Mystery magneto

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Model T Ford Forum: Forum 2015: Mystery magneto
Top of pagePrevious messageNext messageBottom of page Link to this message  By Don Buelke on Saturday, August 29, 2015 - 07:15 pm:

While cruising up the highway with my 26 Coupe this week, it suddenly started backfiring and died while running on mag. As it coasted to the shoulder I tried starting it with the inertia, pulled the choke once or twice, backed off the timing but it wouldn't run. At the last moment I turned the key to battery and it started right off, so I managed to keep my appointments and get back home.

I've read the June thread on this subject and have run every diagnostic I could with the equipment at hand right now. The mag post had a little lint, but cleaning and replacing it didn't change things. The resistance from the mag post to ground was about 1.5 ohms. There was good continuity from there to the terminal block and through to the coil box when the key was on mag. However, this sudden loss of mag power with no other signs still made me suspicious of a loss of continuity. So I tried the jumper test and while running on battery, shut the ignition off, and connected the coil box terminal directly to the mag post, but the engine still died.

There isn't a mag tester in the area and I only have digital meters so they register dysfunctionally when doing the light bulb test, although the bulb glowed at increasing intensity from idle to about half throttle. I'm thinking that even if the mag isn't up to full output, if there is enough to light the bulb, shouldn't it at least run poorly and not just die?

I'm trying to access an analog meter and a mag tester, but in the mean time does anyone have any other suggestions I can pursue. Sure shudder at the thought of having to tear into the hogshead!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Saturday, August 29, 2015 - 07:35 pm:

Don, Rick is visiting me on Tuesday. If you can get it back to me, I can have him take my St. Louis meter to you, if he will do it.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Hap Tucker in Sumter SC on Sunday, August 30, 2015 - 07:22 am:

Don,

Yes, if the mag will light up the light bulb it should at least do something for the ignition. I suspect you may have a switch or wire problem. You can also purchase a very inexpensive analog meter at Walmart and other locations for under $11 dollars. And many folks have one in their tool box you can borrow. The nice thing about the new one – it still has the directions….see walmart add at: http://www.walmart.com/ip/Voltage-Meter-10729W/14521542 And HomeDepot has some for under $9. Just be sure it says analog.

From the 1926 owner's manual at: http://www.mtfca.com/books/1926Inst.htm at the end it has a trouble shooting guide.

One item it mentions for the engine stopping suddenly is:

5. Magneto wire loose at either terminal.

A bad switch contact could also produce a similar result but that was not listed.

And be careful as you are trouble shooting/wiring etc. Do not allow the battery power to go back to the magneto post as it can demagnetize the magnets.

Good luck and let us know what you discover.

Respectfully submitted,

Hap l9l5 cut off


Top of pagePrevious messageNext messageBottom of page Link to this message  By James A. Golden on Sunday, August 30, 2015 - 09:41 am:

I keep a good supply of those 1157 bulbs on hand for my cheapie/quickie Mag test, as a good mag will light that bulb at an idle and burn the bulb out at a fast idle.


Top of pagePrevious messageNext messageBottom of page Link to this message  By joe bell on Sunday, August 30, 2015 - 10:42 am:

Don,
I have had many come in with mag issues and ended up being a brass rivet logged from the wire to the casting and it was usually by the top post? do not know why there unless it was a good place to get thrown in to. Most the time it was removing the hogshead and looking for the short.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John C Codman on Sunday, August 30, 2015 - 11:07 am:

Hi Hap, Why do you insist on an analog meter? I can think of no reason why a digital meter wouldn't work just as well on a T. Analog meters are death on modern cars. Any electrical testing that I have done on my T is with a digital meter, and there have been no issues.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bill Harper - Keene, NH on Sunday, August 30, 2015 - 11:23 am:

Hi John,

I, too, have used a digital meter for "chasing" battery voltage, or it's absence and general continuity checks on Model Ts with success. The magneto voltage is a different animal and only an analog meter will give an accurate reading. I will have to leave the explanation of why and why not to the more electrically knowledgeable. (so many milli-fubs of impudence and those Mean Square Roots)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dave Dufault on Sunday, August 30, 2015 - 11:54 am:

And....

http://www.mtfca.com/discus/messages/179374/183126.html


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Gregush Portland Oregon on Sunday, August 30, 2015 - 11:57 am:

Depends if you are chasing a short with the engine running or not how well a digital meter will work. I too have tried the digital meter to check the output of the magneto, the reading was bouncing all over the place.
"I can think of no reason why a digital meter wouldn't work just as well on a T", try checking your output from the magneto with your digital meter and see what happens.

When I check my output I use my coils for the load instead of the light bulb.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Don Buelke on Sunday, August 30, 2015 - 05:14 pm:

Tom, thanks for the offer of the St. Louis meter. Rick will pick it up and we'll see what we can find.

Joe, I don't like what you had to say, but I'm afraid your the closest! Having bypassed the switch with a jumper, I don't see where the problem could still hide in the circuitry.

For others: This is Montana and the nearest Walmart or Home Depot is 35 miles away. We're enshrouded in dense smoke from forest fires in the region with visibility of a mile at times and I know what I need and where I can get it, but not today! Probably get it as fast from Amazon.


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