Voltage drop on Magneto meter shifting from B to M

[49willard]

My 12 T is equipped with the later Magneto meter. I see a significant voltage drop in indicated on the magneto meter when I shift from Battery to Mag. What should I see? It currently is unhappy running on Mag.

[Moxie26]

What are the A / C numbers that you refer to?.... Possible that the magnets would have to be recharged. You always see a slight decreased voltage when it is used by the ignition system.

[Jerry VanOoteghem]

When you switch to MAG you are now placing an electrical load on the magneto. Either your magneto is not in good enough condition, or your electrical load is too great. Something shorted? Or, you may have a poor electrical connection somewhere that will not carry the load.

[TXGOAT2]

Coils that are out of proper adjustment can draw more current than they should as well as giving poor performance.

[MKossor]

Pat and Jerry both raise relevant points. Voltage may also be dropping because the Magneto output resistance is too high.

Check the Magneto post contact is making good contact internally. Read previous posts where band lint gets caught up in the Mag post contact raising the contact resistance.

[49willard]

Thanks for your responses.
I installed a rebuilt coil pack this winter, recharged magnets (3# measured pull) and checked .006 inch crankshaft thrust and gapped .025-.040 coil pack to magnet. The resistence to ground either with the post in series or directly on the mag ring contact spot is a little high at .6 to .7 ohms. I have tried 2 sets of Ron Paterson rebuilt coils, I do not think coils could be the problem. What I am looking for the driver of a later T with the Magneto Meter how much does the voltage drop going from battery to mag. i.e. 10%, 20% 30% more?

[TXGOAT2]

Some voltage drop is normal, and it would probably be less as a percentage of the voltage reading at higher engine speeds, since available current at higher RPM would be greater than at lower RPM or cranking speed.

[TXGOAT2]

The Ford magneto should show AT LEAST 7 volts on an AC voltmeter with the engine running at 400 RPM and no load on the magneto. 400 RPM engine speed would move the car at about 9 MPH in high gear.

[John.Zibell]

My 12 T is equipped with the later Magneto meter. I see a significant voltage drop in indicated on the magneto meter when I shift from Battery to Mag. What should I see? It currently is unhappy running on Mag.

If unhappy running on Mag, you have a problem. Possibly the mag output is insufficient. Even with recharged magnets, your gap to the coil ring may be to varied to produce sufficient output. The closer to 0.025 inch all your clearances are, the better output you will have from the magneto. Also check wiring and resistances in wiring, mag post, and switch.

[TXGOAT2]

Erratic contact between the coil units and the coil box can cause a lot of rough running issues.

[49willard]

I just took a measurement at above a high idle running on battery with the mag voltage at 15 V if I flip the switch to mag, the indicated voltage drops to 10 v and the engine runs rough. These are the indicated voltages on the Ford T magneto meter. It seems like a lot of voltage drop. I am asking what others with a magneto meter have experienced.

[TXGOAT2]

I don't think it should drop that much, and the engine will normally run better on magneto than battery, especially when running above idle. I don't have a magneto meter, but I do have a tachometer and a VOM. I'll see if I can get some voltage readings at a specific engine speed on battery and then on magneto. My car runs OK on 6 volt battery/generator, but it runs VERY well on magneto, delivering a "smooth, even purr" mile after mile.

** Your problem may be a "weak magneto". I'm not a Ford magneto expert, but I'd think that a magneto with weak magnets might show a fairly high no-load voltage yet suffer an abnormal voltage and current collapse when a load is applied. Of course, a high resistance electrical connection in the wiring or in the switch could present the same symptom.

[MKossor]

Ron Paterson rebuilt coils, I do not think coils could be the problem.

Find someone in your area with an ECCT and have them test and tune your coils for equal and consistent firing time using the ECCT. I think you will be pleaently surprised by a marked improvement in engine performance.

[49willard]

I don't think it should drop that much, and the engine will normally run better on magneto than battery, especially when running above idle. I don't have a magneto meter, but I do have a tachometer and a VOM. I'll see if I can get some voltage readings at a specific engine speed on battery and then on magneto. My car runs OK on 6 volt battery/generator, but it runs VERY well on magneto, delivering a "smooth, even purr" mile after mile.

** Your problem may be a "weak magneto". I'm not a Ford magneto expert, but I'd think that a magneto with weak magnets might show a fairly high no-load voltage yet suffer an abnormal voltage and current collapse when a load is applied. Of course, a high resistance electrical connection in the wiring or in the switch could present the same symptom.

Pat,
I would appreciate the result of any test. Is your VOM an analog meter with AC capability?

[TXGOAT2]

I have a digital VOM and I have a cheap analogue one... if I can find it. The digital meter may work OK with my car since it has the late style metal coil box under the hood. I can run a portable radio around the car with no significant interference on either AM or FM and on both local and distant stations. This meter can test 480 volt 60 HP 3 phase motors under heavy load with no issues, so I doubt a model T magneto would challenge it. The RF from the coil secondaries seems to be well contained by the coil box and hood, and the very short plug wires also limit RF radiation.

[TXGOAT2]

OK, here are some voltage readings when running on magneto and on battery:
I tested with both the cheapie analog meter and a basic Fluke digital hand held meter. Readings were identical.
With the engine running on Magneto at an indicated 1,000 RPM, the magneto voltage reads 19.6 to 20 volts on the AC 0-50 scale, measured at the post.
Switching to Battery and adjusting the RPM back to 1,000 gave a reading magneto voltage reading of 20 to 20.5 volts on the same scale. That's very little difference, if any. For reference 1,000 RPM gives a road speed of 25 MPH in high gear. I doubt if my magneto is performing at 100%, but it's pretty good.

To sum up, I measured a magneto voltage drop when switching from Battery to Magneto of under 5% at a road speed of 25 MPH or 1,000 engine RPM.

[Scott_Conger]

Conductive contaminants in the timer have been known to wreak havoc on MAG and often not so much on BAT

see viewtopic.php?t=38194

[AndreFordT]

Just My opinion.

Connect a 12V 21W light bulb between the magneto post and a bolt on the engine. Connect an analoge voltmeter the same way and set it on 200Vac.
Start the engine on battery. Watch the light bulb and the voltmeter.
The bulb should light up during the start and will become britgher as the RPM getting higher.
6Vac by the start
12Vac at Idle
18 Vac+ at a brisck idle
24Vac at 1000rpm
higher RPM the bulb will blow out and voltmeter will show 30+Vac.
This mean a good magneto

If not try to recharge the magnets in the car and test again.

Andre
Belgium

[49willard]

Pat,
Thanks for performing the test and posting your results. In summary the mag voltage drop switching from battery to mag should be small!
Last night I had a discussion with Bill Lucius in Ohio. Bill had rebuilt my mag ring recently. When I explained my situation Bill said "I think that you have weaken your magnets". This morning I went thru the in vehicle recharging process using 3 12v batteries in series. I now have all kinds of Mag voltage! Problem solved, a big thank you to Bill Lucius!!

[MKossor]

Great news! Congratulations on resolving your magneto performance issue.

Still think it would be well worth the effort to find someone in your area with an ECCT to re-tune your coils for equal and consistent firing time. Would be interested to view the test results before and after coil tuning with the ECCT.

Here is an example of what I would expect to see:
https://mtfca.com/phpBB3/viewtopic.php? ... ts#p354056

Of course, the detail that really matters is how the engine performs in the car on the road.

[Alden]

I seem to have a similar situation. Any idea how--"I think that you have weaken your magnets"?
I may have left the switch in the mag position as I had shut off the gas and let in run out to stop, but there should be no path to battery in that position, (even if it was over the whole winter) or are there gremlins that can slip in somehow?

[49willard]

Alden,
In my case I had observed an arcing from the spark plug wire connections at the firewall to the 12v cloth covered wire supplying 12 v + power to the coil box. I had relocated the 12V supply wire and the mag never performed properly after finding that issue. Hopefully a unique find. Bill indicated that the impressed voltage probably demagnetized the magnets of the magneto. He was right!

A thanks to all that responded.

[Scott_Conger]

knowing that along with your original request would have resulted in fewer and certainly identical suggestions to put things right, with far less speculation.

Good that you are running well and for bringing conclusion to the post

[Jerry VanOoteghem]

Alden,
In my case I had observed an arcing from the spark plug wire connections at the firewall to the 12v cloth covered wire supplying 12 v + power to the coil box. I had relocated the 12V supply wire and the mag never performed properly after finding that issue.

A thanks to all that responded.

This should have been your first posting...

[49willard]

In retrospect I should have posted the arching however in that the mag never worked right from the beginning after a full rebuild including rebuilt coils. I did not consider that the arching would have discharged the magnets that I fully charged individually and pull tested all magnets to exceed 3 # with a spring scale. Now we all know that can occur.

[Jerry VanOoteghem]

In retrospect I should have posted the arching however in that the mag never worked right from the beginning after a full rebuild including rebuilt coils. I did not consider that the arching would have discharged the magnets that I fully charged individually and pull tested all magnets to exceed 3 # with a spring scale. Now we all know that can occur.

All's well that ends well. Happy you've got it licked.