MTFCA Forum

The family took the T out Sunday and had a great drive until she suddenly started to miss and backfire. Switched to battery and she restarted ok but would not run on mag. I have checked the wiring and found no problems. The mag coils show no resistance (complete short). My question is shouldn't there be some resistance?
Another question is what possible problems do I have?? Any help would be greatly appreciated.

Resistance is very low. Should be in the .5 Ohm range. Be sure to zero out your test probes, too.

If it was bucking and snorting, it was putting out current/voltage. I am going to suspect the mag post on top of the hog's head first. May have some band material interfering with good contact (yes this really does happen), and second, I am going to suspect the mag/bat switch having intermittent or high resistance on contacts.

Way too early to start worrying unless there was distinct "crack" or bang sound in the transission, associated with it, and you didn't mention that.

Don’t look at the mag post hole. Instead look in the round mirror. It’s hard to see, but there is a piece of band lint about the size of a short piece of dental floss on top of the blob of solder. The lint got stuck between the solder blob and the brass contact of the mag post. Thus lifting the contact enough to break contact. This lint caused me the exact symptoms that you described.

Another picture, a different incident. Lint interfering with my magneto.

It's hard to do from a distance... but I'll throw some wild guesses...

People have already talked about the mag post, and it's the very first thing I'd check... make sure that contact area is clean clean, because things like band wear fabric will start to interfere with the mag post contact... It's quick and easy...

If you are running a 12V battery and it picks right up, you could be experiencing the beginning of mag problems, either weak magnets (rare) or a failing rear main (more common)... your mag puts out an average of 12V (tho it is AC) up to 34 Volts (wow!) but when the magnets start to pull from the field coils, your voltage will drop dramatically. Thats the first thing I'd think about... tho...

If you have switch probs (you're leaking battery voltage to your mag post) it can start to discharge your magnets.

Since it runs hot on Bat and not on Mag that actually rules out some things (like your timer) and since it's reduced on all cylinders and not missing on one cylinder it rules out one bum coil.

If you're running 6V battery electrics, you could be looking at other issues... but if thats the case we'll regroup once you confirm or deny.

You may find this of interest. If this does not post as a link, copy and paste.

http://fordfarm.net/RCosart19.html

John

the mess that Phil posted is a traumatic destruction of a magneto. It doesn't happen without a lot of noise and drama. The story as presented leaves out an enormous amount of detail. From your description, I would not be jumping to any sort of horrifying conclusions based on the Fordbarn post.

I pulled the mag plug and it appears clean and I used a mirror to look in the case and could not see anything that appears abnormal. Next I am going to reassemble and see if it might run on mag ......... be good if it did but I hate things that fix themselves!

For what it's worth I had a similar issue with my '14 and it turned out to be the switch. Even after cleaning the contacts and not seeing anything unusual the problem persisted. As a last resort I swapped it out for an old switch and the problem went away.

The magneto coil is grounded at one end so it will show as a short. You can not get a resistant reading unless it is out and the grounded end
un-grounded.

Mark...HUH?

think about that again...

You can't measure resistance of the coils on the ring with out undoing the one end that is attached to the plate.

Actually, I need to revise my first post and state that normal resistance on a good coil is about .25 Ohms as a separate assembly and a tad more installed... and not .5 Ohms as I previously stated. Memory isn't what it used to be. To most folks, that is a dead short on their meter if they do not "0" out their leads and understand that a "good" reading is very, very low. This isn't a lightbulb. If you have a good coil, and it is installed in the car, you WILL get a reading between the post and the hog's head. It will, of course not determine if the coil itself has an internal short of a few windings...that can only be accurately tested out of the car with current going into the coil and each pole tested for magnetism (not with one end of lifted).

The notion that a mag ring must be somehow out of the car or have one end lifted from the ground to test for continuity is simply erroneous. In the installed condition, it is not "shorted" it is grounded, which is it's normal operating condition of the complete circuit.

A decent read on this can be found here...for those not wishing to read the entire thread (which would be a shame) go to Zachary Dillinger on Friday, August 29, 2014 - 01:12 pm
http://www.mtfca.com/discus/messages/41 ... 1412882357

First thing, if you don't have one already is to order the book on "Electrical system". It explains the magneto and how to repair it.
Second and easy way to check things out would be to find someone with a good running car and temporarily borrow they coils. Put those coils in your car. If that fixes your problem, then your problem is with the coils. Yes it is possible for coils to work on battery but not on magneto or vice versa! If your car still runs the same way, your problem is most likely with the magneto. Check the output of the magneto across a car light bulb while running on battery. The bulb should be connected from the post to ground. An analog AC voltmeter should be used for this test. It should test at least 6 volts AC at idle and run to almost 30 volts at high speed. If not, you have a problem with the magneto. Next step would be to take out the post and check for lint as explained above. If your voltage is still low, your magnets need re-charging. The book will give you methods to do this. Other causes would be grounded magneto coil ring. If grounded somewhere in the middle of the ring, you could have a low voltage output and only replacement of the coil ring will fix that problem. The other possible cause would be loose connection between the magneto plug and the ignition switch or problem with the switch itself.
A very important thing, NEVER allow the battery to come in contact with the magneto wiring. It Will de-magnetize the magnets.
Norm

And finally, my own sanity check this AM on my '13

1. "0" the leads of my cheapy digital meter
2. For information...measure resistance of car frame at full extension of my leads (4'): reading is "0"
3. For information...measure resistance of brass fan hub, through grease, through fan arm, through block, across block/frame interface, across frame/brass radiator interface: reading is "0" (that surprised me, frankly)
4. measure mag post to hog's head: ".3" Ohms, and since my meter is only capable of .1 Ohm graduation, I'll accept that I am measuring the mag coil resistance and that interfaces including frame and engine do not influence the reading.

If no effort is made to "zero" the test leads or at least subtract them out of the equation (or has an Analogue meter improperly adjusted), and one did not know that the resistance of the mag coil was very near "0", one could easily be persuaded to believe that there is a dead short (unplanned or undesigned path to ground). With a good coil installed in the car, there is not.

The above is meant as guidance to the OP.

The best overall advice given is, I think, Norm's and that is to buy the electrical book and test mag OUTPUT.

.What a day! Reassembled the coil plug assemble (managed to get the base in 60 degrees out the first time and of course all three screws turned hard to the last thread (and in cramped space besides). The old girl fired right up on battery and guess what .... fired good on mag!!! As I said before I hate things that fix themselves since I didn't find anything wrong. I'm thinking it had to be the switch. I cycled it back and forth several times with normal results. I may never know but at least I don't have to pull the motor. Thanks guys for your advice .... what would you do without this forum.

Good for you, John
Good of you to post the results...and don't look a gift horse in the mouth...if it runs, it runs. One day, you may have it buck and snort again and wiggle the key and blow the muffler to kingdom-come. That's life with a T (oh, and a good indication that the switch needs some work!)