MTFCA Forum

You'd think ideally that 0 ohms would be the ideal amount. In my coil box, there are certain things that I, can not for the life of me, get below 0.3 ohms.

I've soldered the posts to the cleaned up bronze contacts, and before and after soldering, 0.3 ohms was the lowest I could get between both contact and screw, and even contact and nuts. I even got 0.3 ohms of resistance from the wire (that will be connected to the switch) that is connected to the bottom strip via an eyelet in one of the rebuild kits. I began to wonder if 0.3 ohms was just the "natural" resistance of the size and type of material.

I then went ahead and cleaned up the contacts as much as I could in the switch as seen here: I cleaned as much surface as possible without removing the rivets, including where the mag and bat terminals are, and the large switch contact. The place I couldn't get was under the contact with the terminal connecting to the bottom strip of the coil box. For fun, I tested the conductivity, and low and behold, I once again reached 0.3 ohms in this position. To my surprise, in the opposite position, I actually reached 0 ohms (my meter goes to the tenth, so less than 0.05ohms). Going all the way across to the last terminal however, it hovered around the 1.0 ohm range. This is likely due to the face that I can't get my emery cloth under the last contact.

So, I'm wondering if 0.3 ohms is within tolerance. Reading the Ford Magneto section in Murray Fahnestock's book, the derived resistance as per Voltage=Resistance*Amperage from the magneto output is as follows:

0200 RPM 5.00V 6.1A 0.8ohm
0400 RPM 9.80V 7.9A 1.2ohm
0600 RPM 14.4V 8.5A 1.7ohm
0800 RPM 18.8V 8.8A 2.1ohm
1000 RPM 22.8V 8.9A 2.6ohm
1200 RPM 26.2V 9.0A 2.9ohm

I suppose my entire point of this, is that I would love to make the switch usable (clean points, new wire) without having to remove the rivets. It appears that the 0.3ohms is more material at lower speeds than higher. However, if I could get to 0ohms for one of my contacts, why am I having so much trouble with the others? Are there any suggestions on cleaning the points?

FWIW, all contacts, nuts, terminals and screws are all original. The wire mentioned above was included in the rebuild kit, and I have bulk wire ready for the switch as soon as it's cleaned.

I am wondering whether your meter may be misleading you somewhat. The voltage it applies and the current flowing will be very small.
I wonder if it's more meaningful to compare the current supplying a bulb directly and via the switch. I suggest a 12v stop or turn lamp bulb. That should take about 1.7A. Most meters have a 10A range available.

Chris Barker wrote: ↑

Fri Jan 15, 2021 6:05 am

I am wondering whether your meter may be misleading you somewhat. The voltage it applies and the current flowing will be very small.
I wonder if it's more meaningful to compare the current supplying a bulb directly and via the switch. I suggest a 12v stop or turn lamp bulb. That should take about 1.7A. Most meters have a 10A range available.

I agree . I work on printing equipment and have for 40 years. Tracing old wiring and troubleshooting. I think you will be fine with what you have. You would probably get close to the same readings on a new switch.

I worked on series stop button circuits sometimes with 25 contacts in series. 24 vdc feeding the circuit and maybe 12 vdc at the trailing end. Enough to pull the contact in, but the cascading resistance of each contact stacks up and drive you crazy. Replacing all contacts new helps some, but out of the box new is not 0 ohms.

Zero resistance isn't really possible, especially with some of the older metals. Remember, amps = voltage ÷ resistance.
So 12 volts ÷ 0.3 ohms = 40 amps.

Greg: Your measured value of 0.3 ohms is reasonable and I'd be comfortable using your switch. If you check the resistance by placing both probe tips on the polished blade portion of the switch (eg, place black probe tip next to red probe shown in second pic), what is the resistance reading? I doubt you will see exactly 0.0 Ohms even doing that. More and less pressure on the probe tips obviously gives different results and probe contact resistance can easily give values approaching what you are seeing.

Chris has the answer.

Greg is correct stating Ohm's law, but there are higher resistance components in series with the switch as Chris notes, making 0.3 Ohms a very small contribution to the over all circuit resistance. Applying 12V directly across the switch is essentially a welder circuit, you aren't restoring a welder here. Best, jb

James
Is your meter calibrated, have you adjusted the meter to zero with the probes shorted directly together and are your probes in good shape and not frayed internally?
Ron Patterson

jab35 wrote: ↑

Fri Jan 15, 2021 12:16 pm

Greg: Your measured value of 0.3 ohms is reasonable and I'd be comfortable using your switch. If you check the resistance by placing both probe tips on the polished blade portion of the switch (eg, place black probe tip next to red probe shown in second pic), what is the resistance reading? I doubt you will see exactly 0.0 Ohms even doing that. More and less pressure on the probe tips obviously gives different results and probe contact resistance ...

My point also. The most important reading you have shown was 1 ohm with a contact on the Bat screw and the other on the switch arm in one position. but not the other. You need to test the entire current path also. The pressure of the coiled contact on the switch arm may not be consistent

Thank you everyone for the replies.

When I connect both probes together, I do usually get the 0.0ohm reading, however, a little bit of readjusting and often the 0.3ohm reading will come up. I don't know the exact age of the meter. I would guess in the 15-20 year range.

I did test on the base instead of the screw as well, and nothing really changed.

If the 1.0 ohm is material, I wonder if there will be excessive heat while driving, especially from the high voltage the magneto produces.

ryanf1023 wrote: ↑

Fri Jan 15, 2021 1:58 pm

Thank you everyone for the replies.

When I connect both probes together, I do usually get the 0.0ohm reading, however, a little bit of readjusting and often the 0.3ohm reading will come up. I don't know the exact age of the meter. I would guess in the 15-20 year range.

I did test on the base instead of the screw as well, and nothing really changed.

If the 1.0 ohm is material, I wonder if there will be excessive heat while driving, especially from the high voltage the magneto produces.

If you got 1 ohm as shown in the photo from the Mag side I would expect that you would get 1 ohm from the Bat position as well. If not then the contact to the switch arm should be checked.

I did as you suggested Frank, and yes. I got 1.3ohm for the mag and 1.2ohm for the battery. I think the difference of 0.1ohm is that the contact on the bat is slightly tighter than the mag one, perhaps due to more mag driving in its lifetime than bat. If this is normal I’m happy to proceed with the wiring.

Hi,
You may be going about this the wrong way. Corrosion is best tested under load. So, you really need to do a voltage drop test while the proper current is passing though a device. For instance, measuring electricity between a battery post and the clamp will change once the starter is moving or even headlights. You might get .01 ohms for a VOM but an oxidized cable under load will give a different value. Just research voltage drop test.