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Old 04-22-2011, 11:48 AM   #1
Tom Wesenberg
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Default The Model A Ignition System

Model A Ignition System


The Model A ignition system consists of the battery, amp guage, ignition switch, coil, distributor, spark plugs, and wires connecting these parts. A failure in any part means no spark to the plugs.

The coil always has 6 volts at the primary terminal and the coil is connected to the points when the ignition switch is turned on. The popout switch in the OFF position grounds the points, so the distributor contact on the popout cable should show continuity to ground and the coil terminal (by the key end) should show no continuity to anything. In the ON position the distributor contact should show continuity to the coil terminal, and neither terminal should show continuity to ground.

Repro switches are simple OFF-ON single pole switches with no connection to ground. The main fault I find in repro switches is the contacts in the back of the switch are not a very positive contact and often don’t make a good connection.

When the car doesn’t start, check the simple things first. Pull the coil wire from the cap and hold it ” from a head nut while cranking the engine with the key on. You should have a good blue spark. If you have a weak yellow spark check the points and condenser first. Condensers are very easy and quick to change if you follow these steps exactly.

1. Have the new condenser laying next to the distributor.
2. From the passenger side, remove the ground screw FIRST.
3. From the driver’s side remove the condenser terminal screw and be sure to keep light pressure on the screw while you slip out the old condenser and slip in the new one.
4. Tighten the terminal screw, then go to the passenger side and tighten the ground screw.

Always set the points to .020” first, then set the timing. Remember that the points setting can affect the timing, but the timing can’t affect the points setting. Be sure to have lube on the points rubbing block. I always carry a small piece of sandpaper in my billfold just in case I need to clean points, or check an engine number at a swap meet, etc. A points file is good to keep in your car’s tool kit and only costs about a dollar. To check the function of the points, stop the engine with the points open and key ON, then short the points contacts off and on using a screwdriver. If this produces a good spark, then the points need to be cleaned. If you are getting no spark, but the coil is getting hot, then something is grounded between the points and coil, which keeps the coil turned on full time. Be sure to oil the distributor often.

The Model A coil primary consists of 250 turns of # 20 enameled wire. The coil secondary consists of 16,000 turns of #38 enameled wire. The condenser is .3 mf.* See page 390 in the SERVICE BULLETINS for a change in the coil connections. 1928-9 coils have the negative terminal on the passenger side, while Nov. 1929 and later coils have the negative terminal on the driver’s side. Date codes can be found on the coil mounting bracket.

When checking cylinders for power, it’s better to SHORT the spark plug, rather than to disconnect the plug wire. When the spark has no place to go it tries to find the easiest path to ground, which may be to jump internally inside the coil to the metal body, or arc at the coil tower.

1928-9 slant pole coils have the metal case around the primary terminals, so make sure the first thing to go on the terminal studs is an INSULATING WASHER, then a flat washer, lock washer, and a hex nut. Barrel nuts are used to secure the wires to the slant pole coils. NO boots are used on the coil wire.

To check coil polarity hold a pencil lead in the spark path with the plug wire about a third of an inch away from the distributor terminal. You’ll see a FLARE on one side of the pencil lead and this flare should be on the spark plug side of the lead. If you do a Google search on “COIL POLARITY”, CHICAGOLAND MG CLUB has an excellent short article about checking spark plug polarity and why it’s important. Check:
http://www.chicagolandmgclub.com/tec...neral/574.html

* Thanks to Vince Falter for the coil and condenser information.

This is the sheet of notes I handed out last night at our club clinic when I gave a talk about the ignition system. This applies to the original ignition system and parts. More notes can be added, especially for after market parts, such as: some aftermarket coils are oil filled and must be mounted with the tower facing UP.

Feel free to add to the list of notes if you can think of something I missed.

Last edited by Tom Wesenberg; 04-22-2011 at 11:55 AM.
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Old 04-22-2011, 12:35 PM   #2
Mitch Bunkin in PA
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Default Re: The Model A Ignition System

Tom,
Thanks so much for the knowledge you are knid enough to share with us.
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Old 04-22-2011, 01:38 PM   #3
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Default Re: The Model A Ignition System

I caught Tom’s posting on Ahooga, he was kind enough to allow me to use his information on my web site. Here is a picture to go along with his words.

"To check coil polarity hold a pencil lead in the spark path with the plug wire about a third of an inch away from the distributor terminal. You’ll see a FLARE on one side of the pencil lead and this flare should be on the spark plug side of the lead."




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Old 04-23-2011, 06:51 AM   #4
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Default Re: The Model A Ignition System

Mitch, You;re welcome!

The Chicagoland MG club article also suggests that you can use an analog voltmeter to test the polarity, which is true and easy to do, but I would suggest also using a high ohm resistor between the meter leads so you don't peg the needle or damage the meter. Something like a 100K ohm resistor will work fine.
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Old 04-23-2011, 08:48 AM   #5
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Default Re: The Model A Ignition System

Thanks Tom, To the electricaly chalenged, it is good to see the "but" enter the discussion. This once again proves that no two people can explain electrical systems without getting more data added to the original post. Its a gift some got at birth, I'm one that was shorted.
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Mitch, You;re welcome!

The Chicagoland MG club article also suggests that you can use an analog voltmeter to test the polarity, which is true and easy to do, but I would suggest also using a high ohm resistor between the meter leads so you don't peg the needle or damage the meter. Something like a 100K ohm resistor will work fine.
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Old 04-23-2011, 10:52 AM   #6
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Default Re: The Model A Ignition System

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Originally Posted by Tom Wesenberg View Post
Mitch, You;re welcome!

The Chicagoland MG club article also suggests that you can use an analog voltmeter to test the polarity, which is true and easy to do, but I would suggest also using a high ohm resistor between the meter leads so you don't peg the needle or damage the meter. Something like a 100K ohm resistor will work fine.
Ya, you'll ruin your digital unit. Ask me how I know.......
Holding the pencil is a shocking experience as well
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Old 04-23-2011, 02:06 PM   #7
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Ya, you'll ruin your digital unit. Ask me how I know.......
Holding the pencil is a shocking experience as well
I slip some plastic tubing over the pencil end. Sometimes if you grab the pencil NOT NEAR THE END, but near the center, you can get by with no shocks.

Last edited by Tom Wesenberg; 05-22-2013 at 09:36 AM.
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Old 05-22-2013, 08:41 AM   #8
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Default Re: The Model A Ignition System

I know this is an old post. Anyone know of a business that will service an old 6 volt beehive coil? Please PM me.

Last edited by skryla; 05-22-2013 at 01:05 PM.
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Old 05-22-2013, 12:12 PM   #9
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Default Re: The Model A Ignition System

You may can still get a NOS beehive coil from Berts. I got several from them in the ninties.
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Old 05-22-2013, 12:16 PM   #10
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Is there such a thing as a diamond-impregnated thin blade that were serve as a point file? Seems like I've heard of something like this.
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Old 05-22-2013, 12:23 PM   #11
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Is there such a thing as a diamond-impregnated thin blade that were serve as a point file? Seems like I've heard of something like this.
i have diamond file sets i use for computer connectors etc they work great
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Old 05-22-2013, 12:25 PM   #12
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Default Re: The Model A Ignition System

My wife had a Diamond Deb fingernail file, really!! I'm not sure that it really had any diamonds impregnated. My wife, Deb, frowned on my using it on the model A points. I'm pretty sure that it would have worked well.
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Old 05-22-2013, 12:34 PM   #13
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Default Re: The Model A Ignition System

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Is there such a thing as a diamond-impregnated thin blade that were serve as a point file? Seems like I've heard of something like this.
Yep! http://www.northerntool.com/shop/too...6157_200396157
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Old 05-22-2013, 12:42 PM   #14
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its not diamond
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Old 05-22-2013, 12:53 PM   #15
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its not diamond
That's a cheap steel one. Most points files are made of tungsten. I did a search and didn't see any diamond coated. The files I have are old and of unknown composition.
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Old 05-22-2013, 09:17 PM   #16
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Default Re: The Model A Ignition System

At the risk of being thrown under the bus. . . If I loosen the cam screw on the distributor and turn the cam, what direction do I have to rotate the cam to retard the timing?

I recently removed the upper plate to replace the wire and adjusted points/timing per Marco's page. I think the timing is too advanced - when I pull the spark control lever down past 1/2 way, the engine begins to misfire. I have re-timed it three times - with the same results. I'd like to loosen the cam and turn it ever so slightly to retard the timing and see if this makes a difference.

I know it's not the correct way, but I'd like to see if my theory is correct.
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Old 05-22-2013, 09:49 PM   #17
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Default Re: The Model A Ignition System

Hard to believe this is a two year old thread, maybe this is the Summer I get Dad's Roadster running again. It will be 30 years since he last drove it. Bob
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Old 05-22-2013, 10:09 PM   #18
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Default Re: The Model A Ignition System

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Originally Posted by Mike in NRN IN View Post
At the risk of being thrown under the bus. . . If I loosen the cam screw on the distributor and turn the cam, what direction do I have to rotate the cam to retard the timing?

I recently removed the upper plate to replace the wire and adjusted points/timing per Marco's page. I think the timing is too advanced - when I pull the spark control lever down past 1/2 way, the engine begins to misfire. I have re-timed it three times - with the same results. I'd like to loosen the cam and turn it ever so slightly to retard the timing and see if this makes a difference.

I know it's not the correct way, but I'd like to see if my theory is correct.
The rotor turns counterclockwise, so turning the cam clockwise will RETARD the timing.

Make sure the wire terminal between the plates is bent against the points mounting nut, so it won't touch the spring or distributor cast body as the advance lever is moved.
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Old 05-22-2013, 11:25 PM   #19
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Default Re: The Model A Ignition System

I may have missed it in the above discussion but: What is the range of advance/retard that you should get using the hand advance lever on the steering column in a strictly stock situation? +38/-0 or what?
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Old 05-23-2013, 07:38 AM   #20
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My wife had a Diamond Deb fingernail file, really!! I'm not sure that it really had any diamonds impregnated. My wife, Deb, frowned on my using it on the model A points. I'm pretty sure that it would have worked well.
I believe the abrasive is actually corundum, bonded with nickel and overplated with bright chrome. Corundum is an industrial abrasive grade of ruby or sapphire. Many grinding wheels are made of this stuff. (Mohs=9). I have a "DiamonDeb" fingernail file ($5) in my carry-along tools. I've used it several times. Like any use of abrasives on points, the dust may be non-conductive so drag some paper or a matchbook cover through the points when finished.
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