Fairbanks Morse FM Mag Troubleshooting
Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=177465
Printed Date: 25 Dec 2025 at 12:49pm
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Topic: Fairbanks Morse FM Mag Troubleshooting
Posted By: Oniondip
Subject: Fairbanks Morse FM Mag Troubleshooting
Date Posted: 11 Jan 2021 at 10:39pm
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Hey all, Im new here and am trying to troubleshooting and repair the mag on a 1951 Allis B. Tractor has no spark. I checked for continuity from p-lead to ground, there was continuity. So I disconnected the wire, and checked for continuity from just the p-lead stud, still had ground. So I removed the mag and brought it into the shop. I took the cap off and inspected, looked alright. I took the cover off and inspected, looked good aswell. Then I removed the three wires (coil, condenser, p-lead) and checked continuity to case from each one. The red wire off the coil is case ground. So here are my two questions for the experts; Is the coil shorted and faulty because the red wire is case ground? How do I time the gears when I remove them to get the coil out? I dont see any timing marks. And I prefer to not just create my own markings incase they were assembled incorrectly in the past.  |
Replies:
Posted By: WF owner
Date Posted: 12 Jan 2021 at 6:16am
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The top gear in your photo should remain in the cap when you remove/reinstall it. Both gears should have a marked tooth (on the side facing out on the bottom and on the side facing in on the top gear). The mark may be a beveled tooth or a small indentation (look like a prick punch mark) on the marked side. The marks can be very hard to find. You may have to clean them up and may need a magnifying glass to find them. After i find them, I mark them with paint so I can find them easier in the future. When you reassemble, it is very easy to get off one tooth. Take your time and be patient.
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Posted By: Oniondip
Date Posted: 12 Jan 2021 at 6:21am
I thought I may have seen a small small mark on the small gear. I've always seen larger marks for timing than that. Do you agree if the red wire is grounding that the coil is bad? Or is the red wire supposed to go to ground? |
WF owner wrote:Posted By: WF owner
Date Posted: 12 Jan 2021 at 6:22am
| It's hard to be sure. but I think your bottom mark is between the 11 and 12 o'clock position. |
Posted By: Steve in NJ
Date Posted: 12 Jan 2021 at 6:54am
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Looks like you have an "X" series Fairbanks where the gears are on the breaker plate. On the steel mainshaft gear one of the teeth on that gear should have either a "dog ear" or a "pip" mark. (dimple) On the fiber gear, if you clean the gear off, you'll find an "A" or a "C". You want the "C" on the fiber gear to line up with the dog ear steel gear. That will put the mag in time internally. Being that's an "X" series, when you remove the lower cover, you don't take the Mag out of time like you would a "J" series. The "J" series mag has the gear in the lower cover which you have to mate with the steel mainshaft gear. A little Brakekleen should clean things up in order for you to see the marks. In your pic, I don't see a dimple, so it probably has one of the teeth with a dog ear. Find the dog ear tooth. Once you determine which one that is, clean off the fiber gear. If you see the "C" mated with the dog ear tooth, they're timed correctly. When I rebuild our customer Mags, I mark the teeth on the fiber gear with blue or red paint. Some of those markings are a bear to see on the fiber gears! The "X" series Mag was Fairbanks better Mag which replaced the quirky "J" Mag before AC changed over completely to Battery ignition. (Distributor) HTH Steve@B&B ------------- 39'RC, 43'WC, 48'B, 49'G, 50'WF, 65 Big 10, 67'B-110, 75'716H, 2-620's, & a Motorhead wife |
Posted By: MACK
Date Posted: 12 Jan 2021 at 7:45am
| I would say if you file or replace the points, you might have spark. MACK |
Posted By: steve(ill)
Date Posted: 12 Jan 2021 at 8:28am
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JEff , concerning the COIL.. just think if it as a winding of wires with TWO END... You have the RED lead coming off the coil to your point... The OTHER END of the wire has to be connected to ground internally... So NO, do not replace the coil. Your coil is not original, it has already been replaced once... Mack is right, clean the point and make sure the wires are not touching ground... and get the gears timed right.
------------- Like them all, but love the "B"s. |
Posted By: Oniondip
Date Posted: 12 Jan 2021 at 10:00am
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Alright, I followed everyone's pointers here and got one check in the box. I googled magneto wiring diagrams to get a better understand of it and the coil. The coil to ground makes sense now. So I buttoned the coil back into place, looked very closely to the small/bottom gear and found a stake mark at the base of a tooth. And it happened to line up with what looked like a small scribe in a valley on the top gear. So I decided that was good. I removed the points, gently sanded them, reinstalled and gapped to 0.014". Good news: I now have spark! Bad news: Spark is intermittent. When the mag is snapping I get spark. But after a few revolutions of the engine the mag stops snapping. So now I need a better understanding of the impulse coupling and any tips or tricks for why the dogs may not be catching and allowing it to continue doing its job. Once I get spark its time to put gas in it and try to get it to fire up. Even coughing on a few squirts of ether would be a good win considering this tractor hasn't been started in 10+ years.
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Posted By: Bill Long
Date Posted: 12 Jan 2021 at 10:27am
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OnionDip, First, Welcome to the unofficial Allis Chalmers Forum. AS you have seen you will get LOADS of excellent knowledge here. Listen closely to what is said. If you continue to have problems ship the Mag to SteveNJ Custom Circuits. He will overhaul it and return it in working order. Also, you may want to view the Knowledge section of this forum. There may be a post there that well describes Magneto adjustments and repair. Appreciate the fact that you have my favorite Allis Chalmers Tractor - The B. Appears like you are taking good care of it. Good Luck! Bill Long
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Posted By: Steve in NJ
Date Posted: 12 Jan 2021 at 10:48am
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The impulse is only going to snap till about 400 rpm. Over 400 rpm or so, the impulse pawls will retract and the engine will run. To inspect the impulse, you'll need a puller to pull off the coupler assy. If the spring in the coupler is rusted, it'll probably need a good cleaning if its not working correctly. Also inspect the pawls are free and moving okay. If not wash up the coupler and get the pawls moving again. Before assembling the coupler, its a good idea to spray some dry graphite lube on the pawls and spring. This way things will work properly. Don't use any oil. That just attracts dust n' dirt and slings everywhere making a mess . You probably either have a very dirty set of points, or you need a new set of points. It would be a good idea to put a new condenser in it also. Then give it a whirl. She'll probably fire off and go. Steve@B&B ------------- 39'RC, 43'WC, 48'B, 49'G, 50'WF, 65 Big 10, 67'B-110, 75'716H, 2-620's, & a Motorhead wife |
Posted By: Fred in Pa
Date Posted: 12 Jan 2021 at 11:11am
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Not all points n condensers out on market are good ones , ------------- He who dies with the most toys is, nonetheless ,still dead. If all else fails ,Read all that is PRINTED. |
Posted By: Mikez
Date Posted: 12 Jan 2021 at 9:29pm
| welcome to forum. Hope you stay around and share your tractor |
Posted By: Andyw
Date Posted: 13 Jan 2021 at 8:14am
| The red wire on the coil should be your positive side. Your ground wire should be running back behind the coil to a screw or stud for your kill switch. Before you pull the impulse off, 90% of most issues of no or intermittent spark is that I've seen or heard about is either points, condenser, or coil. That too check for bare spots in the wiring, it may be shorting it out |