I've been trying to chase down intermittent magneto operation with my '24 Speedster. The car has Coilman Coils and a Total Recoil ring. Crankshaft end play is 0.006 and, when I had the engine apart this summer for a drum replacement, I confirmed the magnet clearance ranges from 0.025-028 at the top and a few thou more at the bottom. When the magneto is functioning, the mag meter pegs to the right at 30 mph+.
We put 50 miles on the re-assembled engine last week with zero problem then got 20 miles into a road trip when the magneto failed (would run on battery). The mag meter swung wildly for a moment then settled down to zero. We turned around for home and 15 miles later the meter pegged again and we drove the rest of the way on mag.
I could not find any continuity problem or shorts within the mag post and I am now running wood bands so there is no fluff collecting at the contact. When we on the road with mag failure I tried bypassing the wiring/switch etc and the car would not run on mag (had the engine running briskly on battery, turned the switch to off, and connected the bypass jumper whilst the engine was still turning).
Back in the shop another round of looking for shorts and opens with no luck. I thought the continuity of the plunger to the terminal post might be iffy so soldered a jumper between the two (third photo).
Everything was fine for another 20 miles and then the same problem: mag failure, intermittent operation for a moment, then nothing.
One thing I have found since — even when the mag has cut out, there is still 0.4 Volt AC at the post whenever the engine is running, and no matter what speed. Always 0.4 Volt.
I do not have easy access to a substitute mag post. but will scare one up if necessary. In the meantime, has anyone here had a similar problem, is this continuous 0.4 Volt AC to be expected, and could there be an internal problem I may have missed/caused during engine assembly?
that is not an original magneto plug. The contact looks a bit short to me. The plug you show looks to be made for an outside oiler. Those have a reputation for not making good contact with the magneto. Try an original spring type plug and see if that fixes the magneto problem. If it does, and you still want an outside oiler, you need to get or make a plug with a longer connection.
Norm
Hi Norman — I do run an outside oiler from the mag post. The coil ring contact is less than 1/2" from the gasket flange, so the plunger is almost fully depressed (spring almost fully compressed) when the plug is installed. I soldered the jumper wire on just in case the plunger-to-post connection was iffy. I have run this particular plug for almost 20 years.
Probably not the case, but make sure your outside oilier fitting on the mag plug can't make contact with the mag post internally. For example the fitting can screw in too far. I have also seen the contact rivet on the top of the mag ring touch the rivet that secures the insulation block.
Is that a replacement mag coil? Or has it been in service for a long time?
Either way I would suspect a problem w/it, possibly a bad connection from the field coil to ground or more likely from the coil to the solder blob where the mag post connects.
A depression may have worn in the mag solder pad and your needle is kissing it intermittently. You have an intermittent connection somewhere in your circuit.
You could measure the voltage directly on the pad with the terminal removed.
If the voltage is solid on the pad the you might consider adding a ball of solder on the point of your needle.
If its intermittent on the pad, the pad or the mag coil ground terminal could be the problem, or crud is shorting it out.
All the tests you describe sound like the proper way to diagnose such a problem. The only unexplained finding left dangling is that 4/10 volts.
Assuming the 4/10 volts you are reading when the mag has "cut out" is real, and not a 'phantom' reading produced by a bunch of magnets spinning two feet away from your meter, then from an electrical standpoint my best guess is bad news.
It sounds like an intermittent short-to-ground in your coil ring, one or two coils away from the pickup point, or maybe halfway through the coil closest to the pickup point. I don't know what voltage a single coil would produce, and it might be more than 4/10 volts, but no other explanation makes a lot of sense to me.
The reason I said "bad news" is that this would imply a remove-and-take-down to find and correct.
I'm sorry!
Thanks all for your comments. The mag coil was new 2000 miles ago, and was left bolted to the block, with no known trauma, hen I had the engine out for drum replacement.
I did find another (non-oiler) post in my junk and just tried it — mag put out fine for 10 sec then nothing except the 0.4 volts. The replacement post compresses a good 1/4" when the screws are tightened, so I doubt there is a poor contact with the solder blob.
I won't get back to this job until later today, will try to get a voltage reading directly from the blob.
Those repro oiler mag posts are trouble. You should keep your eyes open for one of the old original ones from years back, they are made out of cast pot metal and are much more dependable. In the mean time I would do away with the outside oil line (you have an inside one too, right?) and install a more reliable mag post.
Sometimes all it takes to short the magneto is small piece of debris grounding out the field coil. You might try recharging the magnets in the car. If it is a small piece of brass or metal shaving the high amperage of the recharge might burn it up.
If you try to test the voltage with the contact removed, be sure to put rags around your probe to keep oil from spraying out.
Norm
What about the ground between the coil ring and the engine block? What kind of shims were used?
I suspect that the ground end of the magneto ring is not firmly connected to the ring or one of the spliced solder connections is intermittent. The .4V is likely just noise being picked up by your meter since putting a meter on the AC scale makes it a receiver of transient noise which is all around the magneto and its wiring. I don't think it is meaningful. Connect your meter again and when the magneto goes dead, turn off the motor quickly and flip the meter scale to read Resistance on the Rx1 scale once the motor stops turning. My bet is that the mag ring it OPEN at that time and will read a high resistance. Might be hard to catch it that way but I think you have an open in the coil with the most likely points being at the ends or splices between coils. A short to ground is possible but that would be most likely a visible wound that you could see when you had the engine out. A shorted ring or a good ring will be nearly identical in resistance using a typical ohm meter to measure it.
Well, I am cautiously optimistic the problem may have been corrected — drove 23 miles since the repair and the magneto has not faltered.
Whilst getting a voltage reading off the mag ring solder blob I found the probe unexpectedly fussy about just where on the blob it was placed. Using an inspection mirror I determined there was indeed a recess in the top of the blob and the recess was dark at the bottom — possibly some non-conductive matter.
I soldered a little ball of copper wire strands onto the pick-up plunger of my non-oiler mag post and hooked everything back up. Success! The mag worked immediately and ever since. Tomorrow I'll put a similar solder blob on the plunger of my accessory-oiler plunger and, which I should have done today, also try to clean the dark matter out of the recess with a dental pick.
Many thanks to all those who contributed to my understanding and the solution, especially you Ted Dumas. This forum is a wonderful place.