Magneto issue

[Curt]

Working on a 1925. Magneto works fine when the engine is cold and puts out 25 volts. After about a half hour of driving it stops working properly and drops down to about eight volts and will only run on battery. Once the engine cools down, voltage is back up. Has anybody experienced this problem? Any suggestions?
Thank you,
Curt

[BE_ZERO_BE]

Sounds like thermal leakage.
As the engine warms up the insulation starts to breakdown.
If the magneto is original, this is no surprise.

[BE_ZERO_BE]

It may have a short that happens when it it warm.
This would truncate the number of coils and reduce the output.

[Curt]

Thanks B_Zero_B,
This has been occurring for some time (500 miles). It goes from running perfectly on the mag to shutting down. I would have thought that if it was shorting out due to thermal growth at some point it would stay shorted as the insulation is further damaged.

I don't know if the mag is original but this is car was thoroughly restored and if it is original it is the only part on the car that is.

Have you actually experienced this phenomenon?

[Curt]

Sixteen million cars and 100 years - somebody must have experienced this before and have specific info. I am trying to avoid changing the mag only to find out that it is not the issue.

Any specific suggestions for troubleshooting would be much appreciated!

Thanks,

Curt

[Mark Gregush]

While the collective knowledge adds up to a lot of years, the number of people that read, post or visits this forum is small in comparison to the number of people that own them. Keep bumping it up every so often and may be it will be seen by someone that can help. Other thing, we are not there hands on to see what where and why so may be difficult to diagnose over the air. You might also try posting on Royce's forum or even Facebook's Model T page.
https://modeltfordfix.com/
https://www.facebook.com/groups/103574776049/

[Steve Jelf]

You're measuring voltage at the mag post, aren't you, and not downstream of the switch? Old time analog meter, or digital? (You want analog.) Obviously the voltage is dropping if the engine won't run on it, but it's good to have an accurate reading of it. Try removing the mag post and measuring voltage at the solder blob. If the voltage drops off there you know the problem is in the mag ring or the blob and not the post. If the mag ring has some temperature-dependent defect you'll have to pull the engine, remove the transmission, and fix or replace the mag ring. Fixing is covered in the MTFCA Electrical System book. I hope somebody can come up with better news for you.

[Mark Gregush]

Steve I don't think I would want to pull the mag post and check with engine running.
Would not be the first time a restored car had an old tired coil ring installed.

[Steve Jelf]

I don't think I would want to pull the mag post and check with engine running.

I'd put one of the meter leads against the blob and stuff a rag in the hole around it before startup. That should prevent an oil shower.

[Curt]

Steve,
Thanks for your input. Yes, I am checking the voltage at the mag post with an analog meter.

The difficulty in troubleshooting this issue is that it only occurs when hot, so there is limited time for diagnostics. I have found a couple of items that I thought could be the issue, but it takes 1/2 to 1 hour of test driving to prove them out. This may seem like a good problem to have, but it ii not my car! In fact I have not experienced it myself and the car runs great for me - but not for the owner who takes it on lengthy drives.

I am going to change the mag post - don't know how that could be the issue, but I have one, so it is worth a try.

Again, if anyone has any specific experience with this type of issue I would certainly appreciate your sharing of it. I have no problem changing the mag but never want to "through parts" at an issue, especially if it is someone else's money!

It certainly appears that the issue is related to thermal growth of something, whether that is a mag coil, mag ring, crank, etc.

Is it common that an old mag to work perfectly and then stop working, only to repeat the cycle with each cool down? My experience is that they either work or don't work.

Thanks,
Curt

[Steve Jelf]

If this was a common thing you would probably have some definite answers by now. If a change of mag post doesn't cure it, that leaves just the mag ring. I suppose something there is expanding with heat and making a temporary ground that goes away when the heat dissipates.

[R.V.Anderson]

This same, or a very similar, issue surfaced about 8 years ago on the Forum. Turned out that the screw holding the contact button in place was a hair too long. When it got hot, it would expand and touch the iron rivet holding the contact block to the frame, which shorted the whole works. Even though that was the only known case, we immediately switched to shorter screws in all our mags, which still held extremely well in the hard thermoplastic that we make the blocks from.

[Curt]

R.V. - thanks for your reply.

I am not following your explanation (I am not well versed in the details of the mag and terminology. Can you explain further?

Thanks,

Curt

[Steve Jelf]

Here's what R.V is describing.

At the top of the mag ring is this contact. The pointed, spring loaded shaft from the mag post pokes into the contact button (solder blob). Note the little indentations made by the mag posts that have been used with this ring. Coming from the left you see the copper strip that comes from the nearest coil. A small screw attaches the copper strip to the triangular insulator and the contact button is on top of the screw. If the screw is too long it can contact the rivet you see holding the insulator. When that happens most of the current generated by the magneto goes to ground instead of to the mag post.

[Curt]

Steve,
Thanks so much for the photos and explanation, it is very much appreciated.
I am going to measure the mag resistance cold, and then when it is hot and "failing" to see if I can detect a short.
Still amazes me that the problem is so predictable and consistent.

Curt

[John Dow]

Steve and R.V., Your thoughts on Curt's magneto malfunction interests me. I have a problem with my magneto in that it is intermittent and randomly cuts out then begins to work again. (Right now it isn't working.) The problem is with the rebuilt magneto itself. Checked with an analog meter there is no output with the engine running. Using a tiny remote camera I have been able to see well enough to re-melt the solder button in an effort to create a new clean contact point. This produced no positive result. I'm wondering if my problem is with the button screw grounding out on the steel rivet. It seems when I "fiddle" with the contact, the magneto will briefly work then quit again. So your explanation seems possible in my case. Now, without extreme measures is there any way I can fix it?

[Mark Gregush]

The coil ring is grounded at the end. You will be taking a reading that will include all the mass of the iron from the coil ring engine etc. if you take a reading off the button and hogs head. Now if you get no reading, then there is an open spot in the winding's not a short. The only way I could see it being done as you suggest is to be able to reach the probes thru the hole to check between the rivet that holds the block to coil plate and the button, it would be a tight fit but might be doable and might show a short between the two, maybe don't know as I said the end of the ribbon is grounded. You can sure give it a try. The red circle is the grounded end.

[R.V.Anderson]

John, your issue sounds more like it's either in your switch or else a cold solder joint somewhere in the field coil. If it was the rivet/screw issue under discussion here, your mag would work fine when cold and then would fade once it got hot, not work intermittently from the get-go as you describe.