I am working on my 1926 and am having some trouble with the magneto. It ran and started on magneto when I first brought the car back to life, but now it will not run on mag at all. If I test the output using an analog meter / 1156 bulb socket, I get 2 volts at idle, 4 volts at mid-throttle. Clearly this is not going to work. Further, when I check resistance between the output post and ground, the meter pegs out (no resistance).
I find it odd that the magneto would just short itself out so completely for no apparent reason. The only think I can think of is a loose cotter pin / bit of wire or something found its way to the field coils and is shorting them out.
Unfortunately, the access hole is too small to do any meaningful looking, but I really don't want to pull the hogshead for the second time. I cannot see anything if I pull the hogshead cover. Would I have a line of site if I removed the bottom inspection cover? I probably wouldn't have any luck just pulling the drain plug.
Anyone have any tips for accessing / investigating without pulling the hogshead?
Try removing & cleaning the magneto output post & checking it proper spring tension and condition. Are you running a mag post accessory oiler? Sometimes they can be problematic.
Even if you did remove the hogshead, you would most likely not spot the problem and even if you did, there would be very little you could do to correct it without pulling the motor and swapping out the coil ring for a rebuilt one.
BTW, I believe your resistance test would be about the same even if the coil ring wasn't shorted somewhere. The windings of the coil ring are substantial enough that resistance would be very low anyway.
A good or bad magneto ring that does not have a "break" in it anywhere will measure about the same resistance and that is extremely low at 1/4 of one ohm so for all practical purposes it is a dead short to ground whether it is good or shorted to ground. Where you playing with the ignition switch by any chance? Any accidental short between the battery voltage and the magneto wire could have discharged the magnets inside the motor. That can be corrected sometimes by doing an "in-the-car" recharge of the magneto magnets. Since you are getting a small voltage I doubt you have any lint on your magneto plug that is causing anything. I wouldn't pull the magneto plug if you don't know how to put it back correctly since you can bend it in so doing and then have an additional problem. Lint on a mag plug does not cause a low voltage - it causes NO VOLTAGE. Lint on a mag plug causes infinite resistance reading (open circuit) and not a low resistance like you are getting. If the crank shaft has a lot of end play you might have an issue with too much gap between the magnets and the mag coil. If you attempted to pull the starter out before you removed the bendix you could have easily damaged the mag ring and it could then now be shorted. Just looking for the cause of what happened can often then tell you what needs fixing.
I jut ran the car, accelerated the motor to about half throttle with my 1156 hooked up. It started out very very dim, but when I sped up the motor, it suddenly got very bright and would run on the mag. Put out over 20 volts and the bulb popped. I shut off the motor, put the new bulb in, restarted, flipped to mag and it ran fine for a minute, but then died suddenly. When I restarted, the light was dim brown and putting out just three volts. So, I am thinking there must be a bit of debris shorting out the supply intermittently.
At least I know the magnets and coils are functional. Tested switch, wiring, and post first thing.
You may have a defective MAG post. The repros are not so good.............
How would I confirm a bad mag post? I can't handily test voltage while holding a test probe on the contact button can I?
I wouldn't do it in a white shirt.
Hal's right a good deal of oil will be shooting out were the mag plug goes.
I would take the mag connection out and clean it. On a '26 they just unscrew. If you have an outside oil line that needs to come loose also. Likely a wad of lint from the bands causing a short.
If you don't have a transmission screen now is a good time to put one in. Most vendors sell them. They catch most of the band lint and crud.
Sounds to me like a problem with the magneto plug, however, I have found one where the magnets scraped a connection on the coils. This would ground the winding at that point whenever that magnet came in contact with the coil.
Try the plug first. That is an easy fix, and it is also a common cause of your problem. Also check all the wires from the magneto post to the terminal block to the ignition switch and the switch itself. Any of these could cause the problem.
Based on "new information" posted above I believe you have an intermittent short to ground in the magneto field winding shorting the field or excessive crankshaft end play affecting the magneto gap.
Ron the Coilman
Thanks to everyone for providing me with things to troubleshoot. I guess, for now, I will just run the car on battery and enjoy the ride until the end of the year, at which point I'm planning to pull the car apart for paint. With the engine out, I will trace down the magneto problem by removing the hogshead and beginning to look around.
You can check crankshaft end play with the engine in the car. Just use a pry bar on the front of the crank pulley. Pry it one way, then pry it the other. I don't recall the acceptable amount, but it should be SMALL. A magnetic base dial indicator will tell you how much, but if it's enough to really see with the naked eye, that could be where the problem is.
Well, I found one of those mythical roundtoits the other day and pulled off the crankcase cover. Feeling around in the sump, I managed to pull a two inch piece of wire out. I'm assuming that is what was giving me the short. However, I still have no joy. The magneto still puts out about 3 volts at idle and will only increase to 6 volts at speed.
I'm beginning to think I may have excessive crankshaft endplay. I will pick up a dial indicator and check it out. I think that this would account for the tendency for the mag to work and then suddenly stop working with throttle input or after shutoff.
If the endplay proves excessive, I will have no choice but to resolve it as I really don't want to risk any major damage to the crank, rods, or bearings.
Thanks for the ideas.
Zach I had the end play issue on one of my cars and while I was waiting to find the time to pull the motor I put one of those after market spacers up front behind the fan pulley. Lang's sell them and it worked great. It was a good, easy, temporary fix and allowed me to drive the car on Mag for over a year before it wore out.
Here is an excellent article technical article by Ken Foster describing in detail how he repaired excessive crankshaft end play (and poor magneto output)in his Model T without entirely disassembling the engine.
Insufficient Magneto Output Due to Excessive Crankshaft End Play
Ron the Coilman
Ron, I found that article while searching the forum for ideas. It seems to be an excellent fix to the problem, assuming I can get my ham-hands into the crankcase. I need to verify that this is in fact the cause but it makes sense given the symptoms.
Val, I've seen the spacer mentioned. It held up for a year before wearing out?
I did the same thing as described above with the engine out of the car. I used a torch with a small tip to melt the babbitt on. I hand filed mine to fit with good gap (had to work it that way, no lathe). The only problem I see in facing the thrust on the cap back to fit with .004 clearance is that does not take in to account any wear to the flange or front thrust on the crank. I have seen that area under cut so there was a ridge that formed that would require removing more material then what is needed from the cap to install it. As soon as that ridge cut into the babbitt would give you a larger end play reading. This could be why in the how to do he ended up with .008 clearance. But this will put you back in the ball park.
You are very smart to not start fixing the magneto until you know what is wrong. More problems can be inserted into things that are working OK. If you think a starter "extraction" may have been tried before the total bendix was removed, you can see any damage inside the motor there when the starter bendix cover and bendix is removed which isn't terribly difficult to take apart but getting the top screw back in the bendix cover is tricky. The wire you found if bare might have come from one of the piston wrist pin locks. Shine a light up inside each bore and see if any piston is missing a safety wire that holds the wrist pin bolt from coming loose.
IF you shut off the engine when the mag output is low and without moving anything you can measure the resistance to ground at the mag post. If it looks like a short then it probably is NOT anything shorting it out but rather that you have excessive end play in the crank if you can then move the crankshaft forward a bunch. IF you have the lower crankcase still off you can easily put a large screwdriver up there and try to gently pry the crankshaft forward then backward. New should be about .004 but not likely you will have that. My son's T when purchased had about .08-.100 in one position but when the crankshaft shifted the mag worked fine. He drove the car once all day on the magneto then for the return trip it was lights out dead mag like yours. It ran fine with no noise with the crank shaft in either position but only worked the mag in one position. Funny that it would have that much play and yet be perfectly happy to stay in one position all day.
The fault with the repro oiler type mag posts was that the side fitting that threaded the oil line onto the mag post base was threaded too deep and the fitting hit the center contact and shorted out the mag. They might be intermittent but generally they just kill the magneto voltage from the git go. Since the voltage at the magneto is low even at 30V you are then not likely to find that the oiler line is intermittent if you have that fault. If you suspect it is then back any side threaded fitting out a few turns to move it away from the magneto center electrode internally and try to see if that make the thing run but all of this is moot if the crankshaft end play is excessive. Since the magneto does put out enough voltage when it is working, do not try an in-the-car recharge with the idea that you can bring the magneto up from 4 volts to 28 at this point because sometimes it is in fact putting out normal voltage and weak magnets remain weak until recharged and yours are NOT weak or you would never have gotten the test results for even a short time that showed the light being bright. If you have the idea of "I'll just try the recharge anyway" you will regret that idea. It is very easy to do more harm than good and for sure your magnets don't appear to be weak at this point.
Good hunting and let us know what you find.
John, thanks. I'm just following auto repair / troubleshooting 101. Don't fix something until you know, repeat KNOW, it needs fixing. I don't have the "feel" for the Model T yet, so I can't trust my gut like I can with fixing a Datsun inline six (I loved Z cars once upon a time). So I have to be absolutely methodical to make sure I don't screw anything up more than it already is. I feel very lucky to have nearly instantaneous access to the incredible knowledge you all share here.
I don't have an outside oiler although I have contemplated adding one. I have used a visual inspection and compressed air to eliminate the band debris at the contact theory. Since pulling the wire, I get about .3 ohm resistance between the post and ground, which I believe is within the proper range per my forum research.
The fact that my magneto has provided sufficient operating voltage confirms in my head, like you say, that the magnets do not need recharging. And I'm not about to attempt a tricky operation like that unless I prove it to be absolutely necessary. No sense in risking the discharge of perfectly good magnets when it probably isn't the problem. The only way that the magnets could be weak now is if the wire short somehow either beat the charge out of the magnets or if the short exposed the magnets to battery current somehow while running the car. I have been extra careful to not knowingly expose the magneto to battery current, but I suppose anything is possible. Unlikely and nowhere near the top of my troubleshooting list.
There is no evidence of damage from an improper starter removal. There was no starter in place on the car when I got it, it had a blockoff. I did get a new starter motor and installed it following the service manual. Prior to installation I did a visual inspection of the magneto at the suggestion of a T friend. There was no evident damage and I haven't pulled the starter since installing it.
I'm embarrassed to say that the troublesome bit of wire looks suspiciously like what I used when first doing my new band linings, twisted over the band ears to hold them together while installing the nuts on the posts. I must have lost track of it and into the darkness it went. This was before I thought through the consequences of using ferrous metal near magnets and started using dental floss to tie them together.
I will attempt to disprove the end play theory and will report back to the group in the hopes that it helps someone down the road. Or to get additional advice if it turns out I successfully disprove the theory.
You are a rare T guy in that you follow instructions before screwing with something. Usually the truth has to be pried loose ha ha. I make a bit of things for T's and usually I get something back that is not working and the customer says "it was working fine and then quit" I asked what he might have done to resolve the problem and the answer is "nothing at all I just removed the unit and sent it to you". The problem for me then is how is it that internal parts are now held in place minus certain insulators and with phillips head screws which we do not use on anything we manufacture. Makes it hard to help someone if they try to cover their tail ha ha.
If that is steel wire, it could very easily got caught on the magnets and torn up the magneto coil. You might never know just what is the cause of the problem until you remove the engine and are able to see and check the coil. If it is an old coil, the insulation gets very brittle and it doesn't take much to short or ground it.
Zach, it held up for a year but I have 6 other T's and I rotate driving them so I would estimate it held up for a bit over 500 miles of driving. It wore down to the point where the gap was again to large to keep the mag operating well. You could easily braze more brass on it to keep it going but I finally pulled the engine and did a total rebuild. I guess with the later cars you can replace the lower cap on the 3rd main to correct the end play without pulling the engine but with a one piece pan the only option for me was the spacer.
I had post problems with mine. I finally found that the spring was going off to one side when screwing down the post to the trans cover.
I had purchased a new coil ring after doing the above. The contact solder button on the mag ring was off to one side about an 1/8".
I used a mirrow and finally noticed it after I looked at it when assembling the outside oiler and installing it.
Measured the crank end play. It was 0.074. Easily visible by eye. Moves easily when the control lever is put forward and then stays where I shift it to. If I pull the lever all the way back, the crankshaft pulls back with it and when I pry it forward it moves back and forth. I assume I am feeling the clutch spring.
So, as I see it, I can either shim out the pulley with shim stock to make up the difference (would require about 0.322 between the back of the pulley and the crankcase). I could rebabbit the rear main which I am in no way qualified to do. Or I could buy the somewhat short lived commercial shim that Val mentioned.
The magneto is not much of a concern to me right now, but I am worried about the crankshaft sliding around and breaking or bending itself or something else.
You can also buy a rebuild 3rd main cap and make it the right size and replace your third bearing.
(can be done under the car). You can't set up the clearance of your magneto but you will eliminate the crank shaft play.
The best way to repair is taking out the engine and take off the oil pan so you can see if the wire had destroyed other parts.
I somehow think the mag ring is electrically OK but clearly the motor is pretty "tired". If you opt to fix the end play, I would assume the gap is OK on the mag and buy and install a built up main bearing end cap. I think that might fix the magneto and the end play at the same time. If there are no shims left in the motor mains or rods you might want to think about just doing the motor during the cold months ahead but I am being generous with YOUR $$$
I do believe you are correct, that the ring is electrically ok. I jerry-rigged some thin machinery bushings in behind the crank pulley (temporary fix) to see if I could get everything to work properly. After a couple of minutes of warming up and revving the engine from idle to high idle, the mag kicked back on like it used to. So, I'm fairly certain the end play is the only issue. My machinery bushings weren't perfectly set up, so I'm hopeful I can set up the accessory shim to provide the proper clearance for a functional mag all the time, not just sporadic function.
I plan to redo the engine soon. It's already got new rings, new valves, new head gasket, tightened up rod bearings, new cam bushings etc. The only thing I' haven't messed with is the main bearings, so of course that is the source of the problem!
I've been trying to get the car to an enjoyable state off and on for the past year, and was hoping to have her good to go (at least short term) yet this year for some fall touring. I've been through just about everything mechanical and this is the last thing that is bothering me. I've decided to run (again, temporarily) the end play shim accessory from Langs for a while and will rebuild the engine when the old car fund is regenerated.
Well, just an update to this thread. A while back I bought and installed the bronze crankshaft end play repair shim from Langs but it didn't fix the problem.
I figured that the wire I had pulled out of the transmission (from my idiotic use of wire to tie the band ears together after relining them when I first got the car) had scarred up the mag coil leading to an intermittent short. Since I had no interest in pulling the motor or hogshead just to fix the magneto, I was going to live with it.
But, I got to thinking... I could at least verify the damage if I pulled the bendix cover and bendix, and then the starter. I could see the mag ring. So, I proceeded to take the cover off, then the bendix. Just for kicks I shined a light in the opening before removing the starter. Staring back at me was an all-too familiar shape... a loop of wire that once held the band ears together. It was touching the mag ring. There was also another smaller piece. I was able to recover that through the bendix opening with a long flexible magnet.
Before reassembling the bendix, I visually verified, as best I could, that the mag rings aren't scarred up. I saw no copper colored scratches or obvious damage from the wire, so I think I may have gotten extremely, extremely lucky, not only that the damage didn't occur but that the culprits were easily retrievable with a little thought. Anyhow, I put it all back together, started the motor, flipped the switch to mag, and SHE PERKED RIGHT UP AND RAN STRONG ON MAG.
Verified the voltage output at idle, about 5 volts using the Regan test. At speed, I get about 19 volts, which I understand may be a little low to get peak performance from the mag. She may need a recharge, but I will drive a while and see what happens. This weakness could also be the unknown magneto gap, as I'm unsure if it is correct now with the endplay shim in place.
I was also able to finally verify that the coils I rebuilt, my valve replacement job, and the new carb are tuned properly, as the car easily crank started on mag (one sharp pull up) after I advanced the timing a bit. I understand that this ability is a good indicator that the engine is in good fettle, at least tuning wise. The bearings are probably worn but I'm not getting any knocks and I've shimmed the end play.
It is nice to know that the problem is fixed and that I didn't cause any major damage through my stupidity and inexperience. Thank you all for your help, advice, and for (hopefully) not making too much fun of me.
(Message edited by zdillinger on October 09, 2014)
(Message edited by zdillinger on October 09, 2014)
you are one lucky guy
Good for you Zach!
Another testament to the basic simplicity, ruggedness and durability of the basic Model T!
(And your attention to Mechanics 101).
I'm glad to hear the good news.
I use strong string to hold the band ears while installing the hogs head. At least if some is left behind it won't be any more damaging than band lint.
Zip ties work to hold band ears as well and won't hurt the engine if one breaks and a piece falls inside
Thanks guys. Needless to say that I will not be repeating my "wired band ears" escapades and will rely on something like strong string or zip ties in the future. Probably the string.
I got really lucky this time, and don't want to give the Model T gods a chance to whoop me harder next time.