Magneto Problems

Discuss all things Model T related.
Forum rules
If you need help logging in, or have question about how something works, use the Support forum located here Support Forum
Complete set of Forum Rules Forum Rules
User avatar

Topic author
namdc3
Posts: 154
Joined: Sun Jan 06, 2019 4:15 pm
First Name: Nikolaus
Last Name: Martin
Location: Kansas City
MTFCA Number: 32342
MTFCI Number: 23512

Magneto Problems

Post by namdc3 » Sun Jun 16, 2019 6:53 pm

John Regan once said in a post..."A few weak magnetos I have seen will put out correct low RPM voltage but then the position of the crankshaft shifts as the engine speeds up the the magneto never develops the higher AC voltage needed at the higher RPM's. Those will tend to indeed increase AC voltage with higher RPM but will just not be able to get that voltage over 20 volts. That can be problematic for higher speed driving."

This seems to be the problem I'm having. A few months ago, the 1925 ran great on mag and tested over 30 volts (34 if I remember right) using the 1156 bulb test. At low idle it was only about 4 volts, but that was perhaps an excessively low speed idle. It recently started to run terribly on mag when going down the road. It will run fine idling, but miss when running fast at a standstill or driving. It seems to run fine on battery, both at a standstill and driving. I just did the bulb test and can only get about 15 volts with the high lever pulled back. It didn't really change with the car on jackstands in high gear (lever forward). I've taken the mag post apart several times and even tested it reaching through with the mag post removed, which yielded the same results albeit with a little oil in the driveway.

What's my next step? Having the car in high only took the rearward spring tension off of the crank; is there a way to make it go forward for testing? End play is the only thing I can thing of, but I'm unsure why it happened all of a sudden. Are there field coil tests I could do? Am I correct that 15 volts is why it's missing at high speed? If so, why is 6 volts from the battery enough?

User avatar

Topic author
namdc3
Posts: 154
Joined: Sun Jan 06, 2019 4:15 pm
First Name: Nikolaus
Last Name: Martin
Location: Kansas City
MTFCA Number: 32342
MTFCI Number: 23512

Re: Magneto Problems

Post by namdc3 » Sun Jun 16, 2019 10:40 pm

Update: More details. I had recently changed the plugs and timer due to a high speed (25-30 mph) surge. This surge seemed electrical and happened on both battery and mag. I had switched from a used roller timer to a NOS swap meet non-S New Day. This evening I switched back to the roller timer. Now it drives down the road without total power loss and bucking on mag - but the little surge is still there. It also runs a little rougher on mag than battery when sitting in neutral at medium engine speed. Also, it is extremely sensitive to timing when running on mag down the road - as in one or two notches either side of the sweet spot makes it fall on its face. This doesn't seem right for running on mag, as the timing should change by 22 degree increments with broad changes to the lever setting. Something/multiple things aren't right, because it only goes 33 mph tops with factory gears, low mileage rebuild with domed pistons, and roadster body (I'm not that fat), and that feels like pushing it. I'm happy to go 30 mph, but it shouldn't be all she has and stumbling. I'm thinking the coils (pro rebuilt) aren't working well at high speeds on only 15 volts. Sound right? Back to my original question/problem, too, I had 30+ volts from the mag and now only have 15. Obviously something happened.


Piewagon
Posts: 67
Joined: Thu Jan 10, 2019 8:38 pm
First Name: John
Last Name: Regan
Location: St. Charles, IL
MTFCA Number: 0
MTFCI Number: 77
Board Member Since: 1999

Re: Magneto Problems

Post by Piewagon » Mon Jun 17, 2019 9:50 am

15 volts is clearly NOT enough for a good running T. A healthy magneto will idle around 7 or 8 volts and cracking the throttle will quickly and easily put it over 20 volts and usually burn out your 1156 bulb before you finish your testing. The magneto post is not any part of the issue unless the voltage is intermittent between zero and 15 but even then I would think it a separate problem from the low voltage. If you recently had your motor done professionally I would ask the rebuilder what to do since a good working magneto is definitely a part of the billed amount if not otherwise stated. If you were running the car on bat during the test of the magneto then there is no reason the thing should not put out well over 20 volts with the only load then being the 1156 bulb.

Is there any chance you shorted the battery in the car to the magneto post? Since magnets not being fully charged or having been accidentally discharged are one way you get 15V or so and a sick magneto. The other way is to have excessive gap between magnets and magneto coil. New motor work usually includes a new magneto ring but maybe not with your rebuilder. DO pursue this and get it fixed since running the car on magneto produces the performance that you want.

Good luck.

User avatar

Topic author
namdc3
Posts: 154
Joined: Sun Jan 06, 2019 4:15 pm
First Name: Nikolaus
Last Name: Martin
Location: Kansas City
MTFCA Number: 32342
MTFCI Number: 23512

Re: Magneto Problems

Post by namdc3 » Mon Jun 17, 2019 11:31 am

Piewagon, thanks. The engine was built around 2005 by a reputable guy. I bought the car in 2018, so no reach back (plus, he unfortunately has passed away). That being said, the PO didn't drive it much, so I wouldn't think the thrust surface would be worn badly. Plus, it's odd that I had such a step change from 30+ to 15 volts. I would think wearing would be more gradual. I have taken the water pump off, put a new fan belt on, and had the generator on and off several times, so I guess there is potential that the crank's natural axial location has shifted (aka, the wear was already there and the crank's natural axial location has shifted due to other forces that I changed). Alternatively, maybe something went wrong with the field coils. I wish there was an in-car field coil test, but I haven't read about one. I've been very careful not to put battery power to the mag, so I don't think that's it, but beats me. I read somewhere that Ford said you could only get 65% of the magnet strength with an in-car recharge rather than taking everything apart. I guess as long as I was careful and charged everything in the same polarity it is now, I shouldn't "hurt" anything (as in weaken the magnets rather than boost them).

User avatar

Topic author
namdc3
Posts: 154
Joined: Sun Jan 06, 2019 4:15 pm
First Name: Nikolaus
Last Name: Martin
Location: Kansas City
MTFCA Number: 32342
MTFCI Number: 23512

Re: Magneto Problems

Post by namdc3 » Mon Jun 17, 2019 7:21 pm

Update: I realize this is crude and approximate, but I pulled the lever back and inserted a stack of feeler gauges between the belt pulley and timing cover. Then, without rotating the crank, I put the lever in high gear, used a pry bar to pry the crank forward somewhat gently to not bend anything, and adjusted the feeler gauge stack to insert in the same location with the same feel of resistance. This was all done being careful that the crank pulley wasn't moving on the crankshaft. I repeated this procedure several times in the same and opposite order. The difference in feeler stacks was about 0.015". I realize that this is well over the 0.003" new-build setting, but it seems within the allowed functionality of the mag. I also realize that this was a crude way of measuring it, but it seems like it would be okay ballpark-wise. Am I wrong - would an extra 0.012" cause a 19 volt drop from the mag? Is there a way to test the field coils in car, should I try to do an in-car recharge even though I don't think they've seen battery power, is it possible for debris in the trans to have been slung somewhere that is shorting only part of the mag? I have a mag post on order just to cheaply rule that out for good, but I don't think that sounds like my problem. It seems like that would be more intermittent. I've looked in the mag post hole, and the solder blob seems okay and clean.


Scott_Conger
Posts: 2674
Joined: Sun Jan 06, 2019 11:18 am
First Name: Scott
Last Name: Conger
Location: Clark, WY
Board Member Since: 2005

Re: Magneto Problems

Post by Scott_Conger » Mon Jun 17, 2019 7:55 pm

That doesn't sound like enough wear to make a SUDDEN difference. I'm inclined to believe you may have a short on the mag ring. An in-car charge may provide enough current to burn it off.

My '13 has at least that much slop and though I cannot get it to start on MAG, it will sure run and idle on MAG.
Scott Conger

Full Flow Float Valves - deliver fuel like Henry intended!

User avatar

Topic author
namdc3
Posts: 154
Joined: Sun Jan 06, 2019 4:15 pm
First Name: Nikolaus
Last Name: Martin
Location: Kansas City
MTFCA Number: 32342
MTFCI Number: 23512

Re: Magneto Problems

Post by namdc3 » Mon Jun 17, 2019 10:18 pm

I just measured the resistance of the magneto solder blob to ground as 0.9 ohms with a cheap digital multimeter. Shorting just the leads of the meter together was about 0.4 ohms, so the mag was about 0.5 ohms. I gather from reading others' posts that this is higher than it should be (should be 0.25-0.30 ohms for the mag alone). I haven't read anywhere what this means, though.

User avatar

Topic author
namdc3
Posts: 154
Joined: Sun Jan 06, 2019 4:15 pm
First Name: Nikolaus
Last Name: Martin
Location: Kansas City
MTFCA Number: 32342
MTFCI Number: 23512

Re: Magneto Problems

Post by namdc3 » Wed Jun 19, 2019 11:59 pm

No other takers? Thoughts? Does the mag resistance tell anyone anything? FYI, I installed a new mag post, and got the same output at about 15 volts; no change, as expected.

User avatar

Topic author
namdc3
Posts: 154
Joined: Sun Jan 06, 2019 4:15 pm
First Name: Nikolaus
Last Name: Martin
Location: Kansas City
MTFCA Number: 32342
MTFCI Number: 23512

Re: Magneto Problems

Post by namdc3 » Fri Jun 21, 2019 12:36 pm

I am seriously considering an in-car magnet recharge. Although I have been careful and don't recall ever putting battery power to the magneto, something could have happened that I'm not aware of. The end play doesn't seem to be enough to drop from 34 to 15 volts. I was kind of hoping for more forum discussion from the electrical gurus. For example, maybe the field coil voltage being a little high would tell someone something.

User avatar

Novice
Posts: 203
Joined: Tue Jan 08, 2019 12:25 pm
First Name: Jim
Last Name: Davis
Location: Tomball,Texas
MTFCA Number: 49832
MTFCI Number: 24686
Board Member Since: 2017

Re: Magneto Problems

Post by Novice » Fri Jun 21, 2019 1:07 pm

You could check the field coils with a inductance bridge / meter. If You know the inductance for a new set of field coils and compare the two. Checking the A/C magneto output on a oscilloscope and looking at the wave form might indicate a problem if the wave form is different from a new or known to be good magneto. Also the low resistance of the field coils makes it very difficult to get a accurate meter reading unless You are using lab type instruments and have a solid connection to the coils. You can get a fraction of a ohm or more resistance change just by the way You hold the meter probes and the pressure exerted. Good Luck.

My 2 Cents worth maybe less.

User avatar

Topic author
namdc3
Posts: 154
Joined: Sun Jan 06, 2019 4:15 pm
First Name: Nikolaus
Last Name: Martin
Location: Kansas City
MTFCA Number: 32342
MTFCI Number: 23512

Re: Magneto Problems

Post by namdc3 » Mon Jul 08, 2019 12:36 pm

Thanks, Novice. I might need that luck.

User avatar

Susanne
Posts: 313
Joined: Sat Apr 13, 2019 6:06 pm
First Name: Susanne
Last Name: Rohner
Location: Northeastern California
MTFCA Number: 464
MTFCA Life Member: YES
Board Member Since: 1999
Contact:

Re: Magneto Problems

Post by Susanne » Mon Jul 08, 2019 1:34 pm

Worn crank end play allowing your magnets to distance themselves from the field coils? Happened to me, and it's a common enough thing they actually make main caps to make up for it in the short term...

I was thinking parasitic DC voltage, but that usually kills the magnets, not just weakens them... you can try the recharge, if it works, then you need to find out why your otherwise healthy magnets went soft. but my money is on crank end play and magnet to coil spacing.

User avatar

Topic author
namdc3
Posts: 154
Joined: Sun Jan 06, 2019 4:15 pm
First Name: Nikolaus
Last Name: Martin
Location: Kansas City
MTFCA Number: 32342
MTFCI Number: 23512

Re: Magneto Problems

Post by namdc3 » Mon Sep 02, 2019 9:44 pm

Follow-up: I did an in-car magneto recharge, and I think I'm in okay shape. Before recharge output was 15 V; after recharge was 26 V and the bulb blew followed by a new bulb that blew at 24 V. I'm questioning that I may have misread the meter a long time ago when I thought I had 34 V since the same bulb blew at 26 V after the recharge. The wiring on this car was pretty scary, with at least one loom being original with a Ford tag. It had many bare spots, the terminal insulators were shall we say part their prime, and I'm guessing the battery scrambled the magnets. Now with all new wiring and the in-car recharge, I hope to be in the clear. It did let a little magic smoke out of the transmission when I did the recharge (light, wafty curls out the open inspection cover), but I'm hoping that doesn't rear its head later on. I'm hoping that was a little paint or a contaminant on the field coil wiring. I took it out for a test drive, and it ran strong on mag - better than it ever did on battery. Time will tell if the little smoke ends up being a problem. Fingers crossed.

Post Reply Previous topicNext topic