Shorted coil?

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bobt
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First Name: Robert
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* REQUIRED* Type and Year of Model Ts owned: 1915 touring 1926 roadster
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Shorted coil?

Post by bobt » Fri Apr 16, 2021 6:13 am

I drive my T's a lot. Yesterday. after a short drive, I left my 1915 touring idling running on magneto while I ran in the house for no more than two minutes. When I came out, smoke was pouring out of the coil box. I shut her down and removed the coil box lid and found the number one coil had tar bubbling out of it. I'm guessing it shorted out? I had a spare rebuilt coil and everything seems OK now. Has anyone had this problem? What part of the coil would you suspect? Thanks, bobt


Luke
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Re: Shorted coil?

Post by Luke » Fri Apr 16, 2021 6:59 am

The points probably stuck on, you might get lucky and find the coil still works if you repair/replace the points...


Randall strickland
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Re: Shorted coil?

Post by Randall strickland » Fri Apr 16, 2021 7:01 am

I had one do that a few years ago, one of the post on the timer shorted out to the case.


TXGOAT2
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Re: Shorted coil?

Post by TXGOAT2 » Fri Apr 16, 2021 9:52 am

Stuck points could do it. If it was bubbling tar in two minutes, I'd also look for a shorted timer contact or a short anywhere in the wiring between the coils and the timer unit. If a short occurred in the coil itself, it may have damaged the timer and/or the wiring.


Ron Patterson
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Re: Shorted coil?

Post by Ron Patterson » Fri Apr 16, 2021 12:38 pm

Bob T
The problem you describe is a common problem with Model T's and the cause must be found or it will happen again.
The problem is usually in the area around the timer and it's wiring, but can be in other areas too. Anything that causes a timer wire to get shorted to ground ANYWHERE (other normally by the timer) Remember the timer wiring normally moves with the timer case and the wires may be frayed and shorting to ground. It could also be with one of the engine pan bolts immediately under the timer is installed wrong allowing one of the timer terminals shorting to ground at the head of the shank of the bolt during its normal travel during spark lever movement. I once saw a timer wire at the coil box had frayed by rubbing against the firewall or radiator support rod cause a short to ground. .
If one of the timer terminals or flag terminals gets shorted to ground the associated coil in the coilbox will operate continuously on battery (this will happen on magneto too if you wait long enough
Coil Boil.JPG
) regardless of the timer rotor position exceeding its duty cycle and get very hot causing the tar to boil out the top of the coil around the iron core. In some cases the short may only occur when the timer case and spark lever are in one position (remember it rotates as the spark lever is moved).
Here is a way to locate the shorted timer wire. Turn off the gas inlet supply and run the engine till it stops so the engine will not start during this procedure. Set the emergency brake and turn on the key switch to battery position. Now slowly crank the engine over through two complete turns and ensure that ONLY ONE coil is buzzing at any given time. You may have to move the spark lever to several different positions to get the short to exhibit itself and cause one coil to operator continuously.
In any case With a little patience you will find a place where one coil is buzzing continually or two at one time. One of them has a short to ground.
REMEMBER do not let any one coil buzz for extended time without giving it a rest or you have a coil boil.
If any coil is left buzzing continually for a long time on battery you have six volts feeding a 1/3 ohm coil primary winding and there is 20 amps current flowing and the coil gets very hot fast. If you use a 12 volt battery in your Model T that is 40 amps I.E that would be very very fast.
Good luck and if you run into trouble give me a call.
See attached photo. And remember if this happens the tar jams the points closed which makes matters worse.
When this happens the coil is usually NOT badly damaged and can be repaired by someone who knows what they are doing.
Ron Patterson


Topic author
bobt
Posts: 228
Joined: Tue Aug 13, 2019 5:43 am
First Name: Robert
Last Name: Thompson
* REQUIRED* Type and Year of Model Ts owned: 1915 touring 1926 roadster
Location: virginia

Re: Shorted coil?

Post by bobt » Fri Apr 16, 2021 7:33 pm

OK. I tried everything on Ron's post to no avail. I'm using a repro New Day timer and I replaced the timer wiring harness months ago. It is still in great shape with no rot or frays. One thing that is different is my coils. the refurbished ones I've been running for at least seven months have a copper plate on top of the coils and set higher than my old wooden type. The coil box lid is steel so maybe??? I just got in from my daily drive with no problems. The # 1 coil was the one that I smoked so now I'm running a shorter wood top coil on # 1 and copper tops on 2-3-4. Thanks for all the input folks and I'm still open to any more suggestions. bobt

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MKossor
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Re: Shorted coil?

Post by MKossor » Sat Apr 17, 2021 2:41 am

If any coil is left buzzing continually for a long time on battery you have six volts feeding a 1/3 ohm coil primary winding and there is 20 amps current flowing and the coil gets very hot fast. If you use a 12 volt battery in your Model T that is 40 amps I.E that would be very very fast.
Huh? A properly functioning Model T coil that is properly adjusted for equal and consistent firing Time will Not get very very hot very fast operating on 6V or even 12V battery in my experience. Here is how I tested it.

First, start with a properly adjusted coil.
Coil Adjustment.JPG
Coil Adjustment.JPG (33.92 KiB) Viewed 2040 times

Note that this coil has a 13% lower primary inductance (2.88mH versus 3.3mH) which means the coil firing current will be ~13% higher than typical. Power is proportional to coil current squared so this coil represents somewhat of a worse case for power dissipation.

Hook up the coil primary to fully charged 12V Car battery (note the float voltage 14.63V) and wire directly to coil under test (bottom power terminal to side timer terminal) allowing it to buzz continuously with a proper 1/4" spark gap. Coil current is calculated using a HA50-100 (100mV/50A = 0.002 Ohm) current shunt resistance. Voltage is measured across the current shunt using an Lab grade HP 3400A RMS voltmeter with accuracy of better than 5% from 10Hz to 10MHz. Coil current = (shunt voltage)/0.002 Coil current was observed to vary between 2A and 2.25A RMS, much much lower than could flow through the low (0.3 Ohm) primary winding resistance if the points are not opening very periodically and very consistently.
Test Setup.JPG

Temperature is monitored via Type-T thermocouple in direct contact with the iron wires of the coil core and by Flir IR thermometer. Elapsed time is measured on an analog clock with second hand.
Coil Core Temperature Vs Time.JPG
Coil Core Temperature Vs Time.JPG (42.46 KiB) Viewed 2040 times

The coil Iron core reached a temperature of only 102F after 10 minutes of continuous operation buzzing on 14.6V DC battery. The hottest part of the coil was the High Voltage terminal which measured between 130F -140F as shown on the IR scan.

Summary
It is not good practice to allow Model T coils to buzz continuously for long periods (minutes); especially if you don't know if your coils were properly adjusted for equal and consistent firing Time. Coil core melt down can and does occur if the coil points malfunction (broken rivet or contacts) or anything that interrupts the coil contacts from opening normally (e.g. coil box lid) as they are supposed to open when properly adjusted. In the event of such a fault, coil current can easily reach the 10A to 20A range very quickly and cause a melt down and fire hazard. You can easily add fail safe protection to your car wiring by installing a 4A fuse in line with the coil box power wire in a discrete location. This is separate from the 25A fuse recommended to protect the main +6V battery power wire which does not protect from coil melt down.
I-Timer + ECCT Adjusted Coils = Best Model T Engine Performance Possible!
www.modeltitimer.com www.modeltecct.com


TXGOAT2
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Re: Shorted coil?

Post by TXGOAT2 » Sat Apr 17, 2021 11:05 am

As long as a coil is buzzing, beside the fact that it is not "on" all the time, it has impedance as well as DC resistance. Impedance is a property of inductive devices operating on interrupted DC or on AC current, especially high frequency AC current. It has the effect of adding electrical resistance to the device. A buzzing model T coil in good condition probably pulls around 1 A or less during the time the timer contact is grounded or if it is left buzzing due to a shorted timer or timer wire. If the points stick, the impedance mostly disappears and you are left with only the DC resistance, which is quite low, and current draw goes up dramatically. Some impedance would occur when operating on AC magneto current, but I'd think it would still be far less with the points stuck than otherwise. I once had a metal can type coil burst open on an old 6V Plymouth I was working on. I had left the key on, and the points happened to be closed, and after about 5 minutes or so, the coil got so hot it blew the end out of the metal can, spewing hot oil all over the engine. I will be following Mr. Kossor's excellent advice and adding an inline fuse to the coil feed.

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MKossor
Posts: 458
Joined: Sun Jan 06, 2019 7:30 pm
First Name: Mike
Last Name: Kossor
* REQUIRED* Type and Year of Model Ts owned: 1927 Touring
Location: Kenilworth, NJ 07033
MTFCI Number: 22706

Re: Shorted coil?

Post by MKossor » Mon Apr 19, 2021 3:31 pm

Received inquiry on a suitable fuse holder and location to protect against coil melt down. Here is an example available from Advance Auto Parts
https://shop.advanceautoparts.com/p/lit ... ine%20fuse
In-Line Fuse.JPG
In-Line Fuse.JPG (24.64 KiB) Viewed 1863 times
I located this fuse on my 1927 Touring directly under the coil box. Cut the fuse wire in the center then crimp wire terminal on one end for connection to the bottom coil box power terminal nut. Connect the other end to the coil box power feed wire normally connected to the coil box power terminal using a wire to wire crimp connector. If you have soldering equipment, solder the fuse wire to the coil box power wire and insulate with short length of heat shrink tubing. Be sure to install a 4A fuse and keep spares under the seat.
I-Timer + ECCT Adjusted Coils = Best Model T Engine Performance Possible!
www.modeltitimer.com www.modeltecct.com

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