shaft at the extreme right-hand end of the picture. (I have a friend who is more with-it mechanically than I, and he has plans to make a suitable tool to pull the bearing; a good analogy, like pulling a tooth). I might add that the axis of the magneto, shown horizontal, is actually vertical in my car.

JPIndusi. Yes, you may well be right that dwell isn?t a problem, but I think a capacitive discharge system eliminates any doubts, and incidentally probably consumes less current at the modest engine speeds in my Rover (absolute maximum about 3000 rpm).

Stude8. Having spent an arm and leg having an adapter machined to allow the fitting of a modern fan, I am very reluctant to get into the fabrication of special parts. In any case, I see no reason why timing information should not be derived from the top of the magneto, whether I use the contacts or install a magnetic or optical sensor.

Since I prepared my last message, I have searched on the internet. The MSD 6-series capacitive discharge system looks as if it would work using the existing magneto contacts via the brush in the top, so I have sent a query to MSD to see whether they have suggestions. In particular I need to know whether the ohm or so of brush resistance would upset the unit (I doubt it). If anyone knows any reason to avoid MSD, let them speak now or forever hold their peace.

Ken G, 1925 Rover 16/50 (San Francisco)

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Guest BillP

Hi Ken,

When restoring my MG a few years ago I installed an electronic ignition module because the car is so short and leaning over into the engine and setting the points with trifocals was a pain in the boot. (British auto humor)

I think the name of the unit was Pertronix or similar, maybe it could be cannibalized or adapted for your application as it is four cylinder.

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Thanks, BillP. Yes, Pertronix makes electronic ignition systems that fit inside the distributor, which is very neat if of course you have a distributor that includes the contact breaker and has space. (I see they list a part for 1920s Rolls Royces!) I assume the existing contacts are removed, and a magnetic timing system substituted. Alas, my distributor has no spare space, and probably its shaft position is not precise enough to use for timing (there is potential backlash in the gearing). Mere distribution to the four plugs does not require accurate positioning. But I was glad for the reminder. I had looked at their website at some time in the past, but it was good to check.

Ken G, 1925 Rover 16/50 (San Francisco)

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Guest BillP

Hi Ken,

Well, then, what about one of those hot rod crank fire things with a disc with four nubs on it that blips a fixed photocell, sending a signal off to the brain box to make a spark. You stick the disc on the front of the motor somehow (slottedly, to adj timing, or maybe that's all done in the box) and reach over with an aluminum bracket off the block to mount the cell so it can see the nubs. Then you wouldn't have to monkey with the mag or the original spark distributor at all.

I always keep a Summit racing catalog around to do this stuff, they'll likely have something.

Bill

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Guest BillP

I s'pose if you didn't want to put the nubby disc on the front of the engine, you could put it anywhere that turns relative to the engine. Like where the mag is now. Come up out of there with a stalk to mount the disc on and point the photocell at it. Encase the whole business in a little shroud (polished brass to look suitably post-Edwardian) and away you go. Bill

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BillP, Thank you for your further thoughts.

Alas, I don?t think there is anywhere to put a timing disk, short of a major dismantling and marks on the flywheel (and optical detection probably wouldn?t work, since it is a wet clutch, running in the engine oil). The lower fan-belt pulley is not rigidly attached to the crankshaft but has a Lanchester damper. I have only just discovered why timing marks on the pulley were unstable and moved with respect to top dead center. I had been puzzling over that for months. There is no other visible or accessible rotating part.

Hence I think it is in fact most practical to use the top of the magneto, either with the existing contacts or with a magnetic or optical detector. I would in any case have to use the existing distributor, unless there were a system available with separate outputs for each cylinder, requiring four coils (like some recent vehicles, but it seems a silly idea to me). Yes, it would of course be possible to build a new part to go in place of the magneto, but it would have to include the gears and distributor. I don?t see the point. For appearance?s sake, I could put the electronic ignition unit and coil on the drivers side of the ?firewall? (actually plywood) so that opening the engine compartment you wouldn?t see anything different unless you tried to follow the wiring.

Ken G, 1925 Rover 16/50 (San Francisco)

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Guest imported_JPIndusi

Ken:

I see from the attachment that you have a distributor to fire the correct plug at the right time.

Now for a backup, the easiest thing to do is to get an old FORD vibrator ignition box, ground the low voltage side to the engine and run the high voltage output to the center tower on the distributor. You can make your own out of an old coil and a vibrator or relay. Wire the relay in series with the coil primary and as the relay goes on and off at a rate fixed by its mechanical and electrical parameters, it will induce a high voltage in the coil secondary. I would use the current relay from an old voltage regulator and adjust it so it vibrates on and off.

Joe

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JPIndusi. Thank you for the idea, but since I have the 12 volt battery available (I doubt that I am strong enough the start the engine with the handle!), I think it is probably easier to use a readily available capacitive discharge system. I'm not sure that I understand how the timing of the Ford vibrator worked. As you wrote, it sounded as if there would be a continuous series of impulses, and whether they made the engine run would depend critically on the position of the distributor rotor and how fast it approached each cylinder output.

However, your mention of a relay made me realize that one could perhaps use a small relay as a contact "amplifier" so as to avoid putting the much larger coil current through the magneto brush and contacts. The problem might be timing. A relay probably takes several milliseconds to drop out, and that is a variable number of degrees of rotation of the crankshaft, depending on engine speed. In fact, on further thoughts, at maximum revs., about 3000 rpm, one whole revolution is only 20 milliseconds. So, no, that wouldn't work!

When I have solved these problems, I will post the results. However, I would still like to hear from anyone who has actually used an electronic ignition in place of a magneto; I know there's someone out there.

Ken G, 1925 Rover 16/50 (San Francisco)

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Guest imported_JPIndusi

Ken:

You are exactly right on the Ford vibrator ignition box in the first paragraph. You are also right that the mechanical relay would be too slow in it's operation.

I agree that a capacitive discharge ignition can be made to work here. The vibrator supply was suggested as a backup and it bypasses the contact points and brush assembly and just uses the distributor and rotor part of your set-up.

Joe

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