Coil testing

[sdgloo]

[sdgloo]

1937 Evinrude Sport Four 9022;

cleaning and testing the magneto;

what is the proper test procedure and values for the two output coils?
TIA

1 user thanked author for this post.

Tubs

[joecb]

“two output coils” … I call these “ungrounded secondary coils”. There is some confusion / mis-understanding surrounding these coils.  In a single output coil, one end of the secondary winding is grounded at the side of the coil, the other end of the winding is your spark plug wire…. thus creating a “loop” circuit. With the two output coil each end of the secondary winding goes to a spark plug wire… so, you might ask, where is the loop circuit? With this arrangement, the loop  includes the entire engine block. The electrical circuit is this,  those little electron guys leave one end of the secondary winding, jump the gap in the first spark plug, travel through the engine block, jump the other spark plug gap and then back to the opposite end of the secondary winding.

Since the “loop” includes BOTH spark plugs , it is very important that when pulling over a motor with this “ungrounded secondary” ignition system, to be sure that both wires are are connected to form at complete loop circuit. To not do this, risks damaging the coil.

Testing with a ohm meter, connect test leads to both spark plug wires. The reading should be about as in a single output coil…somewhere in the range of 4500 to 7000 ( 4.5 K – 7K) ohms.

As in any coil the primary winding will show a very low resistance… like 1.5 – 2 ohms

Hope this helps…

Joe B

1 user thanked author for this post.

T....in Wisconsin

[sdgloo]

Thank you Joe!
that confirms my thinking. I’m into a  1937 Evinrude Sport Four. One coil has continuity across the spark plug wire terminals and one doesn’t. Neither of course has continuity to the primary.
Looks like I’m going coil shopping. Wish me luck.

[jeff-register]

The secondary winding has two ends, one to each spark plug. To test use an ohm meter & connect each end to the spark plug leads & read resistance. When a “dual fire” coil discharges the end that has the least resistance fires first, then the coil “sees” the path to ground thru the spark plug there is enough electrical energy in the windings to “fire” the second spark plug. NEVER put the two spark plug leads together as the energy has no where to go & might arc to ground thru the windings. Once you make the electrical energy it HAS to go somewhere & will blow a hole in the windings to the mag plate, then the coil has very high resistance from the carbon arc or it’s completely open, either one is coil failure.

If you ever work with relays in a D.C. circuit, there must be a doide across the two winding terminals as once the power is removed the winding now has an electrical charge & must go somewhere. That is why a diode is placed across the relay winding to discharge the stored energy. The diode is a one way electrical valve.

Also must be put in the correct polarty.

1 user thanked author for this post.

T....in Wisconsin

[T….in Wisconsin]

Appreciate this information, I’ve been testing an 23 A  & 27 A-35. I believe this explains why I get no reading across the secondary windings when checking with a multimeter.

If I understand this correctly? To check the secondary winding using a multimeter  both plugs need to be installed and run  a  test leads from both spark plugs leads to the multimeter test lead?

Please advise.

[joecb]

Tim, to clarify… to test the secondary winding with an ohm meter, connect the meter leads to the ends of the spark plug wires.  You should read somewhere like 4 to 7 K ohms.

If you get an infinite ( open) reading the secondary winding has a open, broken wire and is most likely bad…. however, I have seen coils that test open produce a spark. The question is for just how long they will continue to run.    There should be no connection between either spark plug leads and any ground.

Joe B

[T….in Wisconsin]

Joe,

I may be reading to much into this.  When using a multimeter do I connect to both spark plug wires and take my reading or do I test them separately.  Referencing the above information is why I ask. “Since the “loop” includes BOTH spark plugs , it is very important that when pulling over a motor with this “ungrounded secondary” ignition system, to be sure that both wires are are connected to form at complete loop circuit” I understand this in reference to pulling the motor over, does it also apply when testing with a multimeter.

Respectfully, there are no d..b questions applies here….

[Steve Martin]

Disconnect both spark plug wires from the plugs. Connect one ohm meter test lead to one of the plug wires.  Connect the other ohm meter test lead to the other plug wire.  You should read 4 to 7k ohms if the coil is good.

1 user thanked author for this post.

T....in Wisconsin

[Tubs]

A picture might be helpful.
I believe this is a good coil is from a
1923 or 24 Model A Johnson.
Tubs

A "Boathouse Repair" is one that done without having tools or the skills to do it properly.

[Author]

Posts